INTERNATIONAL PROGRAMME ON CHEMICAL SAFETY

    ENVIRONMENTAL HEALTH CRITERIA 86

    Mercury - Environmental Aspects

    This report contains the collective views of an international group of
    experts and does not necessarily represent the decisions or the stated
    policy of the United Nations Environment Programme, the International
    Labour Organisation, or the World Health Organization.

    First draft prepared at the National Institute of Health Sciences,
    Tokyo, Japan, and the Institute of Terrestrial Ecology, Monk's Wood,
    United Kingdom

    Published under the joint sponsorship of the United Nations
    Environment Programme, the International Labour Organisation, and the
    World Health Organization

    World Health Organization
    Geneva, 1989

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    CONTENTS

    ENVIRONMENTAL HEALTH CRITERIA FOR MERCURY - ENVIRONMENTAL ASPECTS

    1. SUMMARY AND CONCLUSIONS

         1.1. Physical and chemical properties
         1.2. Sources in the environment
         1.3. Uptake, elimination, and accumulation in organisms
         1.4. Toxicity to microorganisms
         1.5. Toxicity to aquatic organisms
         1.6. Toxicity to terrestrial organisms
         1.7. Effects of mercury in the field

    2. PHYSICAL AND CHEMICAL PROPERTIES

    3. SOURCES OF MERCURY IN THE ENVIRONMENT

         3.1. Natural and anthropogenic sources and cycling
         3.2. Speciation
         3.3. Levels in the environment
         3.4. Methylation of mercury

    4. UPTAKE, LOSS, AND ACCUMULATION IN ORGANISMS

         4.1. Speciation of mercury
         4.2. Uptake and loss in aquatic organisms
              4.2.1. Microorganisms, plants, and invertebrates
              4.2.2. Fish
                     4.2.2.1  Effects of environmental variables on 
                              uptake by fish
              4.2.3. Studies on more than one type of organism
         4.3. Uptake and loss in terrestrial organisms
         4.4. Accumulation in the field
              4.4.1. General exposure
              4.4.2. Mercury manufacturing and general industrial areas
              4.4.3. Mining activity
              4.4.4. Chloralkali plants
              4.4.5. Mercurial fungicides

    5. TOXICITY TO MICROORGANISMS

         5.1. Toxicity of inorganic mercury
              5.1.1. Single species cultures
              5.1.2. Mixed cultures and communities
         5.2. Toxicity of organic mercury

    6. TOXICITY TO AQUATIC ORGANISMS

         6.1. Toxicity to aquatic plants
         6.2. Toxicity to aquatic invertebrates

              6.2.1. Acute and short-term toxicity to
                     invertebrates
              6.2.2. Behavioural effects
         6.3. Toxicity to fish
              6.3.1. Acute and short-term toxicity to fish
              6.3.2. Reproductive effects and effects on
                     early life stages
              6.3.3. Behavioural effects
              6.3.4. Physiological and biochemical effects
         6.4. Toxicity to amphibia
         6.5. Toxicity to aquatic mammals

    7. TOXICITY TO TERRESTRIAL ORGANISMS

         7.1. Toxicity to terrestrial plants
         7.2. Toxicity to terrestrial animals
              7.2.1. Toxicity to terrestrial invertebrates
              7.2.2. Effects of mercury on birds
                     7.2.2.1  Inorganic and metallic mercury
                     7.2.2.2  Effect of organic mercury on birds
              7.2.3. Effects of mercury on non-laboratory mammals

    8. EFFECTS OF MERCURY IN THE FIELD

    9. EVALUATION

         9.1. The marine environment
         9.2. The freshwater environment
         9.3. The terrestrial environment

    REFERENCES

    WHO TASK GROUP ON ENVIRONMENTAL HEALTH CRITERIA FOR MERCURY -
    ENVIRONMENTAL ASPECTS

     Participants

    Dr L.A. Albert, Director, Environmental Pollution Programme, National
         Institute for Research on Biotic Resources, Xalapa, Mexico

    Professor T.W. Clarkson, Division of Toxicology, The University of
         Rochester, School of Medicine and Dentistry, Rochester, USA
          (Chairman)

    Dr R. Elias, Environmental Criteria and Assessment Office, US
         Environmental Protection Agency, Research Triangle Park, North
         Carolina, USA

    Dr J.H.M. Temmink, Department of Toxicology, Agricultural University,
         Biotechnion, Wageningen, Netherlands

    Dr G. Roderer, Fraunhofer Institute for Environmental Chemistry and
         Ecotoxicology, Schmallenberg-Grafschaft, Federal Republic of
         Germany

    Dr R. Koch, Division of Toxicology, Research Institute for Hygiene and
         Microbiology, Bad Elster, German Democratic Republic

    Professor Y. Kodama, Department of Environmental Health, University of
         Occupational and Environmental Health, Kitakyushu, Japan

    Professor P.N. Viswanathan, Ecotoxicology Section, Industrial
         Toxicology Research Centre, Lucknow, India

     Observers

    Mr D.J.A. Davies, Department of the Environment, London, United
         Kingdom

    Dr I. Newton, The Institute of Terrestrial Ecology, Monks Wood
         Experimental Station, Huntingdon, United Kingdom

     Secretariat

    Dr S. Dobson, The Institute of Terrestrial Ecology, Monks Wood
         Experimental Station, Huntingdon, United Kingdom  (Rapporteur)

    Dr M. Gilbert, International Programme on Chemical Safety, World
         Health Organization, Geneva, Switzerland  (Secretary)

    Mr P.D. Howe, The Institute of Terrestrial Ecology, Monks Wood
         Experimental Station, Huntingdon, United Kingdom

    NOTE TO READERS OF THE CRITERIA DOCUMENTS

         Every effort has been made to present information in the criteria
    documents as accurately as possible without unduly delaying their
    publication. In the interest of all users of the environmental health
    criteria documents, readers are kindly requested to communicate any
    errors that may have occurred to the Manager of the International
    Programme on Chemical Safety, World Health Organization, Geneva,
    Switzerland, in order that they may be included in corrigenda, which
    will appear in subsequent volumes.

                               *   *   *

         A detailed data profile and a legal file can be obtained from the
    International Register of Potentially Toxic Chemicals, Palais des
    Nations, 1211 Geneva 10, Switzerland (Telephone no. 988400 - 985850).

    ENVIRONMENTAL HEALTH CRITERIA FOR MERCURY - ENVIRONMENTAL ASPECTS

         A WHO Task Group on Environmental Health Criteria for Mercury -
    Environmental Aspects met at the Institute of Terrestrial Ecology,
    Monks Wood, UK, from 7 to 11 December 1987. Dr B.N.K. Davis welcomed
    the participants on behalf of the host Institution, and Dr M. Gilbert
    opened the meeting on behalf of the three co-sponsoring organizations
    of the IPCS (ILO/UNEP/WHO). The Task Group reviewed and revised the
    draft criteria document and made an evaluation of the risks for the
    environment from exposure to mercury.

         The first draft of this document was prepared by Dr S. Dobson and
    Mr P.D. Howe, Institute of Terrestrial Ecology. Dr M. Gilbert and
    Dr P.G. Jenkins, both members of the IPCS Central Unit, were
    responsible for the overall scientific content and editing,
    respectively.

                                  *  *  *

         Partial financial support for the publication of this criteria
    document was kindly provided by the United States Department of Health
    and Human Services, through a contract from the National Institute of
    Environmental Health Sciences, Research Triangle Park, North Carolina,
    USA - a WHO Collaborating Centre for Environmental Health Effects.

    INTRODUCTION

         There is a fundamental difference in approach between the
    toxicologist and the ecotoxicologist concerning the appraisal of the
    potential threat posed by chemicals. The toxicologist, because his
    concern is with human health and welfare, is preoccupied with any
    adverse effects on individuals, whether or not they have ultimate
    effects on performance or survival. The ecotoxicologist, in contrast,
    is concerned primarily with the maintenance of population levels of
    organisms in the environment. In toxicity tests, he is interested in
    effects on the performance of individuals - in their reproduction and
    survival - only insofar as these might ultimately affect the
    population size. To him, minor biochemical and physiological effects
    of toxicants are irrelevant if they do not, in turn, affect
    reproduction, growth, or survival.

         It is the aim of this document to take the ecotoxicologist's
    point of view and consider effects on populations of organisms in the
    environment. No attempt has been made to link the conclusions reached
    in this document with possible effects on human health. This will only
    be feasible when Environmental Health Criteria 1: Mercury (WHO, 1976),
    which considered the effects of mercury on human health, has been
    updated. Due attention has been given to the persistence in the
    environment and the bioaccumulation and transport of mercury in
    aquatic food chains. These will have implications for human
    consumption of the metal.

         This document, although based on a thorough survey of the
    literature, is not intended to be exhaustive in the material included.
    In order to keep the document concise, only those data which were
    considered to be essential in the evaluation of the risk posed by
    mercury to the environment have been included. Concentration figures
    for mercury in the environment, or in particular species of organism,
    have not been included unless they illustrate specific toxicological
    points. "Snap shot" concentration data, where a causal relationship
    between the presence of the metal and an observed effect is not
    clearly demonstrated, have been excluded.

         The term bioaccumulation indicates that organisms take-up
    chemicals to a greater concentration than that found in their
    environment or their food. 'Bioconcentration factor' is a quantitative
    way of expressing bioaccumulation: the ratio of the concentration of
    the chemical in the organism to the concentration of the chemical in
    the environment or food. Biomagnification refers, in this document, to
    the progressive accumulation of chemicals along a food chain.

    1.  SUMMARY AND CONCLUSIONS

    1.1  Physical and chemical properties

         Mercury is a metal which is liquid at normal temperatures and
    pressures. It forms salts in two ionic states mercury(I) and
    mercury(II). Mercury(II), or mercuric, salts are very much more common
    than mercury(I) salts, and hence it is mercuric salts which will be
    mainly considered here. Mercury also forms organometallic compounds,
    some of which have found industrial and agricultural use.
    "Organometallic" is used here to indicate a covalently-bonded
    compound, and does not include mercury bound to proteins nor salts
    formed with organic acids. These organometallic compounds are stable,
    though some are readily broken down by living organisms, while others
    are not readily biodegraded. Elemental mercury gives rise to a vapour
    which dissolves only slightly in water.

    1.2  Sources in the Environment

         Natural mercury arises from the degassing of the earth's crust
    through volcanic gases and, probably, by evaporation from the oceans.
    Local levels in water derived from mercury ores may also be high
    (up to 80 µg/litre). Atmospheric pollution from industrial production
    is probably low, but pollution of water by mine tailings is
    significant. The burning of fossil fuels is a source of mercury. The
    chloralkali industry and, previously, the wood pulping industry also
    released significant amounts of mercury. Although the use of mercury
    is reducing, high concentrations of the metal are still present in
    sediments associated with the industrial applications of mercury. Some
    mercury compounds have been used in agriculture, principally as
    fungicides.

    1.3  Uptake, Elimination, and Accumulation in Organisms

         Mercuric salts, and, to a much greater extent, organic mercury,
    are readily taken up by organisms in water. Aquatic invertebrates, and
    most particularly aquatic insects, accumulate mercury to high
    concentrations. Fish also take up the metal and retain it in tissues,
    principally as methylmercury, although most of the environmental
    mercury to which they are exposed is inorganic. The source of the
    methylation is uncertain, but there is strong indication that
    bacterial action leads to methylation in aquatic systems.
    Environmental levels of methylmercury depend upon the balance between
    bacterial methylation and demethylation. The indications are that
    methylmercury in fish arises from this bacterial methylation of
    inorganic mercury, either in the environment or in bacteria associated
    with fish gills, surface, or gut. There is little indication that fish
    themselves either methylate or demethylate mercury. Elimination of
    methylmercury is slow from fish (with half times in the order of
    months or years) and from other aquatic organisms. Loss of inorganic
    mercury is more rapid and so most of the mercury in fish is retained

    in the form of methylmercury. Terrestrial organisms are also
    contaminated by mercury, with birds being the best studied. Sea birds
    and those feeding in estuaries are most contaminated. The form of
    retained mercury in birds is more variable and depends on species,
    organ, and geographical site.

    1.4  Toxicity to Microorganisms

         The metal is toxic to microorganisms. Inorganic mercury has been
    reported to have effects at concentrations of the metal in the culture
    medium of 5 µg/litre, and organomercury compounds at concentrations at
    least 10 times lower than this. Organomercury compounds have been used
    as fungicides. One factor affecting the toxicity of the organometal is
    the rate of uptake of the metal by cells. Mercury is bound to the cell
    walls or cell membranes of microorganisms, apparently to a limited
    number of binding sites. This means that effects are related to cell
    density as well as to the concentration of mercury in the substrate.
    These effects are often irreversible, and mercury at low
    concentrations represents a major hazard to microorganisms.

    1.5  Toxicity to Aquatic Organisms

         The organic forms of mercury are generally more toxic to aquatic
    organisms than the inorganic forms. Aquatic plants are affected by
    mercury in the water at concentrations approaching 1 mg/litre for
    inorganic mercury but at much lower concentrations of organic mercury.
    Aquatic invertebrates vary greatly in their susceptibility to mercury.
    Generally, larval stages are more sensitive than adults. The 96-h
    LC50s vary between 33 and 400 µg/litre for freshwater fish and are
    higher for sea-water fish. However, organic mercury compounds are more
    toxic. Toxicity is affected by temperature, salinity, dissolved
    oxygen, and water hardness. A wide variety of physiological and
    biochemical abnormalities has been reported after fish have been
    exposed to sublethal concentrations of mercury, although the
    environmental significance of these effects is difficult to assess.
    Reproduction is also affected adversely by mercury.

    1.6  Toxicity to Terrestrial Organisms

         Plants are generally insensitive to the toxic effects of mercury
    compounds. Birds fed inorganic mercury show a reduction in food intake
    and consequent poor growth. Other, more subtle, effects on enzyme
    systems, cardiovascular function, blood parameters, the immune
    response, kidney function and structure, and behaviour have been
    reported. Organomercury compounds are more toxic for birds than are
    inorganic.

    1.7  Effects of Mercury in the Field

         Pollution of the sea with organomercury led to the death of fish
    and fish-eating birds in Japan. Except for this incident at Minamata,
    few follow-up studies of the effects of localised release have been
    conducted. The use of organomercury fungicides as seed dressings in
    Europe led to the deaths of large numbers of granivorous birds,
    together with birds of prey feeding on the corpses. Residues of
    mercury in birds' eggs have been associated with deaths of embryos in
    shell. The presence of organochlorine residues in the same birds and
    their eggs makes an accurate assessment of the effects of mercury
    difficult. It is, however, thought to be a contributing factor in the
    population decline of some species of raptors.

    2.  PHYSICAL AND CHEMICAL PROPERTIES

         The physical and chemical properties of mercury have been
    detailed in Environmental Health Criteria 1: Mercury (WHO, 1976). The
    relevant chapter is summarized here.

         Mercury can exist in a wide variety of physical and chemical
    states. The different chemical and physical forms of this element all
    have their intrinsic toxic properties and different applications in
    industry and agriculture, and require a separate assessment of risk.

         Mercury, along with cadmium and zinc, falls into Group IIb of the
    Periodic Table. In addition to its elemental state, mercury exists in
    the mercury (I) and mercury (II) states in which the mercury atom has
    lost one and two electrons, respectively. The chemical compounds of
    mercury (II) are much more numerous than those of mercury (I).

         In addition to simple salts, such as chloride, nitrate and
    sulfate, mercury (II) forms an important class of organometallic
    compounds. These are characterized by the attachment of mercury to
    either one or two carbon atoms to form compounds of the type RHgX and
    RHgR' where are R and R' represent the organic moiety. The most
    numerous are those of the type RHgX. X may be one of a variety of
    anions. The carbon-mercury bond is chemically stable. It is not split
    in water nor by weak acids or bases. The stability is not due to the
    high strength of the carbon-mercury bond but to the very low affinity
    of mercury for oxygen. The organic moiety, R, takes a variety of
    forms, some of the most common being the alkyl, the phenyl, and the
    methoxyethyl radicals. If the anion X is nitrate or sulfate, the
    compound tends to be "salt-like" having appreciable solubility in
    water; however, the chlorides are covalent, non-polar compounds that
    are more soluble in organic solvents than in water. From the
    toxicological standpoint, the most important of these organometallic
    compounds is the subclass of short-chain alkyl mercurials in which
    mercury is attached to the carbon atom of a methyl, ethyl, or propyl
    group.

    3.  SOURCES OF MERCURY IN THE ENVIRONMENT

         The sources of mercury have been detailed in Environmental Health
    Criteria 1: Mercury (WHO, 1976). Relevant data are summarized here.

    3.1  Natural and Anthropogenic Sources and Cycling

         The major source of mercury is the natural degassing of the
    earth's crust and amounts to between 25 000 and 125 000 tonnes per
    year. Anthropogenic sources are probably less than natural sources.
    World production of mercury by mining and smelting was estimated at
    10 000 tonnes per year in 1973 and has been increasing at an annual
    rate of about 2%. The chloralkali, electrical equipment, and paint
    industries are the largest consumers of mercury, accounting for about
    55% of the total consumption. Mercury has a wide variety of other uses
    in industry, agriculture, military applications, medicine, and
    dentistry.

         Several of man's activities, not directly related to mercury,
    account for substantial releases into the environment. These include
    the burning of fossil fuel, the production of steel, cement, and
    phosphate, and the smelting of metals from their sulfide ores.

         Alkylmercury fungicides used as seed dressings are important
    original sources of mercury in terrestrial food chains, although the
    use of these materials has decreased considerably.

         Two cycles are believed to be involved in the environmental
    transport and distribution of mercury. One is global in scope and
    involves the atmospheric circulation of elemental mercury vapour from
    sources on land to the oceans. However, the mercury content of the
    oceans is so large, at least 70 million tonnes, that the yearly
    increases in concentration due to deposition from the global cycle are
    not detectable.

         The other cycle is local in scope and depends upon the
    methylation of inorganic mercury mainly from anthropogenic sources.
    Many steps in this cycle are still poorly understood, but it is
    believed to involve the atmospheric circulation of dimethylmercury
    formed by bacterial action.

    3.2  Speciation

         The following speciation among mercury compounds has been
    proposed by Lindquist et al. (1984), where V stands for volatile, R
    for water-soluble or particle-borne reactive species, and NR for non-
    reactive species (Hg° is elemental mercury):

         V:   Hg°, (CH3)2Hg

         R:   Hg2+, HgX2, HgX3-, and HgX42-,

              with X = OH-, Cl- and Br-.

              HgO on aerosol particles. Hg2+ complexes with organic
              acids.

         NR:  CH3Hg+, CH3HgCl, CH3HgOH and other organomercuric
              compounds, Hg(CN)2. HgS and Hg2+ bound to sulfur in
              fragments of humic matter.

    The main volatile form in air is elemental mercury but dimethylmercury
    may also occur (Slemr et al., 1951).

         Uncharged complexes, such as HgCl2, CH3HgOH etc., occur in
    the gaseous phase, but are also relatively stable in fresh water (snow
    and rain as well as standing or flowing water). HgCl42- is the
    dominant form in sea water.

    3.3  Levels in the Environment

         The following data have been extracted from Lindquist et al.
    (1984) and are included here to indicate background levels of mercury
    in the environment. Considerable local variations can occur and local
    levels close to anthropogenic sources of mercury would be much higher.

         Reliable data on mercury concentrations in the  air are scarce.
    Recent information suggests a background level at about 2 ng/m3 in
    the lower troposphere of the northern hemisphere and about 1 ng/m3
    in the southern hemisphere, at least over oceanic areas. In European
    areas remote from industrial sources, such as the rural parts of
    southern Sweden and Italy, concentrations most often lie in the range
    from 2 to 3 ng/m3 in summer and from 3 to 4 ng/m3 in winter
    (Brosset 1983, Ferrara et al., 1982). In urban air the concentrations
    could be higher.

         Deposition with  precipitation is a major factor in removing
    mercury from the atmosphere. The lowest concentrations of mercury in
    rain water, around 1 ng/litre, have been reported from a coastal site
    in Japan and from the islands of Samoa. Most other values reported lie
    in the range between 5 and 100 ng/litre.

         Recent measurements of mercury in  aquatic systems have given
    the following concentration ranges, which may be considered
    representative for dissolved mercury:

              Open ocean          0.5-3 ng/litre

              Coastal sea water   2-15 ng/litre

              Rivers and lakes    1-3 ng/litre

         Local variations from these values are considerable, especially
    in coastal sea water and in lakes and rivers where mercury associated
    with suspended material may also contribute to the total load.

         The mercury content in minerals forming ordinary rock and  soils
    is usually very low. The normal level in igneous rocks and minerals
    seems to be less than 50 µg/kg, and in many cases is less than
    10 µg/kg. Due to the strong binding of mercury to soil particles,
    including organic matter, only small amounts of the metal are present
    in soil solution; reported averages range between 20 and 625 µg/kg
    soil.

         Background levels in  sediments are approximately the same as
    levels in unpolluted surface soils. Average concentrations in ocean
    sediments probably lie in the range between 20 and 100 µg/kg.

    3.4  Methylation of Mercury

         The methylation of inorganic mercury in the sediment of lakes,
    rivers and other waterways, as well as in the oceans, is a key step in
    the transport of mercury in aquatic food chains.

         It was first demonstrated by Jensen & Jernelov (1967) that
    microorganisms in lake sediments could methylate mercury. They later
    showed that the degree of methylation correlated well with the overall
    microbial activity in the sediment (Jensen & Jernelov, 1969). Detailed
    mechanisms of methylation in microorganisms have been proposed by Wood
    (1971) and Landner (1971). Some soil organisms capable of methylating
    mercury have also been isolated (Kitamura et al., 1969; Yamada &
    Tonamura, 1972).

         The following general conclusions have been drawn by Bisogni &
    Lawrence (1973) concerning methylation by microorganisms:

         (a)  mono-methylmercury is the predominant product of biological
              methylation near neutral pH,

         (b)  the rate of methylation is greater under oxidising
              conditions than under anaerobic conditions,

         (c)  the output of methylmercury doubles for a ten-fold increase
              in inorganic mercury,

         (d)  temperature affects methylation as a result of its effect on
              overall microbial activity,

         (e)  higher microbial growth rate increases mercury methylation,

         (f)  methylation rates are inhibited by the addition of sulfide
              to anaerobic systems.

         The formation of new or enlarged artificial lakes considerably
    increases the production of methylmercury, although this increase was
    found to be short-lived in new lakes in Finland (Simola & Lodenius,
    1982; Alfthan et al., 1983). A similar problem of increased mercury in
    new lakes, which was taken up by fish and fish-eating mammals,
    occurred in the scheme to divert the Churchill River in Manitoba,
    Canada (Canada-Manitoba, 1987). Methylation rates in one lake, which
    had been flooded 20 years previously, had returned to normal.
    Methylation rates in the new lake, which had flooded arboreal forest,
    were high and were expected to remain high for decades. The source of
    mercury in all of these artificial lakes appeared to be natural rather
    than anthropogenic in origin. Anaerobic conditions after the flooding
    of large amounts of organic material and the subsequent increase in
    microbial activity are thought to be the causes of the increased
    availability of mercury through methylation.

    4.  UPTAKE, LOSS, AND ACCUMULATION IN ORGANISMS

          Background levels of naturally-occuring mercury in the
     environment are generally low, except in the immediate vicinity of
     mining sites and chloralkali plants for the industrial extraction of
     mercury. The majority of mercury in the environment is natural rather
     than the result of human activities. Inorganic mercury can be
     methylated in the environment and the resultant methylmercury is
     taken up into organisms readily; more readily than inorganic mercury.
     Although environmental levels are low, the high capacity of organisms
     to accumulate mercury means that the metal is found widely in both
     aquatic and terrestrial animals and plants. Methylmercury is released
     more slowly by aquatic organisms than inorganic mercury. Aquatic
     invertebrates, and particularly aquatic insects, accumulate mercury
     to a greater extent than fish.

          Speciation of mercury is of great importance in determining the
     uptake of the metal from water and soil. Much of the mercury in
     natural waters and in soil is strongly bound to sediment or organic
     material and is unavailable to organisms.

          Mercury has been found in many terrestrial organisms, birds
     being the subjects of most of the monitoring.

          In many experimental studies, the concentrations of mercury
     quoted are nominal rather than measured. Few attempts have been made
     to estimate available mercury in experimental studies.

          Because of the very extensive literature on the uptake of metals
     into organisms, this section contains illustrative examples and is
     not exhaustive.

         Bioconcentration factors for mercury, determined in laboratory
    experiments, are summarized in Tables 1 and 2.

         Bioconcentration factors are simple ratios between the
    concentration of mercury in an organism and the concentration in the
    medium to which the organism was exposed. This means that results
    should be treated with caution. A relatively low body burden resulting
    from exposure to very low levels of mercury in the medium can give a
    high bioconcentration factor. Conversely, exposure to very high
    mercury levels in the medium can lead to a low bioconcentration
    factor. Exposure to mercury under static test conditions will lead to
    the removal of mercury during the course of the test, whereas flow-
    through conditions maintain a constant level of exposure. Since
    mercury is strongly bound to sediment in the field, it is unclear
    which of these two exposure regimes is the most realistic. It is
    probable that static exposure underestimates and flow-through exposure
    overestimates mercury uptake. Most studies have failed to distinguish

        Table 1.  Accumulation of mercury into aquatic organisms

    Organism         Lifestagec  Stat/  Organb     Temperature  Compounde                 Duration   Exposure   Bioconcentration    Reference
                                 flowa                (°C)                                 (days)   (µg/litre)      factorg

    Alga                                                        mercuric chloride           2         164         8537h          Parrish & Carr
    (Croomonas                                                                                                                   (1976)
    salina)

    Filamentous algae            stat                           phenyl mercuric acetate     35        10          1200f          Hannerz (1968)
    (Oedogonium sp.)             flow                  16.5     methoxyethylmercuric OH     18        0.58        2610f          Hannerz (1968)
                                 flow                  15.2     mercuric chloride           54        0.05        871f           Hannerz (1968)

    Duckweed                     flow                           methylmercuric OH           32        3           2950f          Hannerz (1968)
    (Lemna minor)                flow                  16.5     methoxyethylmercuric OH     24        0.58        480f           Hannerz (1968)
                                 flow                  15.2     mercuric chloride           41        0.05        70f            Hannerz (1968)

    Water hyacinth     mature    stat   roots          23-27    mercuric chloride           16        1000        580            Muramoto & Oki
    (Eichhornia                                                                                                                  (1983)
    crassipes)

    Reed                         stat   emergent                phenyl mercuric acetate     35        10          0f             Hannerz (1968)
    (Phragmites                  stat   submerged               phenyl mercuric acetate     35        10          850f           Hannerz (1968)
    communis)                    flow   emergent                methylmercuric OH           32        3           25f            Hannerz (1968)
                                 flow   submerged               methylmercuric OH           32        3           530f           Hannerz (1968)
                                 flow   emergent       16.5     methoxyethylmercuric OH     24        0.58        74f            Hannerz (1968)
                                 flow   submerged      16.5     methoxyethylmercuric OH     24        0.58        139f           Hannerz (1968)
                                 flow   emergent       15.2     mercuric chloride           41        0.05        56f            Hannerz (1968)
                                 flow   submerged      15.2     mercuric chloride           14        0.05        149f           Hannerz (1968)

    Bulrush                      stat   emergent                phenyl mercuric acetate     35        10          90f            Hannerz (1968)
    (Scirpus                     stat   submerged               phenyl mercuric acetate     35        10          790f           Hannerz (1968)
    lucustris)                   flow   emergent                methylmercuric OH           32        3           8f             Hannerz (1968)
                                 flow   submerged               methylmercuric OH           32        3           1250f          Hannerz (1968)

    Table 1 (cont'd)

    Organism         Lifestagec  Stat/  Organb     Temperature  Compounde                 Duration   Exposure   Bioconcentration    Reference
                                 flowa                (°C)                                 (days)   (µg/litre)      factorg

                                 flow   emergent       16.5     methoxyethylmercuric OH     24        0.58        39f            Hannerz (1968)
                                 flow   submerged      16.5     methoxyethylmercuric OH     24        0.58        190f           Hannerz (1968)
                                 flow   emergent       15.2     mercuric chloride           21        0.05        77f            Hannerz (1968)
                                 flow   submerged      15.2     mercuric chloride           41        0.05        70f            Hannerz (1968)

    Yellow iris                  stat   emergent                phenyl mercuric acetate     35        10          20f            Hannerz (1968)
    (Iris                        stat   submerged               phenyl mercuric acetate     35        10          40f            Hannerz (1968)
    pseudacorus)                 flow   emergent                methylmercuric OH           32        3           18f            Hannerz (1968)
                                 flow   submerged               methylmercuric OH           32        3           34f            Hannerz (1968)
                                 flow   emergent       16.5     methoxyethylmercuric OH     18        0.58        31f            Hannerz (1968)
                                 flow   submerged      16.5     methoxyethylmercuric OH     18        0.58        90f            Hannerz (1968)
                                 flow   emergent       15.2     mercuric chloride           49        0.05        18f            Hannerz (1968)
                                 flow   submerged      15.2     mercuric chloride           49        0.05        23f            Hannerz (1968)

    Bloodworm                    stat                           phenyl mercuric acetate     35        10          12 700f        Hannerz (1968)
    (Chironomidae)               flow                           methylmercuric OH           32        3           3070f          Hannerz (1968)
                                 flow                  16.5     methoxyethylmercuric OH     89        0.58        988f           Hannerz (1968)

    Annelid                      stat                           phenyl mercuric acetate     35        10          2030f          Hannerz (1968)
    (Haemopis                    flow                           methylmercuric OH           32        3           450f           Hannerz (1968)
    sanguisuga)                  flow                  16.5     methoxyethylmercuric OH     89        0.58        1148f          Hannerz (1968)

    Annelid                      flow                           methylmercuric OH           32        3           110f           Hannerz (1968)
    (Glossosiphonia              flow                  16.5     methoxyethylmercuric OH     89        0.58        640f           Hannerz (1968)
    complanata)                  flow                  15.2     mercuric chloride           65        0.05        670f           Hannerz (1968)

    Worm                         flow                           methylmercuric OH           32        3           1780f          Hannerz (1968)
    (Oligochaeta)                flow                  16.5     methoxyethylmercuric OH     65        0.58        690f           Hannerz (1968)
                                 flow                  15.2     mercuric chloride           65        0.05        517f           Hannerz (1968)

    Table 1 (cont'd)

    Organism         Lifestagec  Stat/  Organb     Temperature  Compounde                 Duration   Exposure   Bioconcentration    Reference
                                 flowa                (°C)                                 (days)   (µg/litre)      factorg

    Freshwater leech             flow                  15.2     mercuric chloride           65        0.05        534f           Hannerz (1968)
    (Herpobdella
    octoculata)

    Mussel                              WB                      mercuric chloride           4         50          664            Tsuruga (1963)
    (Mytilus edulis)             flow   WB                                                  4         0.06        236f           Hannerz (1968)

    Short-necked clam                   WB                      mercuric chloride           8         50          190            Tsuruga (1963)
    (Venerupis
    philippinarum)

    Pond snail                   stat                           phenyl mercuric acetate     35        10          1280f          Hannerz (1968)
    (Planorbis sp.)              flow                           methylmercuric OH           32        3           3570f          Hannerz (1968)
                                 flow                  16.5     methoxyethylmercuric OH     31        0.58        1970f          Hannerz (1968)
                                 flow                  15.2     mercuric chloride           49        0.05        795f           Hannerz (1968)

    Giant pond snail             stat                           phenyl mercuric acetate     35        10          1800f          Hannerz (1968)
    (Lymnaea                     flow                           methylmercuric OH           32        3           3480f          Hannerz (1968)
    stagnalis)                   flow                  16.5     methoxyethylmercuric OH     31        0.58        1178f          Hannerz (1968)
                                 flow                  15.2     mercuric chloride           14        0.05        297f           Hannerz (1968)

    Snail                        flow                  16.5     methoxyethylmercuric OH     24        0.58        4266f          Hannerz (1968)
    (Physa                       flow                  15.2     mercuric chloride           14        0.05        637f           Hannerz (1968)
    fontinalis)

    Water flea                   stat                           phenyl mercuric acetate     35        10          3570f          Hannerz (1968)
    (Daphnia sp.)

    Cladoceran                   flow                  16.5     methoxyethylmercuric OH     89        0.58        286f           Hannerz (1968)
    (Eurycerus)

    Table 1 (cont'd)

    Organism         Lifestagec  Stat/  Organb     Temperature  Compounde                 Duration   Exposure   Bioconcentration    Reference
                                 flowa                (°C)                                 (days)   (µg/litre)      factorg

    Copepod                      stat   WB             21-26    mercuric chloride           1         0.1         7600           Hirota et al.
    (Acartia clausi)                                                                                                             (1983)
                                 stat   WB             21-26    methyl mercuric chloride    1         0.1         249 000        Hirota et al.
                                                                                                                                 (1983)

    Grass shrimp                        WB                      mercuric chloride           3         1.5         333            Ray & Tripp
    (Palaemonetes                                                                                                                (1976)
    pugio)

    Mayfly             naiad     stat                           phenyl mercuric acetate     35        10          900f           Hannerz (1968)
    (Ephemeridae)      naiad     flow                           methylmercuric OH           32        3           3290f          Hannerz (1968)
                       naiad     flow                  16.5     methoxyethylmercuric OH     24        0.58        680f           Hannerz (1968)
                       larva     flow                  15.2     mercuric chloride           65        0.05        138f           Hannerz (1968)

    Lesser water                 stat                           phenyl mercuric acetate     35        10          4200f          Hannerz (1968)
    boatman                      flow                           methylmercuric OH           32        3           8470f          Hannerz (1968)
    (Corixa sp.)                 flow                  16.5     methoxyethylmercuric OH     89        0.58        740f           Hannerz (1968)
                                 flow                  15.2     mercuric chloride           65        0.05        414f           Hannerz (1968)

    Water boatman                flow                           methylmercuric OH           32        3           2460f          Hannerz (1968)
    (Notonecta                   flow                  16.5     methoxyethylmercuric OH     89        0.58        674f           Hannerz (1968)
    glaaca)                      flow                  15.2     mercuric chloride           65        0.05        483f           Hannerz (1968)

    Midge              larva     flow   WB                      mercuric chloride           30        5.5         19 600         Rossaro et al.
    (Chironomus                                                                                                                  (1986)
    riparius)          pupa      flow   WB                      mercuric chloride           30        5.5         15 600         Rossaro et al.
                                                                                                                                 (1986)
                       adult     flow   WB                      mercuric chloride           30        5.5         7500           Rossaro et al.
                                                                                                                                 (1986)

                                                                                                                                              

    Table 1 (cont'd)

    Organism         Lifestagec  Stat/  Organb     Temperature  Compounde                 Duration   Exposure   Bioconcentration    Reference
                                 flowa                (°C)                                 (days)   (µg/litre)      factorg

    Caddisfly          larva     flow                  16.5     methoxyethylmercuric OH     89        0.58        710f           Hannerz (1968)
    (Trichoptera sp.)  larva     flow                  15.2     mercuric chloride           49        0.05        513f           Hannerz (1968)

    Dragonfly          nymph     flow                  16.5     methoxyethylmercuric OH     89        0.58        1296f          Hannerz (1968)
    (Odonata sp.)

    Damselfly          nymph     flow                  16.5     methoxyethylmercuric OH     89        0.58        1186f          Hannerz (1968)
    (Odonata sp.)      nymph     flow                  15.2     mercuric chloride           65        0.05        655f           Hannerz (1968)

    Alderfly           larva     flow                  16.5     methoxyethylmercuric OH     89        0.58        1270f          Hannerz (1968)
    (Sialis lutaria)

    Cranefly           larva     flow                  16.5     methoxyethylmercuric OH     18        0.58        625f           Hannerz (1968)
    (Tipula sp.)       larva     flow                  15.2     mercuric chloride           41        0.05        840f           Hannerz (1968)

    Great diving       imago     flow                  16.5     methoxyethylmercuric OH     89        0.58        800f           Hannerz (1968)
    beetle
    (Dytiscus
    marginalis)

                       larva     flow                  16.5     methoxyethylmercuric OH     89        0.58        3134f          Hannerz (1968)
                       larva     flow                  15.2     mercuric chloride           65        0.05        603f           Hannerz (1968)
                       imago     flow                  15.2     mercuric chloride           65        0.05        862f           Hannerz (1968)

    Pond skater                  flow                  16.5     methoxyethylmercuric OH     89        0.58        754f           Hannerz (1968)
    (Gerris najas)               flow                  15.2     mercuric chloride           65        0.05        431f           Hannerz (1968)

    Aquatic saw bug              flow                  16.5     methoxyethylmercuric OH     89        0.58        954f           Hannerz (1968)
    (Asellus
    aquaticus)

    Table 1 (cont'd)

    Organism         Lifestagec  Stat/  Organb     Temperature  Compounde                 Duration   Exposure   Bioconcentration    Reference
                                 flowa                (°C)                                 (days)   (µg/litre)      factorg

    Water spiders                flow                  16.5     methoxyethylmercuric OH     89        0.58        624f           Hannerz (1968)
    (Hydracnidae)

    Pike                         stat   liver          17.2     methoxyethylmercuric OH     10        0.4         7673f          Hannerz (1968)
    (Esox lucius)                stat   kidney         17.2     methoxyethylmercuric OH     10        0.4         7230f          Hannerz (1968)
                                 stat   liver                   methylmercuric OH           10        0.3         2002f          Hannerz (1968)
                                 stat   kidney                  methylmercuric OH           10        0.3         2198f          Hannerz (1968)

    Rainbow trout      juv       flow   WB             5        methyl mercuric chloride    84        0.263       4525           Reinert et al.
                                                                                                                                 (1974)
    (Salmo gairdneri)  juv       flow   WB             10       methyl mercuric chloride    84        0.258       6628           Reinert et al.
                                                                                                                                 (1974)
                       juv       flow   WB             15       methyl mercuric chloride    84        0.244       8033           Reinert et al.
                                                                                                                                 (1974)
                                        WBd            5        mercuric chloride           4         50          5              MacLeod &
                                                                                                                                 Pessah (1973)
                                        WBd            10       mercuric chloride           4         50          12             MacLeod &
                                                                                                                                 Pessah (1973)
                                        WBd            20       mercuric chloride           4         50          26             MacLeod &
                                                                                                                                 Pessah (1973)

    Bluegill sunfish             stat   WB             9        methyl mercuric chloride    28.6      0.5         222f           Cember et al.
                                                                                                                                 (1978)

    Table 1 (cont'd)

    Organism         Lifestagec  Stat/  Organb     Temperature  Compounde                 Duration   Exposure   Bioconcentration    Reference
                                 flowa                (°C)                                 (days)   (µg/litre)      factorg

    (Lepomis                     stat   WB             21       methyl mercuric chloride    28.6      0.5         1138f          Cember et al.
    macrochirus)                                                                                                                 (1978)
                                 stat   WB             33       methyl mercuric chloride    28.6      0.5         2454f          Cember et al.
                                                                                                                                 (1978)

    a  stat = static conditions (water unchanged for duration of experiment); flow = flow-through conditions (mercury concentration in water
       continuously maintained).
    b  WB = whole body.
    c  juv = juvenile.
    d  muscle, skin & bone.
    e  OH = hydroxide.
    f  radiometrically calculated.
    g  bioconcentration factor = concentration in organism/concentration in medium.
    h  dry weight.

    Table 2.  Accumulation of mercury into terrestrial organisms

    Organism              Route   Organ     Compound                         Duration   Exposureb   Bioconcentration   Reference
                                                                             (days)     (mg/kg)        factorc

    Broccoli              soil    leaves    mercuric chloride                60         20              0.002d        John (1972)
    (Brassica oleracea)   soil    roots     mercuric chloride                60         20              0.09d         John (1972)

    Pea                   soil    roots     mercuric chloride                95         20              0.07d         John (1972)
    (Pisum sativum)

    Cauliflower           soil    leaves    mercuric chloride                70         20              0.003d        John (1972)
    (Brassica oleracea)   soil    roots     mercuric chloride                70         20              0.12d         John (1972)

    Spinach               soil    leaves    mercuric chloride                55         20              0.03d         John (1972)
    (Spinacia oleracea)   soil    roots     mercuric chloride                55         20              0.05d         John (1972)

    Chicken               diet    muscle    methyl mercury dicyandiamide     35-42      8               1.25           Borg et al. (1970)
                          diet    liver     methyl mercury dicyandiamide     35-42      8               5              Borg et al. (1970)

    Mallard               diet    liver     methyl mercury dicyandiamide     14         8               2.1            Stickel et al. (1977)
    (Anas platyrhynchos)  diet    kidney    methyl mercury dicyandiamide     14         8               2.2            Stickel et al. (1977)

    Redwinged blackbird   diet    liver     methyl mercury dicyandiamide     11         40              2.3            Finley et al. (1979)
    (Agelaius             diet    kidney    methyl mercury dicyandiamide     11         40              2.1            Finley et al. (1979)
    phoeniceus

    Cowbird               diet    liver     methyl mercury dicyandiamide     11         40              1.7            Finley et al. (1979)
    (Molothrus ater)      diet    kidney    methyl mercury dicyandiamide     11         40              1.5            Finley et al. (1979)

    Grackle               diet    liver     methyl mercury dicyandiamide     11         40              1.3            Finley et al. (1979)
    (Quiscalus quiscula)  diet    kidney    methyl mercury dicyandiamide     11         40              1.1            Finley et al. (1979)

    Table 2 (cont'd)

    Organism              Route   Organ     Compound                         Duration   Exposureb   Bioconcentration   Reference
                                                                             (days)     (mg/kg)        factorc

    Mink                  diet    liver     ceresan La                       5          32              11.1           Aulerich et al.(1974)
    (Mustela vision)      diet    kidney    ceresan La                       5          32              7.4            Aulerich et al.(1974)
    (adult)               diet    liver     mercuric chloride                10         135             0.3            Aulerich et al.(1974)
                          diet    kidney    mercuric chloride                10         135             3.2            Aulerich et al.(1974)

    a  ceresan L = methylmercury 2,3-di-hydroxy propyl mercaptide + methylmercury acetate.
    b  exposure as mg/kg of mercury soil or diet according to route.
    c  concentration factors calculated on a wet weight basis unless otherwise stated; bioconcentration factor = concentration in
       organism/concentration in medium.
    d  dry weight.
    
    between mercury taken into the tissues of the organism and mercury
    adsorbed on external surfaces. This should also be taken into account
    when interpreting results.

         Taking these factors into account, it is still clear that
    organisms take up both inorganic and organic forms of mercury from the
    medium. This uptake can result in high concentration factors. Under
    identical conditions, organic mercury is taken up by organisms to a
    greater degree than inorganic mercury, although the latter may often
    be strongly adsorbed to the outer surfaces.

    4.1  Speciation of Mercury

    Appraisal

          Different species of mercury differ greatly in their
     physicochemical properties: in their solubility, rates of
     accumulation by organisms, and behaviour in ecosystems. It is in its
     methyl form that mercury is most hazardous. Although not all sites of
     methylation in the environment are fully known, several have been
     identified in the aquatic environment.

         Mercury accumulated in the tissues of fish is usually in the form
    of methylmercury, while the source is usually inorganic mercury
    (Huckabee et al., 1979). Several hypotheses of how and where
    methylation occurs have been proposed. The main hypotheses are:

         (a)  biological methylation, bacterial in origin, which produces
              methylmercury in the environment (methylmercury is taken up
              by fish more readily than inorganic mercury),

         (b)  methylation by microorganisms associated with branchial
              mucus of the fish or in the fish gut, and

         (c)  methylation in the fish's liver (Thellen et al, 1981).

    It is generally agreed that methylation by fish, other than by
    bacteria associated with the fish, either does not occur or accounts
    for only an insignificant amount of the methylmercury produced. There
    is good evidence for methylation by bacteria in aquatic systems.

         Jernelov (1968) suggested that fish could not methylate mercury
    themselves and this is generally accepted (Huckabee et al., 1979),
    though not universally. Jernelov & Lann (1971) showed that 60% of the
    mercury content of predator fish (northern pike) arose from prey fish.
    This mercury was already methylated in the prey. The concentration of
    mercury in predator species was similar to that in their prey. They
    also measured the mercury content of organisms that were the food of
    the prey fish. Mercury levels in benthic fauna were very low and
    contributed less than 25% of the mercury in bottom-feeding fish. Most
    of the mercury accumulated by non-predator species was, therefore,

    accumulated directly from water. This conclusion was also reached by
    Fagerstrom & Asell (1973). The question of where the methylation,
    which gives rise to methylmercury residues in fish, occurs is still
    unresolved. It is also generally accepted that fish do not demethylate
    mercury either.

    4.2  Uptake and Loss in Aquatic Organisms

    Appraisal

          The data presented on uptake by aquatic invertebrates are
     difficult to interpret because most studies do not differentiate
     between external adsorption and actual uptake into the organism. This
     is especially important for methylmercury compounds for which uptake
     seems to be correlated with surface adsorption capacity, as expressed
     by the relative size of the organism.

          The extrapolation of data on uptake to other organisms appears
     risky because of a lack of knowledge regarding the mechanisms of
     uptake. This is even true for phenomena that are apparently fairly
     universal, e.g., the facilitating influence of chelators upon
     uptake.

          Most data on uptake by fish support the notion that uptake
     correlates positively with available concentration, with exposure
     time, and with temperature, although hardly any investigation
     differentiates between nominal and available concentrations. The
     importance of this distinction seems to be illustrated by the
     positive influence of lowered pH upon uptake.

          None of the studies address the problem of distinguishing
     between adsorption to gills and slime on the one hand and real uptake
     into the body on the other. Studies of mercury distribution between
     organs are valuable for the potential effects of the total body
     burden, but they give no reliable insight into the time-dependent
     process of accumulation.

          Data consistently show a higher uptake of methylmercury than of
     inorganic mercury. However, other organic mercury compounds exhibit a
     lower uptake, since they are adsorbed to a lesser extent.

    4.2.1  Microorganisms, plants, and invertebrates

         When Glooschenko (1969) exposed the marine diatom  Chaetoceros
     costatum to labelled mercury, he found no difference between uptake
    in the light or the dark in non-dividing cells. Dead cells took up
    twice as much mercury as living cells, presumably by surface
    adsorption. As dividing cells in the light accumulated the labelled
    mercury for longer than non-dividing cells, the author suggested the
    possibility of some active uptake.

         Hannerz (1968) demonstrated that there was no appreciable
    assimilation of mercury into the tissues of aquatic plants. Although
    concentrations were 10-20 times higher in submerged parts compared to
    emergent parts, this was attributed to surface adsorption differences.
    De et al. (1985) grew the plant  Pistia stratiotes in nutrient
    solution to which mercuric chloride had been added at concentrations
    ranging from 0.05 to 20 mg/litre. They found that uptake gradually
    increased with an increase in the mercury concentration. Maximum
    accumulation occurred within one day. Maximum removal (approximately
    90%) was recorded at 6 mg/litre or less, only 20% being lost from
    plants receiving the highest concentration. Mercury accumulation into
    the roots was about 4 times higher than into the shoots at lower
    concentrations and about twice as high at 20 mg/litre.

         Zuberik & O'Connor (1977) studied the accumulation of mercury in
    aquatic organisms from the Hudson River, USA. The organisms were
    maintained in filtered river water that contained mercury
    concentrations of < 0.1 µg/litre (less than levels normally found in
    the Hudson River). Planktonic organisms were exposed to various forms
    of labelled mercury, and the concentration factors after 24 h ranged
    from 102 to 106. Mercury uptake was greater in microplankton and
    algae than in macroplankton and fish larvae. An amphipod  (Gammarus
     sp.) was exposed for one day to each of four types of mercury, two
    organic (phenylmercuric acetate and methylmercury chloride) and two
    inorganic (mercurous nitrate and mercuric chloride). No differences in
    uptake were found, but when the amphipod was exposed for a week the
    organic forms were accumulated to 3 times the concentration of the
    inorganic forms.

         Riisgard et al. (1985) transferred mussels  (Mytilus edulis)
    from clean water to an area chronically polluted with mercury. The
    mussels accumulated mercury readily during 3 months of exposure. They
    were then transferred to clean water in the laboratory and the
    elimination of the mercury was measured. The biological half-life was
    293 days, but was only 53 days in the case of mussels contaminated by
    a temporary massive mercury contamination. In both cases, 75% of the
    mercury in the mussels was inorganic, but both inorganic and organic
    species were immobilized in the mussels from the chronically polluted
    area. In another study, only 6% of the total mercury in  Macoma
     balthica, a sediment-feeding bivalve, was methylated, a much lower
    percentage than in  Mytilus from the same area.

         Hirota et al. (1983) exposed the copepod  Acartia clausi to
    inorganic (mercuric chloride) and organic (methylmercury chloride)
    mercury at concentrations of 0.05-0.5 µg/litre for 24 h. The
    bioconcentration factor for inorganic mercury was nearly constant
    (approximately 7500), regardless of the mercury concentration in the
    water or the density of the copepods. In contrast, the concentration
    factor of methylmercury fluctuated, showing an inverse relationship
    with density but no relationship with the mercury concentration in the
    water.

         DeFreitas et al. (1981) found a net assimilation of 70%-80% for
    methylmercury and 38% for inorganic mercury when fed in the diet to
    the shrimp  Hyalella azteca. From water, inorganic mercury was
    assimilated 2 to 3 times more slowly than methylmercury. Khayrallah
    (1985) found that the accumulation of ethylmercuric chloride was
    almost twice as rapid as that of mercuric chloride in the amphipod
     Bathyporeia pilosa, although death occurred at similar levels of
    mercury.

         Ray & Tripp (1976) exposed the grass shrimp  (Palaemonetes pugio)
    to radioactively labelled methylmercury chloride and mercuric chloride
    for 24 and 72 h. After 24 h, the methylated form was mostly
    concentrated in the ventral nerve cord and to a lesser extent in the
    gills. The reverse was true for mercuric chloride. The concentrations
    of mercury accumulated in the other tissues (exoskeleton, foregut, and
    remainder) were similar for both compounds, and were in decreasing
    order of the above list. After 72 h the tissue distribution had
    changed, and there was no consistent order of the relative tissue
    concentrations. There was an increase in the mercury levels of the
    exoskeleton, foregut, and remainder tissues, while that in the gills
    remained about the same and that in the ventral nerve cord decreased.

         Vernberg & O'Hara (1972) measured the uptake of labelled mercury
    into the gills and hepatopancreas of fiddler crabs  (Uca pugilator)
    maintained in a solution containing 0.18 mg mercury/litre (as mercuric
    chloride) for 72 h. Uptake was determined under various temperature
    (5°C to 33°C) and salinity (5 and 30 g/litre) regimes. The total
    mercury taken up by the gills and hepatopancreas pooled together was
    unaffected by the different regimes. However, the ratio of uptake into
    the two tissues was affected. At higher temperatures, the crabs seem
    able to transport mercury from gill tissue to the hepatopancreas more
    effectively than at low temperatures.

         When Rossaro et al. (1986) exposed various life stages of the
    midge  Chironomus riparius to mercuric chloride for a period of
    30 days, the levels were still increasing at the end of the
    experiment. Both larvae and pupae accumulated mercury to about the
    same levels, some accumulation being due to passive adsorption. In a
    small experiment to illustrate this, larvae kept in a solution of
    5 µg/litre for only 1 min accumulated 9.32 mg mercury/kg. The adults
    accumulated only 40% of the levels found in the larval stage. The
    authors suggested that this is because the adults have some means for
    eliminating the mercury.

         Getsova & Volkova (1964) reported concentration factors for the
    accumulation of radioactively labelled mercury in four insect species.
    A midge,  Glyphotaelius punctatolineatus, accumulated 5240 times the
    water concentration within 16 days, while a dragonfly,  Leucorrhinia
     rubicunda, accumulated 8310 times the concentration over 16 days.
    Another dragonfly,  Aeschna grandis, accumulated 4000 times the
    waterborne mercury in 8 days, while a waste-water inhabiting fly,

     Eristalis tenax, accumulated only 640 times the water concentration
    after 4 days and the concentration factor had fallen to just 266 after
    8 days. The authors stated that the concentration factors that they
    found were in agreement with other Russian work on mercury
    accumulation.

    4.2.2  Fish

         When Birge et al. (1979) exposed rainbow trout eggs to an
    inorganic mercury concentration of 0.1 µg/litre in a flow-through
    system, the eggs accumulated 42.4, 68.2, and 96.8 µg mercury/kg after
    1, 4, and 7.5 days, respectively. Control eggs contained
    18.6 µg mercury/kg. The bioconcentration factor over 7.5 days was 782,
    taking into account the degree of contamination of controls. This
    represented a daily uptake rate of about 20 µg/kg. There was no
    evidence to suggest that the mercury penetrated the outer covering of
    the eggs and there was a high probability that most of the "uptake"
    was surface adsorption.

         Backstrom (1969) found that the uptake by fish of various mercury
    compounds was similar to that observed with birds (where methylmercury
    is rapidly absorbed compared with phenylmercury, methoxyethylmercury,
    and inorganic mercury), but the difference in uptake between
    methylmercury and the other mercury compounds was less pronounced.
    Mercury uptake into the spleen and the thyroids was greater than for
    birds. Phenylmercury was also retained in the wall of the gall
    bladder. In general the uptake of mercury into fish was far more
    localized than in birds. The levels of methylmercury steadily
    increased in the muscles and in the brain, whereas the other compounds
    accumulated primarily in the kidneys, spleen, and liver. More mercury
    accumulated in red flesh than white. There was also a high uptake of
    mercury into the gills and pseudobranch.

         Kramer & Neidhart (1975) demonstrated that methylmercury was
    taken up from water by guppies  (Lebistes reticulatus) 17 times
    faster than inorganic mercury. Organic mercury was also eliminated
    more slowly than inorganic. The authors suggested that some
    methylation of mercury occurred in the fish.

         Ribeyre & Boudou (1984) examined the uptake of mercury over time
    into specific organs of the rainbow trout. The uptake was sigmoid with
    a linear phase and a plateau. The majority (55% for inorganic and 60%
    for methylmercury) of the metal was found in muscle and gills, while
    blood contained 3%-12%, liver 2%-5%, and kidneys 2%-7%. Brain,
    posterior intestine, and spleen together accounted for only 2% of
    total mercury. Those organs which would eventually contain most
    mercury accumulated their mercury exponentially. After the exposure,
    some organs lost their mercury while others (the ones with most
    mercury) continued to increase their mercury content. The organs which
    lost mercury in clean water had accumulated the metal with a flatter
    sigmoid curve.

         Schindler & Alberts (1977) found that the mosquitofish  (Gambusia
     affinis) readily accumulated metallic mercury during short-term
    continuous exposure. Within 24 h, 20 mg/kg wet weight had been taken
    up from a solution containing 0.1 mg total mercury/litre. The uptake
    curves for metallic mercury and mercuric chloride were very similar.
    The authors suggested that uptake in the short-term is largely the
    result of physical adsorption. This rate of uptake closely agrees with
    that found by McKone et al. (1971) in goldfish  (Carassius auratus)
    where 22 mg mercuric chloride/kg was accumulated from a solution
    containing 0.25 mg/litre over a period of 24 h.

         When Schindler & Alberts (1977) periodically exposed (2 h/day for
    10 days) mosquitofish to metallic mercury and mercuric chloride
    (in separate experiments) at 100 µg/litre, the uptake of metallic
    mercury was 5 times greater than that of the chloride. The authors
    suggested that the metallic mercury remained unchanged and that its
    high lipid solubility enabled it to penetrate the gill membrane,
    whereas the salt bound more tightly to the mucoproteins of the gills
    and penetration was restricted. The rate of elimination in mercury-
    free water was about the same for both, with the half-time calculated
    to be about 45 days.

         McKim et al. (1976) exposed 3 generations of brook trout
     (Salvelinus fontinalis) to methylmercury at concentrations measured
    at < 0.01-2.93 µg/litre. The uptake was rapid and 2-week
    concentration factors ranged from 1000 to 12 000, depending on the
    tissue. There was a tendency for the uptake to reach a steady state
    (that is the tissue content reached a constant level) over 20-28
    weeks. There was no significant elimination over this period.

         In studies by Pentreath (1976), the thornback ray  (Raja clavata)
    readily absorbed both inorganic mercuric chloride and organic
    methylmercuric chloride from sea water. Methylmercury, in contrast to
    inorganic mercury, was readily absorbed from food and slowly
    eliminated. The half-lives of elimination of mercury taken up from
    food were 61.6 days for inorganic and 323 days for organic components.

         Thellen et al. (1981) found that methylmercuric chloride rapidly
    accumulated in the organs and muscular tissue of rainbow trout exposed
    to 1 mg/kg diet. However, mercuric chloride, at the same
    concentration, did not accumulate. During exposure to a continuous
    sublethal concentration of 0.25 µg mercury/litre, both organic and
    inorganic mercury accumulated, primarily in the internal organs and to
    a lesser extent the muscle tissue. Mercuric chloride was detected in
    the muscle at half of the concentration of organic mercury. Wobeser
    (1975b) fed rainbow trout fingerlings a diet containing methylmercuric
    chloride (at 4, 8, 16, or 24 mg mercury/kg) over a 15-week period. The
    total accumulation of mercury in muscle tissue was directly related to
    the concentration of mercury in the food, as was the rate of
    accumulation. Mercury was accumulated in muscle to a higher
    concentration than there had been in the diet.

         When Amend (1970) exposed juvenile sockeye salmon (1 h per day
    for 12 to 15 days) to 1 mg/litre of lignasan (6.25% ethylmercury
    phosphate), the fish contained highest levels in the kidneys and
    liver. One week after the cessation of treatment, these levels were
    36.5 and 20.4 mg/kg for the kidney and liver, respectively. Three
    years later, the fish having migrated, levels were still higher than
    normal but had returned to normal after 4 years. Similar studies using
    coho and chinook salmon yielded similar results. When Kendall (1975)
    injected channel catfish intraperitoneally with methylmercury chloride
    at 15 mg/litre, the mean concentration of mercury in the kidneys was
    51.03 µg/g after 24 h and fell to 14.24 mg/kg after 96 h.

    4.2.2.1  Effects of environmental variables on uptake by fish

    Appraisal

          Environmental variables such as temperature and pH increase the
     uptake of mercury, particularly methylmercury, by fish. This is of
     potentially considerable importance in the field.

         Reinert et al. (1974) found that yearling rainbow trout  (Salmo
     gairdneri) exposed to methylmercury chloride for 12 weeks accumulate
    more mercury at 15°C than at 5°C (Table 1). When Cember et al. (1978)
    exposed bluegill sunfish  (Lepomis macrochirus) to methylmercury
    chloride at concentrations ranging from 0.2 to 50 µg/litre for up to
    688 h, mercury accumulation was not affected by the different mercury
    concentrations. It did, however, increase when the temperature was
    increased from 9°C to 33°C (Table 1). MacLeod & Pessah (1973) found an
    increase in mercury accumulation, in response to an increase in
    temperature (from 5 to 20°C), in rainbow trout exposed to
    concentrations of between 50 and 200 µg/litre for 4 days. The authors
    also interpolated (from 7-day data) a 4-day bioconcentration factor
    for phenylmercuric acetate of 100, when the fish were exposed to
    5 µg/litre mercury at 10°C. Tsai et al. (1975) studied the effect of
    pH on the accumulation of inorganic mercury (mercuric chloride) at a
    concentration in water of 1500 µg mercury/litre for 15 min. The
    accumulation increased as pH decreased. At pHs of 5, 6.5, and 7.5,
    fathead minnow accumulated whole body residues of 2.7, 1.8, and
    0.4 mg mercury/kg, calculated on a wet weight basis, respectively. A
    similar result was found for the emerald shiner  (Nicropterus
     atherinoides).

         Rodgers & Beamish (1981) found that the uptake of methylmercury
    by rainbow trout was increased when the hardness of the water was
    decreased from 385 mg/litre to 30 mg/litre. The addition of inorganic
    mercuric chloride increased the uptake of methylmercury in both hard
    and soft water. Kudo & Mortimer (1979) exposed guppies to mercury in a
    double chambered system, with an exchange of water. Only in one
    chamber did the fish have access to sediment. After being exposed for

    20 days to a sediment mercury concentration of 1.023 mg/kg, the fish
    without direct access to the sediment showed a concentration factor of
    57 and those with access a factor of 570.

    4.2.3  Studies on more than one type of organism

         Cultures of the alga  Croomonas salina, grown for 48 h in the
    presence of mercuric chloride (164 µg mercury/litre), retained about
    half of the mercury (1400 mg/kg dry weight) (Parrish & Carr 1976).
    When the alga was fed to the copepod  Acartia tonsa for 5 days,
    neither the copepods nor their eggs or faeces retained mercury in
    detectable amounts.

         Boudou et al. (1979) exposed mosquitofish  (Gambusia affinis) to
    methylmercury directly from the water and via food organisms and water
    in a simple model ecosystem. More mercury was taken up at higher
    temperatures. The authors calculated mercury uptake from water as a
    percentage of the "global" uptake from both water and food. This
    percentage varied with temperature, being 83% at 10°C, 40% at 18°C,
    and 11% at 26°C.

         In studies by Boudou & Ribeyre (1984), alevins of rainbow trout
     (Salmo gairdneri) were exposed to a constant water concentration of
    methylmercuric or mercuric chloride at 1 µg/litre for 83 days. Mercury
    uptake was faster with organic than inorganic and both were initially
    linear. A plateau was eventually achieved in both cases. Uptake was
    negatively related to fish weight, although the authors pointed out
    that in the field there is usually a positive relationship.

         Fang (1973) maintained the pond weed  Elodea canadensis, snail
     Helisoma campanulata, coontail plant  Ceratophyllum demersum, and
    guppy  Lebistes reticulatus in solutions containing labelled
    phenylmercuric acetate (PMA) at concentrations between 5 × 10-8 and
    5 × 10-7mol/litre. All of the organisms readily accumulated PMA and
    the uptake was related to the length of exposure and the
    concentration. The absorbed PMA was largely converted to inorganic
    mercury. Although the uptake curves were very similar, pond weed and
    coontail both accumulated much more PMA than guppy or snail. The half-
    life of Hg203 residues ranged from 43 to 58 days. When Fang (1974)
    exposed  Lebistes reticulatus and  Ceratophyllum demersum to
    labelled ethylmercuric chloride (EMC), the uptake was positively
    related to the time of exposure over 200 h and the concentration up to
    5 × 10-7mol/litre. Highest concentrations were accumulated in the
    internal organs. The half-life of EMC was 20-23 days. Both organisms
    converted EMC to inorganic mercury, 34% being converted by the
    coontail and 29% by the guppy over a 7-day period. When the same
    organisms were exposed to methylmercury chloride, little or no
    breakdown to inorganic mercury occurred.

    4.3  Uptake and Loss in Terrestrial Organisms

    Appraisal

          The accumulation of mercury in plants increases with increasing
     soil mercury concentration. Soil type has a considerable influence on
     this process, a high organic matter content decreasing the uptake.
     Generally, the highest concentrations of mercury are found at the
     roots, but translocation to other organs (e.g., leaves) occurs. In
     contrast to higher plants, mosses take up mercury via the
     atmosphere.

          In exposed birds, the highest mercury levels are generally found
     in liver and kidneys. Methylmercury is more readily absorbed than
     inorganic mercury and it exhibits a longer biological half-time.
     Depending on speciation, mercury occurs in different compartments of
     birds' eggs; methylmercury tends to concentrate in the white and
     inorganic mercury in the yolk.

         Huckabee & Janzen (1975) found that the mat-forming moss
     Dicramum scoparium did not take up radioactively labelled mercury
    from substrate. The authors concluded that the uptake of mercury into
    this point was mostly from the atmosphere. This is commonly true for
    mosses, which have been used extensively as monitor organisms for
    atmospheric pollutants in the field. Weaver et al. (1984) maintained
    bermuda grass  (Cynodon dactylon) in three types of soil (clay, silt
    loam, and fine sand) treated with mercuric chloride (1-50 mg/kg).
    Mercury was accumulated into the roots from silt loam, clay, and sand
    in increasing order. The accumulation increased with increasing
    mercury concentration. At 50 mg/kg the concentration of mercury in
    (and on the surface of) the roots was 800 mg/kg, when the grass was
    grown in sand.

         John (1972) grew eight types of food crop in soil treated with
    mercuric chloride at 4 or 20 mg mercury/kg, and uptake was measured
    after 35 to 130 days, depending on the plant species. Higher
    concentrations of mercury were found in the roots compared to the
    above-ground samples. At the highest treatment level the mercury
    content of the roots, calculated on a dry weight basis, ranged from
    0.387 mg/kg for lettuce to 2.447 mg/kg for cauliflower. Of the edible
    plant parts, spinach leaves and radish tubers contained the highest
    concentrations (0.695 and 0.663 mg/kg mercury, respectively).

         Siegel & Siegel (1985) found that the seed-pods of several
    leguminous species exposed to soil mercury concentrations of
    10-69 µg/kg lost 75-85% of their tissue water during maturation but
    showed no loss of mercury content. However, the seeds not only lost
    most of their water but also at least 75% of their mercury. The
    authors suggested that the elimination was by "bio-volatilisation",
    i.e., loss of elemental mercury as vapour rather than by
    translocation.

         Nuorteva et al. (1980) reared blowfly  (Lucilia illustris) on
    trout flesh contaminated with mercury (0.66 mg/kg). Levels rose from
    0.14 to 1.18 mg/kg during the larval feeding period, whereas pupae and
    freshly emerged adults contained 0.99 and 1.01 mg/kg, respectively.
    When adults were then fed honey, mercury levels were reduced to a
    third within 2 days. The authors found that it was easier for the
    flies to eliminate inorganic mercury than methylmercury. Nuorteva &
    Nuorteva (1982), after rearing blowfly larvae on mercury-contaminated
    fish flesh and obtaining mercury levels of 2, 6.3, and 13.3 mg/kg in
    different groups, fed the flies to staphylinid beetles  (Creophilus
     maxillosus) for a 1-week period. This gave residues of 6.9, 17.4,
    and 33.4 mg/kg, respectively, in the beetles.

         Kiwimae et al. (1969) fed white leghorn hens for 140 days on a
    diet containing 400 or 1600 µg of mercury per day as either mercury
    nitrate, phenylmercury hydroxide, or methoxyethylmercury hydroxide.
    The total mercury accumulated in the egg-whites of eggs laid was 0.31,
    0.53, and 0.46 mg/kg, respectively, for the lower dose and 0.44, 0.85,
    and 0.88 mg/kg for the higher dose. At the higher dose, the mercury
    residue in the egg yolks was 2.12, 4.53, and 2.89 mg/kg, for the three
    mercury compounds, respectively.

         Backstrom (1969) administered labelled mercury compounds, either
    parenterally or perorally, to Japanese quail and studied the tissue
    uptake and elimination. The route of administration did not affect the
    final uptake or subsequent elimination. Methylmercury was readily
    absorbed and was stable, while the other compounds, phenylmercury,
    methoxyethylmercury, and inorganic mercury, were less well absorbed,
    and the phenylmercury was rapidly decomposed to inorganic mercury.
    Methylmercury was characterized by an even tissue distribution and a
    slow excretion, which was enhanced in egg-laying hens. The author
    attributed this to an increased concentration of methylmercury in the
    egg-white. Little of the other compounds were taken up into the brain,
    but methylmercury slowly reached a high concentration. The other
    mercury compounds were accumulated in the  yolks of the eggs laid,
    and also in the liver and kidneys of the adult birds, and were rapidly
    excreted. The plumage and other keratinised structures strongly
    concentrated mercury, irrespective of the compound. These structures
    seem to be an important excretion route, especially for methylmercury.

         Nicholson & Osborn (1984) fed juvenile starlings  (Sturnus
     vulgaris) on a mercury-contaminated synthetic diet
    (1.1 mg mercury/kg) and analysed the birds after 8 weeks. The highest
    mercury levels were found in the kidneys and the liver (36.3 and
    6.55 mg/kg dry weight, respectively).

         In studies by Finley & Stendell (1978), black ducks  (Anas
     rubripes) were fed a diet containing 3 mg mercury/kg (as
    methylmercury dicyandiamide) for periods of 28 weeks over two
    consecutive breeding seasons, during which time any ducklings that
    hatched were also fed the dosed diet. Mercury levels were highest in

    the feathers of the adult birds (61 mg/kg wet weight), followed by the
    liver and kidneys (22 and 14 mg/kg, respectively). Similarly the
    highest levels were also found in the feathers, liver, and kidneys of
    first-year ducklings. Eggs and embryos analysed during the first year
    revealed mercury levels of 6.14 and 9.62 mg/kg, respectively. Mercury
    residues in eggs, embryos, and ducklings were, on average, about 30%
    lower during the second year. Stickel et al. (1977) dosed mallard
     (Anas platyrhynchos) with 8 mg mercury/kg for 2 weeks, and found
    that the highest levels of mercury were accumulated in the liver
    (16.5 mg/kg wet weight) and the kidney (17.6 mg/kg wet weight). One
    week later the liver and kidney had retained 64 and 66%, of the
    mercury, respectively. No significant additional loss was noted during
    the next 8 weeks.

         Adams & Prince (1976) showed that ring-necked pheasants
     (Phasianus colchicus) accumulated more mercury in the tissues after
    consuming methylmercury dicyandiamide than after consuming the
    corresponding mass of phenylmercuric acetate. This reflects the
    greater toxicity of alkyl mercury compounds than aryl ones.

         When Borg et al. (1970) fed goshawks  (Accipiter gentilis) liver
    and muscle from chickens dosed with methylmercury (average dietary
    mercury content 13 mg/kg), the hawks died within 6-7 weeks. The
    highest residues of mercury were found in the liver at 113 mg/kg wet
    weight (102 mg methylmercury/kg), and the kidneys at 129 mg/kg
    (98 mg methylmercury/kg). Substantially higher levels of mercury were
    found in the skeletal muscle and brain of treated birds than in those
    of controls. The reproductive organs also showed an ability to
    accumulate mercury.

    4.4  Accumulation in the Field

    Appraisal

          Observations on given species of marine and freshwater fish
     indicate that all tissue concentrations of mercury increase with
     increasing age (as inferred from length) of the fish. In certain
     species males have been found to have higher levels than females.

          In aquatic systems, fish-eating birds tend to have higher
     mercury levels than non-fishing birds. In terrestrial systems, seed-
     eating birds, small mammals, and their predators can have high levels
     in areas where methylmercury fungicides are used.

          Bird feathers are useful for biological monitoring for
     methylmercury exposure. Analysis of feathers, especially using
     neutron activation, can allow recapitulation of past exposure. In
     general liver and kidney have higher levels than other bird tissues.

          Sea mammals are reported to have a wide range of total mercury
     concentrations in liver (0.4 to over 300 mg/kg), only a small
     fraction (2-17%) being in the methylated form. Selenium and mercury
     have been found in seal livers in a consistent 1:1 atomic ratio. A
     number of studies have indicated that selenium plays a protecting
     role.

          Point sources of mercury pollution often lead to elevated
     mercury levels in organisms living in the affected area. There are
     some circumstances where toxic effects have been produced. These
     effects should be taken into account in various countries during the
     process of industrialization.

    4.4.1  General exposure

         Gilmartin & Revelante (1975) analysed Northern Adriatic anchovy
     (Engraulis encrasicholus) and sardine  (Sardina pilchardus) for
    mercury content. Seasonal distribution of mercury in various tissues
    of both anchovy and sardine ranged between 5 and 610 ng/g wet weight,
    the highest concentrations of mercury being in the liver and kidney.
    Perttila et al. (1982) found that mercury levels in the Baltic herring
     (Clupea harengus) increased significantly with age. Bache et al.
    (1971) observed that concentrations of both total mercury and
    methylmercury increased with the age of lake trout  (Salvelinus
     namaycush), the proportion of methylmercury to total mercury
    increasing with age. However, Westoo (1973) did not find that the
    proportion of methylmercury to total mercury in salmon  (Salmo salar)
    and sea trout  (Salmo ocla) was dependant on age.

         Forrester et al. (1972) found a correlation between length and
    mercury concentration in  Squalus acanthias (the spurdog, an
    elasmobranch fish). Olsson (1976) analysed northern pike  (Esox
     lucius) in 1968 and 1972 and found a correlation between mercury
    levels and length of fish, and that males contained significantly more
    mercury than females. It was considered that, during a general
    decrease of mercury levels within pike population, the age of the fish
    is not a suitable parameter for estimating mercury levels. This is
    because uptake and retention of mercury is dependant on body size but
    loss of accumulated mercury is less dependent on fish size. May &
    Mckinney (1981) sampled freshwater fish, in 1976 and 1977, from
    selected sites throughout the United States, and found mercury levels
    of 0.01-0.84 mg/kg wet weight.

         Berg et al. (1966) analysed feathers from Swedish birds collected
    over a period of 100 years, and found roughly constant levels of
    mercury during the period 1840 to 1940. However, a well documented
    increase of 10-20 times appeared in the 1940s and 1950s, which the
    authors concluded was due to the use of alkylmercury seed dressings.
    Martin (1972) and Martin & Nickerson (1973) sampled starlings
    throughout the United Slates in 1970 and 1971 and found that most of
    the birds had mercury levels of < 0.5 mg/kg (76% of the birds

    analysed in 1971 contained levels of < 0.05 mg/kg). Lindsay & Dimmick
    (1983) found mercury in the liver, breast muscle, and body fat of wood
    duck taken from the area of the Holston River, Tennessee, USA. The
    highest levels were in juveniles (0.42, 0.15, and 0.1 mg/kg, for the
    three tissues, respectively. Local sediment contained
    0.76 mg mercury/kg, black fly larvae and aquatic plants < 0.1 mg/kg.

         Osborn & Nicholson (1984) sampled puffin from the islands of
    St. Kilda and May, off the British coast, and found liver and kidney
    mercury levels of approximately 1.25 mg/kg dry weight (in both
    tissues) for the Isle of May, and 3.75 and 5 mg/kg dry weight,
    respectively, for St. Kilda. Braune (1987) analysed tissues of nine
    species of sea birds sampled in New Brunswick, Canada, for total
    mercury content, and found highest levels in the liver (0.046 to
    0.606 mg/kg) and kidney (0.242 to 5.345 mg/kg). Birds which fed on
    benthic invertebrates or fish had the highest levels, while those
    feeding mainly on pelagic invertebrates had the lowest.

         Fimreite et al. (1982) sampled eggs from a Norwegian gannet
    colony for mercury in 1972, 1978, and 1979, and obtained values of
    0.58, 0.8, and 0.36 mg/kg, respectively. Ohlendorf (1986) analysed
    eggs from three Hawaiian seabird species in 1980, and found mercury in
    all eggs, ranging from 0.122 to 0.359 mg/kg wet weight. Koeman et al.
    (1975) analysed oiled seabirds (guillemot and razorbill) from the
    Dutch coast for mercury residues and reported levels ranging from 1.8
    to 2.4 mg/kg wet weight. Hoffman & Curnow (1979) analysed the levels
    of mercury in the tissues of herons, egrets, and their food collected
    from two sites near Lake Erie, USA. One population fed on Lake Erie
    (food items, 0.02-0.81 mg/kg wet weight; bird livers, 3.0-16.5 mg/kg
    wet weight). The other population fed predominantly on bordering
    marshland (food items, up to 0.24 mg/kg; bird livers,
    1.03-8.22 mg/kg).

         Honda et al. (1986) sampled striped dolphin  (Stenella
     coeruleoalba) and found that the accumulation of total mercury in
    bone correlated significantly with age. Levels rose to 1.44 and
    1.55 mg/kg for adult male and female, respectively, and similar trends
    were seen for methylmercury, levels reaching 0.27 mg/kg in adults.
    Falconer et al. (1983) found that in common porpoise  (Phocoena
     phocoena) highest mercury levels were in the liver, where mean
    levels for females were 6.03 mg/kg and for males 3.42 mg/kg.
    Heppleston & French (1973) analysed tissues of common and grey seals,
    from the British coast for mercury and found highest levels in the
    livers (4.9-113 mg/kg). Koeman et al. (1975) determined mercury levels
    of 0.37-326 mg/kg in the livers of marine mammals (seals, dolphins,
    and porpoises) and also reported an almost perfect correlation between
    mercury and selenium content of these mammals (1:1 ratio between
    mercury and selenium concentrations). The authors suggested that
    selenium uptake may protect marine mammals from the toxic effects of

    mercury. Gaskin et al. (1974) found liver total mercury levels ranging
    from 13 to 157 mg/kg in short-finned pilot whales and long-snouted
    dolphins from the Lesser Antilles. Between 2% and 17% of the total
    mercury was methylated.

    4.4.2  Mercury manufacturing and general industrial areas

         Yeaple (1972) analysed bryophytes from various localities of
    eastern USA for mercury content and found that highest levels
    (1.45 mg/kg) were in plants from a large city. Levels in cities and
    industrial areas were higher than those in rural areas (e.g.,
    < 0.05 mg/kg in a high, isolated mountain area). Kraus et al. (1986)
    collected leaves of the salt marsh cordgrass  (Spartina alterniflora)
    from two sites in the USA, one site near a heavily industrialized area
    and the other in a non-industrialized area. The mean soil
    concentrations of mercury for the two sites were 18.17 and 0.22 mg/kg,
    respectively, while the residues in the leaves were 0.16 and
    0.02 mg/kg, respectively. Salts collected from the surface of plants
    in the contaminated area contained 0.11 mg mercury/kg; laboratory
    studies have shown the plant capable of mercury excretion.

         Nuorteva et al. (1980) analysed trout  (Salmo trutta) from the
    Idrijca River, Yugoslavia, about 3 km downstream from a mercury
    distillation plant. The fish had a mercury content of 0.66 mg/kg in
    the flesh, and highest levels were found in the spleen and kidney
    (17.5 and 24 mg/kg, respectively). Three samples of ephemerids, taken
    6 km from the plant, contained 0.27, 0.36, and 0.56 mg/kg wet weight,
    and a sample containing 4.28 mg/kg was found 1 km from the plant.
    These were lower levels than those reported previously, presumably
    because of six months inactivity at the plant. The same authors
    analysed blow flies from various polluted and non-polluted localities.
    From an unpolluted area mercury levels were < 0.1 mg/kg, near a
    Finnish pulp factory, 0.2 mg/kg, and near a caustic soda factory,
    0.3 mg/kg. Higher levels (0.8 mg/kg) were found close to a mercury
    mine and distillation plant in Yugoslavia, whereas levels were near
    normal 1 km upstream or downstream from the mine.

         Doi et al. (1984) analysed feathers from birds collected over a
    period of 25 years from the mercury-polluted shores of the Shiranui
    Sea, Japan. Relatively high levels were found until the late 1970s
    even though the draining of water containing methylmercury from a
    local factory was stopped in 1968. Mean mercury levels were: fish-
    eating birds, 7.1 mg/kg; omnivorous waterfowl, 5.5 mg/kg; predatory
    birds, 3.6 mg/kg; omnivorous terrestrial birds, 1.5 mg/kg; and
    herbivorous waterfowl, 0.9 mg/kg.

         Fimreite et al. (1971) analysed 156 fish and 48 bird livers from
    the Great Lakes area of Canada in 1968 and 1969. Elevated mercury
    levels were found in all fish samples, highest levels occurring in
    lake trout, pumpkinseed sunfish, and walleye (10.5, 7.09, and

    5.01 mg/kg, respectively). Levels were generally highest in fish
    collected downstream from suspected sources. The highest mercury level
    in a fish-eating bird was found in a red-necked grebe, where the liver
    level was 17.4 mg/kg. Three grebes sampled showed a range of
    0.45-17.4 mg/kg. Lower concentrations were found in cormorants,
    herons, murrelets, terns, kingfisher, and other fish-eating birds, but
    mean mercury liver burden was greater in these birds than in non fish-
    eating species.

    4.4.3  Mining activity

         Huckabee et al. (1983) monitored levels of mercury in vegetation
    in the vicinity of the mercury mine at Almaden in Spain. Mean
    concentrations of total mercury in vegetation ranged from > 100 mg/kg
    within 0.5 km of the mine to 0.20 mg/kg 20 km from the mine. There was
    still a significantly higher mercury content in vegetation 25 km
    upwind from the mine (about 10 times the background level). Mosses
    were found to contain the greatest concentration of mercury
    (7.58 mg/kg), and woody plants accumulated less of the metal
    (0.72 mg/kg) than herbaceous plants (2.25 mg/kg). The figures given
    are for samples collected in spring. There was a correlation between
    distance from the mine and plant mercury content for woody plants and
    mosses but not for herbaceous plants. No methylmercury, at
    quantifiable levels, was found in any of the plants analysed, although
    traces were seen in several samples indicating a methylmercury content
    of less than 10 pg per sample.

         When Phillips & Buhler (1980) analysed rainbow trout  (Salmo
     gairdneri), stocked in a reservoir contaminated by a disused mine,
    for mercury, they found that lateral muscle tissue levels increased
    linearly during the first five months that the fish were in the
    reservoir. Trout sampled 7, 19, or 31 months after being introduced
    showed levels that did not differ significantly (mean level = 1.25 mg
    mercury/kg). Matsunaga (1975) analysed crucian carp, dace, and zacco
    temmincku from two rivers receiving discharge from mercury mines in
    Japan. Total mercury levels in the fish were approximately
    0.2-4.5 mg/kg and reflected the levels of mercury in the water
    (4-50 ng/kg).

         Hesse et al. (1975) determined total mercury concentrations in
    the muscle, liver, and kidney of 22 species of birds collected from a
    western South Dakota watershed contaminated by mining activity.
    Elevated mercury levels were found in fish-eating birds, especially
    double-crested cormorants. Levels in non fish-eating birds were lower
    but still significantly higher than background. In general, greater
    accumulations occurred in the livers of fish-eating birds (0.89 to
    30.9 mg/kg) and in the kidneys of non-fish-eating birds (0.27 to
    0.60 mg/kg).

    4.4.4  Chloralkali plants

         Gardner et al. (1978) analysed sediment, plants, and animals from
    a salt marsh contaminated by a chloralkali plant in Brunswick,
    Georgia, USA. Chloralkali plants produce metallic mercury from salts.
    Sediment levels ranged from 0.27 to 1.7 mg/kg dry weight for the top
    5 cm and they varied according to distance from plant and depth of
    sample. The roots of  Spartina alterniflora, the marsh grass,
    contained the highest levels (0.07-1.47 mg/kg dry weight) within the
    plant. Of the animals analysed from the contaminated marsh and nearby
    river, the invertebrates contained 0.3-9.4 mg/kg dry weight, the fish
    0.3-1.9 mg/kg dry weight, the birds 2.4-37.0 mg/kg dry weight (liver)
    and the mammals 3.8-15 mg/kg dry weight (liver). Methylmercury levels
    were low (< 0.002 mg/kg) in sediment and plants but accounted for
    most of the mercury found in the tissues of higher organisms.

         Hildebrand et al. (1980) sampled fish and invertebrates from the
    Holston River, USA, above and below an inactive chloralkali plant.
    Rock bass and hog sucker contained total mercury levels at less than
    1 mg/kg above the plant, and 1-3 mg/kg immediately below it. Benthic
    invertebrates gave a similar pattern, lower levels being found above
    the plant and the higher levels below it. Total mercury concentrations
    in the individual taxonomic groups of the invertebrates ranged from a
    maximum of 3.75 mg/kg  (Hydropsychidae, 3.7 km below the plant) to a
    minimum of 0.016 mg/kg  (Psepheridae, 5.5 km above the plant). Total
    mercury concentrations in fish and invertebrates decreased with
    distance down stream of the plant. Mercury in the methyl form
    comprised 91.7% of total mercury in the fish and 50% in the
    invertebrates.

         Wallin (1976) reported that samples of the carpet-forming moss
     Hypnum cupressiforme from sites around six Swedish chloralkali
    plants all contained similar mercury levels. Levels were highest
    (1-15 mg/kg) close to the plants and decreased with increasing
    distance from each plant. Background levels for the region
    (90-150 µg/kg) were reached at distances of 9-15 km from the plants.
    The author calculated that only a small part of the annual fallout
    (< 10%) was deposited locally. Shaw & Panigrahi (1986) analysed soil
    and five species of dwarf plants, from an area adjacent to a
    chloralkali factory, for mercury content. Soil from around the roots
    of the plants was analysed, and the mercury content was found to be
    very variable (2.13-893 mg/kg dry weight). Uptake into the roots,
    stem, leaf, and fruit of all plants in the area was significant.
    Leaves contained the highest levels of mercury, ranging between 2.32
    and 38.8 mg/kg dry weight. Greater accumulation of mercury was found
    in the stem than roots of  Croton sp. and  Jatropha sp.; similar
    amounts in both stem and roots of  Argemone sp., and more mercury in
    the roots than the stem of  Ipomoea sp. and  Calotropis sp. No
    correlation was found between the soil mercury level and plant uptake.
    Bull et al. (1977) measured mercury in soil, grass, earthworms, and

    small mammals near a chloralkali factory. At a distance of < 0.5 km
    from the factory, mean mercury levels in surface soil (3.81 mg/kg dry
    weight), grass (4.01 mg/kg dry weight), earthworms (1.29 mg/kg wet
    weight) and moss bags (63 ng/dm2 per day) were significantly higher
    than levels found 10 to 30 km from the works. Levels of mercury at
    this distance were comparable with those found at sites not associated
    with mercury sources. Mercury levels in all tissues analysed, except
    muscle of bank voles  (Clethrionomys glareolus) and woodmice
     (Apodemus sylvaticus) were significantly higher in the study area
    than control areas. The authors also found elevated levels of
    methylmercury in small mammals and earthworms in the study area,
    suggesting methylation of the inorganic mercury fall-out.

    4.4.5  Mercurial fungicides

         Fimreite et al. (1970) found that seed-eating birds, and their
    avian predators, had higher liver mercury levels in areas where
    treated grain (mercurial fungicide) had been sown compared with areas
    using untreated grain. Jefferies & French (1976) analysed specimens of
    the long-tailed field mouse  (Apodemus sylvaticus) taken from a wheat
    field that had been drilled two months previously with wheat dressed
    with dieldrin and mercury. Whole body mercury concentrations were much
    higher (0.83 ± 0.44 mg/kg wet weight) than those found immediately
    after drilling (0.39 ± 0.04 mg/kg wet weight).

    5.  TOXICITY TO MICROORGANISMS

         Mercury in an inorganic form is toxic to microorganisms. It is
    much more toxic in an organic form, owing to increased availability of
    the metal to cells. The following are illustrative examples, rather
    than an exhaustive cover, of research into the effects of mercury on
    microorganisms.

         Wood (1984) discussed six protective mechanisms available to
    microorganisms (and certain higher organisms) that increase their
    resistance to metal ions in general, and specifically to mercury.
    These mechanisms are biochemical in nature and, generally, render the
    mercury ion ineffective in disturbing the normal biochemical processes
    of the cell. The mechanisms are: (a) efflux pumps that remove the ion
    from the cell, a process which requires energy, (b) enzymatic
    reduction to the less toxic elemental form; (c) chelation by
    intracellular polymers (not firmly established for mercury);
    (d) binding of mercury to cell surfaces; (e) precipitation of
    insoluble inorganic complexes, usually sulfides and oxides, at the
    cell surface; and (f) biomethylation with subsequent transport through
    the cell membrane by simple diffusion. It is this last mechanism,
    biomethylation, which renders the mercury more toxic to higher life-
    forms.

    5.1  Toxicity of Inorganic Mercury

    Appraisal

          Inorganic mercury is toxic to microorganisms over a wide range
     of concentrations. Its effects on development and survival are
     modified by environmental factors such as temperature, light
     intensity, pH, and chemical composition of the medium, and by cell-
     related factors such as genetic variation. Through selective effects
     on particular species, it can change the composition of a plankton
     community. The mechanism of action is not fully understood.

    5.1.1  Single species cultures

         Kamp-Nielson (1971) demonstrated a time-dependent effect of
    mercuric chloride, added at 300 µg/litre, on the photosynthesis of
     Chlorella pyrenoidosa. There was little effect in the first hour of
    incubation, a pronounced drop in photosynthetic rate in the second
    hour, and a period of little further effect between 2 and 5 h. An
    overall rate reduction of about 50% occurred after 5 h with a cell
    density of 6.5 × 107 cells/litre. There was a greater effect on
    photosynthesis at lower cell densities. It was also found that
    photosynthesis had to occur for the effect to develop, since exposure
    to mercuric chloride for 2 h in the light had the same effect as
    exposure to the same concentration of mercury for 2 h in the dark
    followed by 2 h in the light. Similar results were found after 1-h
    exposures in light and darkness followed by light. There was an effect

    of light intensity; in short-term experiments mercury had a
    deleterious effect on photosynthesis only at high light intensities.
    Mercury also affected photosynthesis at low light intensity, but only
    after 20-h exposures. Mercury affected photosynthesis adversely at
    concentrations between about 50 and 300 µg/litre, but had no greater
    effect at concentrations up to 1000 µg/litre (the highest tested). The
    effect was dependant on cell density, pH, light intensity, and
    duration of exposure. Potassium and sodium in the growth medium had no
    effect on mercury toxicity to  Chlorella. Increasing the
    concentration of mercuric chloride in the medium increased the
    "leakage" of potassium from the cells of  Chlorella. This was maximal
    at a mercury concentration of about 300 µg/litre and was considered to
    be the main toxic effect of mercury. The effect on potassium leakage
    occurred equally in darkness and light and was, therefore, independent
    of the photosynthetic effect. Mercury increased the length of the 
    lag-phase during the growth of  Chlorella pyrenoidosa cultures. A
    greater effect was seen at 660 than at 330 µg/litre, the only two doses
    tested. This effect was also demonstrated by Osokina et al. (1984) in
    the green alga  Scenedesmus quadricauda. The effect was highly
    dependant on the cell density of the original inoculum.

         Rai et al. (1981) exposed  Chlorella vulgaris to mercuric
    chloride concentrations between 100 and 1000 µg/litre for 3 weeks, and
    monitored growth and survival. LC50 for survival was at 400 µg/litre
    of mercuric chloride. The growth rate was 92% of the control value at
    100 µg/litre and 31% at 800 µg/litre, and there was no growth at
    1000 µg mercuric chloride/litre. The chlorophyll content of the cells
    was reduced throughout the dose range. There was a greater toxic
    effect of mercuric chloride at low pH, with the greatest amelioration
    of toxicity at pH 9. There was also a protective effect of calcium and
    phosphate in the medium and, to a lesser extent, of magnesium. Both
    calcium and phosphate increased the yield of algae, in the presence of
    sublethal concentrations of mercury, when added at concentrations up
    to 20 mg/litre. At higher concentrations of both calcium and
    phosphate, the protection was less marked. Den Dooren de Jong (1965)
    determined the no-observed-effect-level (NOEL) for mercuric chloride
    on  Chlorella vulgaris to be 50 µg/litre. Hannon & Patouillet (1972)
    emphasized the irreversibility of the effects of mercuric chloride on
     Chlorella pyrenoidosa. If mercury was present in sufficient
    concentration to affect growth of the alga, then no recovery was found
    following transfer in clean medium. Similar effects were reported for
    three species of marine unicellular algae. Mercury toxicity was
    dependant on cell numbers in the initial inoculum (Kuiper, 1981). In
    studies with unialgal cultures of  Chlamydomonas sp., there was a
    relationship between cell concentration and mercury toxicity. The
    author attributed this to a surface area effect, the metal is being
    adsorbed onto cell walls to cause its effect on the unicellular algae.

         Huisman et al. (1980) investigated the effect of temperature on
    the toxicity of mercuric salts to the green alga  Scenedesmus acutus.
    Mercury concentration in the cultures was kept constant by a

    mercury(II) buffer system, and the growth and photosynthesis of the
    alga were monitored. Toxicity increased with increasing temperature
    over the range 15-30°C. There was no effect observed in this study on
    the lag phase, no later increase in growth, and no effect of initial
    cell numbers. This was attributed to the buffer system which prevented
    changes in free mercury concentrations over time. The authors also
    examined the binding of mercury to algal cells. Metal bound to the
    cell wall consists of two fractions: one which can be washed off with
    cysteine solution and one which cannot. The amount of mercury which
    can be washed off the cell wall increase with increasing temperature.
    The mercury bound to cell walls, but washable with cysteine, appears
    to be the toxic fraction. The total mercury content of algal cells
    does not correlate with effect. A total mercury content not lethal at
    15°C causes complete inhibition of growth and photosynthesis at 30°C.
    Recovery occurs under circumstances where the cells retain the non-
    washable mercury, indicating that the washable fraction is the toxic
    component. The authors suggested that the reversibility of the action
    of cysteine-washable mercury indicates that the metal is bound to
    carboxyl or phosphate groups and not to sulfhydryl groups. These
    mercury ions can be readily exchanged for other metal ions, leading to
    a decreased inhibition by mercury. Therefore, in media with a high
    concentration of dissolved salts, mercury appears to be less toxic.
    The authors postulated another mechanism by which mercury might be
    toxic to algal cells. Interference with potassium-sodium-dependent
    ATPase in the cell membrane influences the active transport of
    nutrients. This would give rise to disturbances of nitrogen metabolism
    and also of photosynthesis. The delayed action of mercury on cultures
    could be ascribed to their being initially rich in nitrogen, and,
    therefore, less susceptible to nitrogen starvation.

         Nuzzi (1972) exposed  Phaeodactylum tricornutum, Chlorella sp.,
    and  Chlamydomonas sp., isolated from the lower Hudson River, New
    York, USA, to mercuric chloride. The growth of all three organisms was
    severely inhibited by mercury at 7.5 µg/litre (to between 50% and 75%
    of control growth). The growth of  Chlamydomonas sp. was completely
    inhibited by 15 µg mercuric chloride/litre and the other two species
    by 22 µg/litre.

         Gray & Ventilla (1971) found no effect of mercuric chloride on
    growth of the marine ciliate  Cristigera spp. at a concentration of
    100 µg/litre, but growth was affected after exposure to 200 or
    500 µg/litre. There was a synergistic interaction between mercury and
    lead on this ciliate. Gray & Ventilla (1973) reported reductions in
    growth rate of between 8% and 12% after exposing  Cristigera to
    mercuric chloride at 25 or 50 µg/litre. Persoone & Uyttersprot (1975)
    found no effects of mercuric chloride, at concentrations up to
    100 µg/litre, on the survival or reproduction of the marine ciliate
     Euplotes vannus. However, all cells died after exposure to
    1000 µg/litre.

    5.1.2  Mixed cultures and communities

         Singleton & Guthrie (1977) investigated the effects of inorganic
    mercury, added as mercuric chloride at 40 µg/litre, on populations of
    bacteria from fresh and brackish water. Water was taken from the two
    sources and kept for 1 week in the laboratory before the metal salt
    was added. Results were assessed by measuring total colony-forming
    units (viable bacteria), percentage of chromagenic organisms, and
    numbers of different colony types (species diversity). Control systems
    maintained constant numbers of viable bacteria throughout the 14-day
    test period. When mercury was added, the numbers of viable bacteria
    from test samples increased and remained elevated throughout the test.
    The effect was greater in brackish than in fresh water. Diversity
    declined at the same time as total numbers increased. Some genera of
    bacteria disappeared from the community, notably  Flavobacterium and
     Brevibacterium. Other organisms which disappeared or were greatly
    reduced included  Sarcina sp., Enterobacter sp., Achromobacter sp.,
    and  Escherichia sp. After mercury treatment, the percentage of
    chromagenic species decreased in the population. Controls maintained
    chromagens at a steady 20-25% of total bacteria. Chromagen percentage
    declined most markedly after 9-10 days of mercury exposure.

         Kuiper (1981) exposed a mixed community of marine plankton to
    mercuric chloride (at 0.5, 5.0, or 50 µg mercury/litre) in 1400-litre
    plastic bags suspended from a raft in a Netherlands harbour. The
    addition of 50 µg mercury/litre resulted in complete inhibition of
    phytoplankton activity. There was a decrease in phytoplankton biomass
    because of settling of cells to the bottom of the bag. Phytoplankton
    growth resumed after about 20 days when mercury concentrations were
    still at 18 µg/litre. There was evidence for two possible mechanisms
    for this: either mercury-resistant species were growing or mercury was
    being adsorbed to inanimate particles or removed by chelation.
    Addition of 5.0 µg/litre reduced phytoplankton growth rate. Biomass
    decreased initially but began to increase again when the mercury
    concentration decreased to about 1.5 µg/litre. Mercury at 5.0 µg/litre
    delayed the phytoplankton peak by 9 days but relative carbon
    assimilation by only 1 day. One possible explanation is that mercury
    affected cell division more than carbon assimilation. Both 5.0 and
    50 µg/litre altered the species composition of the growth peak; higher
    mercury concentrations favoured the selection of larger species. The
    first stage in the uptake and toxicity of mercury in phytoplankton is
    adsorption on cell surfaces (e.g., cell walls); the smaller surface to
    volume ratio of larger cells may explain why larger cells are more
    resistant to higher mercury concentrations. Another possible
    explanation involves predation; reduced numbers of predatory
    zooplankton might favour larger phytoplankton cells which might be
    preferred by predators. There was some evidence to support both
    hypotheses. Zooplankton were also affected by mercury. There was
    immediate death of most copepods after the addition of mercuric

    chloride at 50 µg mercury/litre. Development of the copepods  Temorus
     longicornis and  Pseudocalanus elongatus was delayed by
    5.0 µg/litre. The results suggest that the major effect on these
    zooplankton is a retardation of development rather than an increase in
    mortality. Laboratory experiments simulating conditions in the bags
    suggested that zooplankton grazing on phytoplankton was an important
    factor in the productivity of the bags during the second, but not the
    first, half of the experimental period. On day 10 of the experiment,
    viable bacteria numbers were higher in bags with 5.0 and 50 µg
    mercury/litre than in controls. This was probably due to the high
    mortality of phytoplankton increasing the food source for bacteria.
    Conversion rate of organic matter into ammonia was reduced. The author
    concluded that the toxicity of mercury to plankton depends on mercury
    concentration, total surface area for adsorption of mercury (and,
    therefore, on the ratio between living and non-living particles
    present and on absolute cell size), and on the metal species present
    (Kuiper, 1981).

         Hongve et al. (1980) added mercuric salt, alone or in combination
    with humus or sediment, to cultures of a natural phytoplankton
    community in lake water, and monitored photosynthetic carbon fixation
    using a radiolabelled tracer. Mercury reduced carbon fixation by 50%
    at the lowest dose tested (5 × 10-9 mol/litre) and to less than 10%
    of control levels at the highest dose tested (2 × 10-7 mol/litre).
    Addition of either humus or sediment to the cultures reduced mercury
    toxicity presumably by binding the metal to surfaces.

         Zelles et al. (1986) conducted a complex and comprehensive
    experiment to compare different methods for assessing the overall
    ecotoxicological effects of chemicals on soil microorganisms. Three
    soil types were used in an 18-week experiment which investigated ATP,
    heat production, respiration (as measured by carbon dioxide output),
    and iron reduction in the soils under dry and moist conditions. Two
    different dose levels of mercuric chloride were added to the soils
    (2 and 20 mg/kg). Averaging the results obtained in the different
    tests, adverse effects on microorganisms were least in peat soil and
    greatest in sandy soil. Some stimulation of microbial activity
    occurred in peat soil with both low and high concentrations of
    mercuric chloride. At both 2 and 20 mg/kg mercury there was inhibition
    of microbial activity in sandy soil. Effects were generally inhibitory
    in clay soil at both concentrations of mercuric chloride. The authors
    pointed out that it is not possible to assess the ecotoxicological
    effects of mercury on soil by using a single method to assess soil
    function.

    5.2  Toxicity of Organic Mercury

    Appraisal

          Methylmercury is more toxic to microorganisms than are inorganic
     mercury salts. This is probably because greater surface adsorption
     enhances the availability and subsequent uptake of methylmercury.
     This may explain why the toxicity of organomercury is inversely
     correlated with cell density. As the surface area of the total cells
     in the culture increases, so less mercury is available for uptake per
     cell. In organomercury compounds, it is the mercury-containing
     moiety, as opposed to the dissociable anion, which determines the
     toxicity. A common toxic effect in phytoplankton is the inhibition of
     growth, which may in turn often be due to reduced photosynthesis.

         Methylmercury in water at 1 µg/litre has adverse effects on
    microorganisms.

         Ukeles (1962) tested the effect of Lignasan (ethylmercuric
    phosphate 6.25%) on a variety of algae in pure culture. The cultures
    were exposed to Lignasan at 0.6, 6.0, and 60 µg/litre for 10 days. The
    highest dose of 60 µg/litre prevented all growth of cultures, and at
    the end of the exposure, all cells were killed by the treatment. Three
    out of the five algae tested were also killed by Lignasan at
    6.0 µg/litre:  Protococcus sp., Chlorella sp., and  Monochrysis
     lutheri. Growth of the other two species was reduced;  Dunaliella
     euchlora showed 31% of the growth of controls and  Phaeodactylum
     tricornutum 17% of control growth. At 0.6 µg/litre, Lignasan reduced
    growth of four of the five cultures to between 55% and 86% of control
    levels,  Monochrysis alone being unaffected.

         Nuzzi (1972) exposed  Phaeodactylum tricornutum, Chlorella sp.
    and  Chlamydomonas sp. to phenylmercuric acetate (PMA) at
    concentrations of 0.06-15.0 µg mercury/litre.  P. tricornutum was
    also tested against phenylacetate equivalent to the phenylacetate
    content of the PMA, but this had no effect. All three organisms were
    adversely affected by the mercury in PMA, growth being inhibited even
    at the lowest dose tested.  Chlamydomonas was totally inhibited by
    3 µg mercury/litre.  Chlorella sp. showed a steep decline in growth
    as exposure increased from 0.06 to 3 µg/litre, where growth was about
    25% of the control value.  Phaeodactylum growth declined rapidly as
    dose increased to 9 µg/litre, where growth was minimal.

         Holderness et al. (1975) cultured the green alga  Coelastrum 
     microporum with methylmercuric chloride (MMC) at 0.8, 3, 6, 12.6,
    and 250 µg/litre. There was no significant effect on cell
    concentration, as determined by transmittance, at 0.8 µg/litre, but
    higher concentrations were inhibitory. There was a steady reduction in
    cell concentration between 0 and 3 µg MMC/litre and a marked decline
    between 3 and 6 µg/litre, with cell concentration changing from

    125 µlitre/litre, at 3 µg MMC/litre to 31 µlitre/litre at
    6 µg MMC/litre. It was noted, in three series of experiments, that MMC
    caused increased storage of starch in the cells. A slight increase in
    photosynthesis was found after exposure to 0.6 µg MMC/litre.

         Delcourt & Mestre (1978) exposed cultures of  Chlamydomonas
     variabilis to concentrations of phenylmercuric acetate (PMA) between
    10-9 and 7.5 × 10-8 mol/litre. Growth curves of the control
    cultures were linear, with no evident lag phase, irrespective of the
    cell concentration (which varied between 2000 and 100 000 cells/ml) in
    the initial inoculum. The effect of mercury as PMA was initially
    tested with cell concentration at 20 000 cells/ml. Under these
    conditions, cultures exposed to PMA at 10-9 or 2.5 × 10-9 mol/litre
    grew exactly the same as controls. However, at PMA concentrations of
    5 × 10-9 mol/litre or more, there was a dose-related lag phase. When
    exponential growth did start, the curves were parallel to those of the
    control. Final cell numbers were not affected, only the time taken to
    reach maximum growth. Changing the initial cell concentration in the
    cultures changed the toxic threshold of the PMA, PMA toxicity being
    higher at lower algal cell concentrations. The authors considered that
    there are a limited number of binding sites for mercury on the cell
    surface and that this was the reason for the effect of cell
    concentration on toxic threshold. Whilst the toxic threshold was
    higher than likely exposure levels in natural waters at high algal
    cell concentrations, the authors pointed out that the threshold would
    be exceeded at low, spring algal concentrations.

         Harriss et al. (1970) exposed a pure culture of the marine diatom
     Nitzschia delicatissima and a natural phytoplankton community from a
    freshwater lake to four organomercurial compounds at concentrations
    between 0 and 50 µg/litre. The four compounds (PMA, methylmercury
    dicyandiamide [Panogen],  N-methylmercuric- 1,2,3,6-tetra hydro-3,6-
    methano-3,4,5,6,7,8-hexachlorophthalimide [MEMMI], and
    diphenylmercury) showed broadly similar effects on photosynthesis at
    the same concentrations expressed in terms of mercury content. The
    diphenylmercury was slightly less toxic than the other compounds. The
    diatom was exposed to the mercurials for 24 h, and the phytoplankton
    community was exposed for 24, 72, or 120 h, before estimating the
    photosynthetic uptake of labelled hydrogen carbonate over 5 h. At
    concentrations of 1 µg/litre, all four mercurials inhibited
    photosynthesis of the natural phytoplankton. Photosynthetic uptake of
    labelled carbon was between 35% and 55% of control levels for the four
    compounds. At 50 µg/litre, all uptake of carbon stopped and cell
    counts indicated cessation of growth in the case of all compounds
    except diphenylmercury. Photosynthetic carbon uptake was about 40% of
    control levels after exposure for 120 h to 50 µg diphenylmercury/
    litre. The authors stated that the toxicity of diphenylmercury to the
    natural phytoplankton was similar to that of mercuric chloride, but no
    details of studies with inorganic mercury were given.  Nitzschia was
    similarly inhibited by all mercurials tested, except diphenylmercury,
    at 1 µg/litre. The diatom showed virtually no carbon, at assimilation

    in the presence of PMA, methylmercury dicyandiamide, or MEMMI
    10 µg/litre. At 1 µg/litre, the carbon assimilation was 95% of the
    control value with diphenylmercury, 60% with PMA, 23% with
    methylmercury dicyandiamide, and < 10% with MEMMI. The authors noted
    that the toxicity of mercurials to the natural phytoplankton community
    decreased with increasing cell numbers, but no details were given.

    6. TOXICITY TO AQUATIC ORGANISMS

         Mercury is toxic to aquatic organisms, organic forms of the metal
    being generally more toxic than inorganic forms. Effects are more
    likely to be observed in soft freshwater, since the toxicity of the
    metal is reduced in the presence of high salt concentrations. The
    concentration of mercury that produces effects varies considerably
    from one species to another.

    6.1  Toxicity to Aquatic Plants

    Appraisal

          As in the case of microorganisms, mercury, at a wide range of
     concentrations, has effects on various aspects of performance,
     including development and survival. These are partly the result of
     adverse effects on photosynthesis.

          The presence of sediment or humic material reduces the
     availability of mercury to aquatic plants because of adsorption. In
     studies involving a dual medium, such as soil-water, actual exposures
     are more difficult to determine than in studies with a single medium,
     such as water alone.

          Organic forms of mercury, such as methyl- or butylmercury
     chloride are more toxic to aquatic plants than inorganic forms.

         Boney (1971) exposed 2-day-old sporelings of the red alga
     Plumaria elegans to mercuric chloride in solution, and found that
    50% growth inhibition occurred after 6 h, approximately 12 h, and
    approximately 24 h at concentrations of 1.0, 0.5, and 0.25 mg/litre,
    respectively. Organic forms of mercury (methyl, butyl, and
    propylmercuric chlorides) were also investigated, and found to be much
    more toxic than inorganic mercury. Methylmercury gave 50% inhibition
    after 17.5 and 25 min of exposure to 0.08 and 0.04 mg/litre,
    respectively. Propylmercury, at 0.5 mg/litre, produced 50% growth
    inhibition after 2.5 min of exposure and 70% inhibition after 5 min.
    Butylmercury produced more marked inhibition than propylmercury (no
    detailed results given). Hopkin & Kain (1978) found that the survival
    of germinating gametophytes of the macroalga  Laminaria hyperborea,
    in culture, was reduced by 0.01 mg mercury/litre. The lowest effective
    toxic level of mercury for the sporophyte culture was 0.05 mg/litre.

         Stanley (1974) determined EC50s, in the presence of a mercuric
    salt, for various growth parameters of Eurasian watermilfoil
     (Myriophyllum spicatum) grown in soil with water above. EC50s
    (in mg/litre) were 3.4 for root weight, 4.4 for shoot weight, 12 for
    root length, 1.2 for shoot length. The author added mercury to the
    water, to the soil, or to the water in a system containing ferric
    silicate instead of soil. Comparison of the tissue concentrations of
    mercury when the metal salt was added in these different ways

    indicated a very strong tendency for mercury to be adsorbed onto soil.
    There was no indication that the presence of soil affected mercury
    uptake in any way other than by simple adsorption, i.e., no soil
    component interacted with the mercury ions.

         De et al. (1985) exposed the floating plant water cabbage
     (Pistia stratiotes) for 2 days to mercuric chloride at
    concentrations between 0.05 and 20.0 mg/litre. The highest dose of
    mercury promoted plant senescence by decreasing chlorophyll content,
    protein, RNA, dry weight, and catalase and protease activities, and by
    increasing free amino acid content. Lesser, mostly non-significant,
    effects on these parameters were recorded at lower doses. In studies
    by Brown & Rattigan (1979), the aquatic macrophyte  Elodea canadensis
    (Canadian pond weed) and the free-floating duckweed  Lemna minor were
    exposed for 28 days and 14 days, respectively, to a range of
    concentrations of mercuric chloride. Damage to the plants was assessed
    visually on a coded scale ranging from 0 (no damage) to 10 (plant
    killed). Water concentrations of 7.4 and 1.0 mg/litre produced 50%
    damage to the two plants, respectively. In a separate study,  Elodea
    was exposed to mercury for 24 h in the dark and then oxygen evolution
    in the light was measured. Levels of 0.8 and 1.69 mg mercury/litre
    reduced photosynthetic oxygen evolution by 50% and 90%, respectively.
    Czuba & Mortimer (1980, 1982) exposed plants of  Elodea densa,
    growing in flowing water, to concentrations of methylmercuric chloride
    at 7.5 × 10-10, 7.5 × 10-9, or 7.5 × 10-8mol/litre, for 25 days.
    Toxicity was assessed by gross morphological examination and from the
    examination of histological sections embedded in paraffin wax. There
    was a difference in toxic effect between tissues. Apical cells were
    most sensitive to the mercury and developed aberrant nuclear and
    mitotic characteristics at lower concentrations than did roots. Root
    meristems showed total inhibition of mitotic activity at the middle
    concentration but no effect at the lowest concentration used. Mitotic
    activity in bud meristems was absent in controls, but increased in the
    presence of methylmercury; divisions were abnormal. Higher
    concentrations of methylmercury chloride, up to 2.5 × 10-6mol/litre,
    stimulated the development of additional buds. The development of root
    and bud initials was inhibited by methylmercury at 7.5 × 10-8 and
    2.5 × 10-6mol/litre, respectively.

    6.2  Toxicity to Aquatic Invertebrates

    Appraisal

          Factors which affect the toxicity of mercury to aquatic
     invertebrates include the concentration and species of mercury, the
     developmental stage of the organisms, and the temperature, salinity,
     water hardness, and flow rate. Methylmercury is more toxic than aryl
     or inorganic mercury. The larval stage is apparently the most
     sensitive stage of the organism's life cycle. Mercury toxicity
     increases with temperature and decreases with water hardness.
     Toxicity, appears to be higher in flow-through systems than in static

     systems. This effect is probably due mostly to the actual
     concentration of mercury available to the organism, which is lower in
     static systems. The fact that lower salinity seems to increase
     toxicity may be due more to the stress that is placed on the
     organism.

          Levels of 1 to 10 µg/litre normally causes acute toxicity for
     the most sensitive developmental stage of many different species of
     aquatic invertebrates.

         The acute toxicity of mercury to aquatic invertebrates is
    summarized in Tables 3 and 4.

    6.2.1  Acute and short-term toxicity to invertebrates

         Wisely & Blick (1967) determined the concentration of mercury in
    water required to kill 50% of larvae for some species of bryozoans
     (Watersipora cucullata and  Bugula neritina), tubeworms  (Spirorbis
     lamellosa and  Galeolaria caespitosa), bivalve molluscs  (Mytilus
     edulis and  Crassostrea commercialis), and the brine shrimp
     (Artemia salina). The 2-h LC50s for the larvae of these species
    were 5 × 10-7, 1 × 10-6, 7 × 10-7, 6 × 10-6, 6.5 × 10-5,
    9 × 10-4, and 9 × 10-3 mol mercuric chloride/litre, respectively.

         Howell (1984) exposed two species of marine nematodes, one
    euryhalinea  (Enoplus brevis) and one stenohalineb  (Enoplus
     communis) to mercuric chloride.  E. brevis was collected from two
    sites, one nonpolluted and one polluted with heavy metals. The
    stenohaline species was more sensitive to mercuric chloride than the
    related euryhaline species. At a concentration of 0.01 mg mercuric
    chloride/litre,  E. communis showed an LT50 of approximately 65 h,
    whereas 50%  E. brevis collected from the nonpolluted site survived
    for approximately 415 h at the same concentration.  E. brevis from
    the polluted area was even less sensitive, with an LT50 of more than
    600 h, suggesting the selection of resistant strains.

         When Best et al. (1981) exposed the planarian  Dugesia
     dorotocephala to concentrations of methylmercury chloride of between
    0 and 2 mg/litre, 100% deaths were reported at 0.5, 1, and 2 mg/litre
    within 5 days, 1 day, and 5 h, respectively. No deaths occurred at
    0.2 mg/litre over a 10-day-period, but other, non-lethal toxic 

    a  tolerant of a wide range of salinity
    b  tolerant of only a narrow range of salinity

        Table 3.  Toxicity of inorganic mercury (as mercuric chloride) to marine invertebrates
                                                                                                                                               
    Organism           Lifestage    Stat/    Temperature    pH        Salinity       Dissolved   Parameter     Water            Reference
                                    flowa    (°C)                     (%)            oxygen                    concentration
                                                                                     (mg/litre)                (µg/litre)

    Starfish           adult        stat     20             7.8       20             > 4         24-h LC50     1800             Eisler & Hennekey
                                                                                                                                (1977)
    (Asterias          adult        stat     20             7.8       20             > 4         96-h LC50     60               Eisler & Hennekey
    forbesi)                                                                                                                    (1977)
                       adult        stat     20             7.8       20             > 4         168-h LC50    20               Eisler & Hennekey
                                                                                                                                (1977)

    Hard clam          embryo       stat     25-27          7-8.5     25                         48-h LC50     4.8              Calabrese & Nelson
    (Mercenaria                                                                                                (3.8-5.6)        (1974)
    merceneria)

    Softshell clam     adult        stat     20             7.8       20             > 4         24-h LC50     5200             Eisler & Hennekey
                                                                                                                                (1977)
    (Mya arenaria)     adult        stat     20             7.8       20             > 4         96-h LC50     400              Eisler & Hennekey
                                                                                                                                (1977)
                       adult        stat     20             7.8       20             > 4         168-h LC50    4                Eisler & Hennekey
                                                                                                                                (1977)

    American oyster    embryo       stat     25-27          7-8.5     25                         48-h LC50     5.6              Calabrese et al.
    (Crassostrea                                                                                               (4.2-6.8)        (1973)
    virginica)

    Pacific oyster     embryo       stat     19-21          7.9-8.3   33.7-33.8      6.5-8.0     48-h EC50c    5.7              Glickstein (1978)
    (Crassostrea
    gigas)

    Oyster             larvae       stat     15                                                  48-h LC50     1.0-3.3          Connor (1972)
    (Ostrea edulis)    adult        stat     15                                                  48-h LC50     4200             Portmann & Wilson
                                                                                                                                (1971)

    Table 3 (cont'd)
                                                                                                                                               
    Organism           Lifestage    Stat/    Temperature    pH        Salinity       Dissolved   Parameter     Water            Reference
                                    flowa    (°C)                     (%)            oxygen                    concentration
                                                                                     (mg/litre)                (µg/litre)

    Cockle             adult        stat     15                                                  48-h LC50     9000             Portmann & Wilson
    (Cardium edule)                                                                                                             (1971)

    Mud snail          adult        stat     20             7.8       20             > 4         24-h LC50     32 000           Eisler & Hennekey
                                                                                                                                (1977)
    (Nassarius         adult        stat     20             7.8       20             > 4         96-h LC50     32 000           Eisler & Hennekey
    obsoletus                                                                                                                   (1977)
                       adult        stat     20             7.8       20             > 4         168-h LC50    700              Eisler & Hennekey
                                                                                                                                (1977)

    American lobster   stage I      stat     18-22                    29.5-31.5      7.6-8.6     96-h LC50     20               Johnson & Gentile
    (Homarus           larvae                                                                                                   (1979)
    americanus)

    European lobster   larvae       stat     15                                                  48-h LC50     33-100           Connor (1972)
    (Homarus
    gammarus)

    Pink shrimp        adult        stat     15                                                  48-h LC50     75               Portmann & Wilson
    (Pandalus                                                                                                                   (1971)
    montagui)

    White shrimp       post-        stat     21-24                    25                         96-h LC50     17               Green et al.
    (Penaeus           larval                                                                                  (13-21)          (1976)
    setiferus)

    Brown shrimp       larvae       stat     15                                                  48-h LC50     10               Connor (1972)
    (Crangon           adult        stat     15                                                  48-h LC50     3300-10 000      Portmann & Wilson
    crangon)                                                                                                                    (1971)
                       adult        statb    15                                                  96-h LC50     100-330          Portmann & Wilson
                                                                                                                                (1971)

    Table 3 (cont'd)
                                                                                                                                               
    Organism           Lifestage    Stat/    Temperature    pH        Salinity       Dissolved   Parameter     Water            Reference
                                    flowa    (°C)                     (%)            oxygen                    concentration
                                                                                     (mg/litre)                (µg/litre)

    Grass shrimp       stage I               26.5-27        6.3-6.9   32.73-33.29    5.6         48-h LC50     10               Shealy & Sandifer
    (Palaemonetes      larvae                                                                                  (7.8-12.7)       (1975)
    vulgaris)                                                                                                  unfed
                       stage I               27             6.4-6.7   33.99          5.8-7.6     48-h LC50     15.6             Shealy & Sandifer
                       larvae                                                                                  (12.7-19.3)      (1975)
                                                                                                               fed

    Dungeness crab     1st stage             14-16          7.9-8.3   33.72-33.86    6.5-8.0     48-h LC50     21.1             Glickstein (1978)
    (Cancer            zoeae                                                                                   (19.7-22.5)
    magister)

                       1st stage             14-16          7.9-8.3   33.72-33.86    6.5-8.0     96-h LC50     6.6              Glickstein (1978)
                       zoeae                                                                                   (5.6-4.6)

    Shore crab         larvae       stat     15                                                  48-h LC50     14               Connor (1972)
    (Carcinus          adult        stat     15                                                  48-h LC50     1200             Portmann & Wilson
    maenus)                                                                                                                     (1971)

    Hermit crab        adult        stat     20             7.8       20             > 4         24-h LC50     2200             Eisler & Hennekey
                                                                                                                                (1977)
    (Pagurus           adult        stat     20             7.8       20             > 4         96-h LC50     50               Eisler & Hennekey
    longicarpus)                                                                                                                (1977)
                       adult        stat     20             7.8       20             > 4         168-h LC50    50               Eisler & Hennekey
                                                                                                                                (1977)

    Crab               adult        stat     26.5-29.5      7-7.2                                24-h LC50     930              Krishnaja et al.
                                                                                                                                (1987)
    (Scylla serrata)   adult        stat     26.5-29.5      7-7.2                                48-h LC50     800              Krishnaja et al.
                                                                                                               (740-860)        (1987)

    Table 3 (cont'd)
                                                                                                                                               
    Organism           Lifestage    Stat/    Temperature    pH        Salinity       Dissolved   Parameter     Water            Reference
                                    flowa    (°C)                     (%)            oxygen                    concentration
                                                                                     (mg/litre)                (µg/litre)

                       adult        stat     26.5-29.5      7-7.2                                72-h LC50     680              Krishnaja et al.
                                                                                                                                (1987)
                       adult        stat     26.5-29.5      7-7.2                                96-h LC50     680              Krishnaja et al.
                                                                                                               (600-760)        (1987)

    Polychaete         juv          stat                    7.8                                  96-h LC50     100d             Reish et al. (1976)
    (Neanthes          adult        stat                    7.8                                  96-h LC50     22d              Reish et al. (1976)
    arenaceodentata)   juv          stat                    7.8                                  28-day LC50   90d              Reish et al. (1976)
                       adult        stat                    7.8                                  28-day LC50   17d              Reish et al. (1976)

    Polychaete         larva        stat                    7.8                                  96-h LC50     14d              Reish et al. (1976)
    (Capitella         adult        stat                    7.8                                  96-h LC50     >100d            Reish et al. (1976)
    capitata)          adult        stat                    7.8                                  28-day LC50   100d             Reish et al. (1976)

    Sandworm           adult        stat     20             7.8       20             > 4         24-h LC50     3100             Eisler & Hennekey
                                                                                                                                (1977)
    (Nereis virens)    adult        stat     20             7.8       20             > 4         96-h LC50     70               Eisler & Hennekey
                                                                                                                                (1977)
                       adult        stat     20             7.8       20             > 4         168-h LC50    60               Eisler & Hennekey
                                                                                                                                (1977)
                                                                                                                                               
    a  stat = static conditions (water unchanged for duration of test).
    b  static conditions but test water changed every 24 h.
    c  abnormal development.
    d  with food.

    Table 4.  Toxicity of inorganic mercury to freshwater invertebratesc

    Organism/           Stat/     Temperature    Alkalinityf   Hardnessf    pH       Dissolved     Parameter     Water           Reference
    weight (g)          flowa     (°C)                                               oxygen                      concentration
                                                                                     (mg/litre)                  (µg/litre)

    Mussel              stat      28-32          9.5-9.9       32-38        7-7.3    5.38-6.2      24-h LC50     7390            Ramamurthi
    (Lamellidens        stat      28-32          9.5-9.9       32-38        7-7.3    5.38-6.2      48-h LC50     5910            et al.
    maginalis)          stat      28-32          9.5-9.9       32-38        7-7.3    5.38-6.2      72-h LC50     3690            (1982)
      32-35

    Snail (egg)         stat      17                           50           7.6      6.2           24-h LC50     6300            Rehwoldt
    (Amnicola sp.)      stat      17                           50           7.6      6.2           96-h LC50     2100            et al.
      (adult)           stat      17                           50           7.6      6.2           24-h LC50     1100            (1973)
                        stat      17                           50           7.6      6.2           96-h LC50     80              Rehwoldt et
                                                                                                                                 al. (1973)

    Snail               stat      28-32          9.5-9.9       32-38        7-7.3    5.38-6.2      24-h LC50     1108            Ramamurthi
    (Pila globosa)      stat      28-32          9.5-9.9       32-38        7-7.3    5.38-6.2      48-h LC50     369             et al.
      20-25             stat      28-32          9.5-9.9       32-38        7-7.3    5.38-6.2      72-h LC50     296             (1982)

    Pulmonate snail               25.5-29.5      240-278       290-335      7.4-8.1  6.0-8.1       24-h LC50     330             Mathur et al.
                                                                                                                 (297-360)       (1981)
    (Lymnaea luteola)             25.5-29.5      240-278       290-335      7.4-8.1  6.0-8.1       48-h LC50     188             Mathur et al.
      0.46-0.72                                                                                                  (163-210)       (1981)
                                  25.5-29.5      240-278       290-335      7.4-8.1  6.0-8.1       96-h LC50     135             Mathur et al.
                                                                                                                 (112-186)       (1981)

    Crab                stat      28-32          9.5-9.9       32-38        7-7.3    5.38-6.2      24-h LC50     739             Ramamurthi
    (Oziotelphusa       stat      28-32          9.5-9.9       32-38        7-7.3    5.38-6.2      48-h LC50     591             et al.
    senex senex)        stat      28-32          9.5-9.9       32-38        7-7.3    5.38-6.2      72-h LC50     443             (1982)
      35-88

    Table 4 (cont'd)

    Organism/           Stat/     Temperature    Alkalinityf   Hardnessf    pH       Dissolved     Parameter     Water           Reference
    weight (g)          flowa     (°C)                                               oxygen                      concentration
                                                                                     (mg/litre)                  (µg/litre)

    Crayfish            flowb     15-17                                     7.0                    96-h LC50     20              Boutet &
    (Austropotamobius   flowb     15-17                                     7.0                    30-day LC50   2               Chaisemartin
    pallipes pallipes)  flowb     15-17                                     7.0                    30-day LC50   < 2d            (1973)

    Crayfish            flowb     15-17                                     7.0                    96-h LC50     50              Boutet &
    (Orconectes         flowb     15-17                                     7.0                    30-day LC50   2               Chaisemartin
    limosus)            flowb     15-17                                     7.0                    30-day LC50   < 2d            (1973)

    Scud                stat      17                           50           7.6      6.2           24-h LC50     90              Rehwoldt
    (Gammarus sp.)      stat      17                           50           7.6      6.2           96-h LC50     10              et al. (1973)

    Copepod             stat      10             0.58 meq/                  7.2                    48-h LC50     2200            Baudouin &
    (Cyclops abyssorum)                          litre                                                           (1500-3300)     Scoppa (1974)

    Water flea          stat      10             0.58 meq/                  7.2                    48-h LC50     5.5             Baudouin &
    (Daphnia hyalina)                            litre                                                           (3.1-9.8)       Scoppa (1974)

    Water flea          stat                                                                       48-h LC50     1.8-4.3         Canton &
                                                                                                                                 Adema (1978)
    (Daphnia magna)     stat      17-19          41-50         44-53        7.4-8.2                48-h LC50     5               Biesinger &
                        stat      17-19          41-50         44-53        7.4-8.2                21-day LC50   13e             Christensen
                                                                                                                 (9-19)          (1972)

                        stat      11.5-14.5      390-415       235-260      7.4-7.8  5.2-6.5       24-h LC50     4890            Khangarot &
                                                                                                                 (4190-5890)     Ray (1987)
                        stat      11.5-14.5      390-415       235-260      7.4-7.8  5.2-6.5       48-h LC50     3610            Khangarot &
                                                                                                                 (2830-4400)     Ray (1987)

    Table 4 (cont'd)

    Organism/           Stat/     Temperature    Alkalinityf   Hardnessf    pH       Dissolved     Parameter     Water           Reference
    weight (g)          flowa     (°C)                                               oxygen                      concentration
                                                                                     (mg/litre)                  (µg/litre)

    Water flea          stat                                                                       48-h LC50     3.0             Canton &
    (Daphnia pulex)                                                                                                              Adema (1978)

    Water flea          stat                                                                       48-h LC50     3.2             Canton &
    (Daphnia cucullata)                                                                                                          Adema (1978)

    Copepod             stat      10             0.58 meq/                  7.2                    48-h LC50     850             Baudouin &
    (Eudiaptomus                                 litre                                                           (710-1020)      Scoppa (1974)
    padanus)

    Bristle worm        stat      17                           50           7.6      6.2           24-h LC50     1900            Rehwoldt
    (Nais sp.)          stat      17                           50           7.6      6.2           96-h LC50     1000            et al. (1973)

    Stone fly           stat      16-20          40            44           7.25     9.2           96-h LC50     2000            Warnick &
    (Acroneuria                                                                                                                  Bell (1969)
    lycorias)

    May fly             stat      16-20          40            44           7.25     9.2           96-h LC50     2000            Warnick &
    (Ephemerella                                                                                                                 Bell (1969)
    subvaria)

    Caddis fly          stat      16-20          40            44           7.25     9.2           96-h LC50     2000            Warnick &
    (Hydropsyche                                                                                                                 Bell (1969)
    betteni)

    Caddis fly          stat      17                           50           7.6      6.2           24-h LC50     5600            Rehwoldt
    (unidentified sp.)  stat      17                           50           7.6      6.2           96-h LC50     1200            et al. (1973)

    Table 4 (cont'd)

    Organism/           Stat/     Temperature    Alkalinityf   Hardnessf    pH       Dissolved     Parameter     Water           Reference
    weight (g)          flowa     (°C)                                               oxygen                      concentration
                                                                                     (mg/litre)                  (µg/litre)

    Damsel fly          stat      17                           50           7.6      6.2           24-h LC50     3200            Rehwoldt
    (unidentified sp.)  stat      17                           50           7.6      6.2           96-h LC50     1200            et al. (1973)

    Midge               stat      17                           50           7.6      6.2           24-h LC50     60              Rehwoldt
    (Chironomus sp.)    stat      17                           50           7.6      6.2           96-h LC50     20              et al. (1973)

    Midge               flowb     20                           50           6.8                    24-h LC50     1074            Rossaro
    (Chironomus                                                                                                  (760-1520)      et al. (1986)
    riparius)
      4th instar        flowb     20                           50           6.8                    48-h LC50     316             Rossaro
      larvae                                                                                                     (230-440)       et al. (1986)

                        flowb     20                           50           6.8                    96-h LC50     100             Rossaro
                                                                                                                 (50-180)        et al. (1986)

                        stat      20                           50           6.8                    24-h LC50     1028            Rossaro
                                                                                                                 (880-1200)      et al. (1986)

                        stat      20                           50           6.8                    48-h LC50     750             Rossaro
                                                                                                                 (660-850)       et al. (1986)

                        stat      20                           50           6.8                    96-h LC50     547             Rossaro
                                                                                                                 (480-630)       et al. (1986)
                                                                                                                                              
    a  stat = static conditions (water unchanged for duration of test).
    b  intermittent flow-through conditions.
    c  mercuric chloride was used except in the studies of Rehwoldt et al. (1973) and Khangarot & Ray (1987) (salt used unspecified).
    d  with food.
    e  extrapolated value from three concentrations less than the LC50, daphnids were fed at the beginning of each week.
    f  alkalinity & hardness expressed as mg CaCO3/litre unless otherwise stated.
    
    responses, including varying degrees of head resorption, were observed
    within 1 day. This was followed by some head regeneration within 10
    days. After some animals were decapitated, regeneration was retarded
    at 0.1 and 0.2 mg methylmercury chloride/litre. Although no deaths,
    malformations, visible lesions, or gross behavioural abnormalities
    were seen at 20 µg/litre or less, significant changes in fissioning
    were noted, even at the lowest mercury concentration tested
    (0.03 µg/litre). Fissioning was almost completely suppressed after
    3 days in 0.1 µg/litre.

         When Dorn (1974) exposed the bivalve mollusc  Congeria
     leucophaeata for 48 h to mercuric chloride at concentrations of
    0, 0.001, 0.01, 0.1, and 1.0 mg/litre, there was a significant
    increase, compared with controls, in respiration rate at all dose
    levels. The effect was dose related over the entire range. Stromgren
    (1982) exposed the mussel  Mytilus edulis to mercuric chloride and
    found after 5 days a significant reduction in growth rate at 0.3 µg
    mercury/litre. At concentrations > 1.6 µg/litre, growth almost ceased
    within 3 to 4 days of exposure, while at 25 µg/litre acute lethal
    effects were observed within 24 h. Breittmayer et al. (1981)
    investigated the effects of metal concentration, size of organism, and
    seasonal differences on the toxicity of mercury to  Mytilus edulis.
    The most important factor for mercury toxicity was season, though all
    factors interacted. MacInnes (1981) studied the effect of mercury on
    embryos of the American oyster  Crassostrea virginica. The test was
    initiated 2 h after fertilization and continued for 48 h, the embryos
    then being checked for abnormal development (they did not undergo
    embryogenesis). The percentage of abnormal development for the test
    concentrations of 5 and 10 µg/litre were 6 and 15.7% for the chloride
    salt, and 2.9 and 9.8% for the nitrate. Dillon (1977) found that the
    96-h LC50 for the estuarine marsh clam  Rangia cuneata exposed to
    mercuric chloride was reduced from 0.122 to 0.058 mg/litre with an
    increase in salinity from 2 to 15%. The pre-exposure of clams to
    8.56 µg mercury/litre, followed by a period in clean water,
    significantly enhanced the survival of  Rangia experimentally exposed
    to 0.87 mg mercury/litre. Results showed an LT50 of 135 h for
    unexposed clams compared to an LT50 of 210 h for pre-exposed clams.

         Biesinger & Christensen (1972) found that in waterfleas  (Daphnia
     magna) reproductive impairment was a more sensitive measure of the
    toxicity of mercuric chloride than survival. EC16 and EC100 values
    were 3.4 and 6.7 µg mercury/litre, respectively, for a 3-week
    exposure. Biesinger et al. (1982) exposed  Daphnia magna to mercury
    (as mercuric chloride, methylmercuric chloride, or phenylmercuric
    acetate) in a chronic experiment over 3 weeks. The lowest
    concentrations of the three compounds to affect survival were 1.92,
    0.2-0.98, and 2.25 µg/litre, respectively. Lowest concentrations
    affecting reproduction were 0.72, 0.04, and 1.90 µg/litre,
    respectively. All figures are in terms of mercury concentration in
    water.

         Pyefinch & Mott (1948) studied the effect of mercuric chloride on
    the barnacles  Balanus balanoides and  Balanus crenatus. The
    toxicity of mercury to cyprids of  B. balanoides was reduced by
    dilution of the sea water to reduce salinity. Older (11-12 day) larvae
    were less resistant than 1-day-old larvae. A mercury concentration of
    0.01 mg/litre reduced the number of cyprids settling. Exposure of
     B. balanoides and  B. crenatus after metamorphosis yielded median
    lethal concentrations, over 6 h, of 0.36 and 1.35 mg/litre,
    respectively.

         Barnes & Stanbury (1948) found the median lethal concentration of
    mercuric chloride to the harpacticoid copepod  Nitocra spinipes to be
    0.6 mg mercury/litre. When the mercuric salt was added with copper
    sulfate, the chemicals acted synergistically. Lalande & Pinel-Alloul
    (1986) collected  Tropocyclops prasinus mexicanus from three
    different Quebec lakes, two of low water hardness
    (10 mg CaCO3/litre) and one of high (120 mg CaCO3/litre). The lake
    with the high water hardness was polluted with human effluent. Animals
    from the two unpolluted lakes showed mean 48-h EC50s
    (immobilization) of 0.015 and 0.045 mg/litre, whereas those from the
    polluted lake with a high water hardness showed an EC50 of
    0.199 mg/litre.

         When Sheally & Sandifer (1975) exposed newly-hatched grass shrimp
     (Palaemonetes vulgaris) larvae to mercury, a concentration of
    56 µg/litre was lethal to all larvae within 24 h. No deaths occurred
    within 48 h when the shrimps were exposed to concentrations of
    3.2 µg/litre or less. At 5.6 µg/litre, there were no deaths in fed
    larvae but some deaths occurred among unfed animals. The authors found
    that feeding slightly increased the resistance of  P. vulgaris larvae
    to mercury. In surviving larvae some delayed effects of mercury were
    noted. Concentrations of 10 to 18 µg/litre caused a significant
    reduction in survival to the post-larval stage, a delayed moult, an
    extended development time, an increase in the number of larval
    instars, and an increase in the occurrence of deformities.

         Portmann (1968) found that a reduction in temperature from 22°C
    to 5°C increased 5-fold the tolerance of brown shrimps to mercury
    (added as mercuric chloride) within 48 h. With cockles the effect was
    even more pronounced, increasing the 48-h LC50 by a factor of 130.
    It was also found that starving the animals reduced their tolerance to
    mercury. The 48-h LC50 for brown shrimps was halved (from 1.3 to
    0.65 mg/litre) and reduced by a third in cockles (from 15.5 to
    9.6 mg/litre). Larger shrimps were more resistant to mercury; the
    LC50 for the largest shrimps was 1.26 mg/litre, whereas that for the
    smallest was 0.58 mg/litre.

         Brown & Ahsanullah (1971) studied the effects of mercuric
    chloride on the mortality of the adult brine shrimp  (Artemis salina)
    and the worm  (Ophryotrocha labronica). After exposure to

    1 mg mercury/litre, the LT50s were 25 h for  Artemia and 0.5 h for
     Ophryotrocha. Green et al. (1976) found that a 60-day exposure of
    post-larval white shrimp  (Penaeus setiferus) to mercuric chloride
    (at either 0.5 or 1.0 µg mercury/litre) did not significantly affect
    respiratory rate, growth, or moulting rate.

         In studies by Chinnayya (1971), mercuric chloride in freshwater
    (at 1 × 10-7mol/litre) reduced oxygen consumption of the shrimp
     Caridina rajadhari from a control level of 0.485 ml/h per g wet
    weight of shrimps to 0.377 ml/h per g. This concentration of mercury
    caused no mortality over 10 days. The lowest concentration causing
    mortality in this species was 2.5 × 10-7mol/litre.

         Barthalamus (1977) found that concentrations of 2 and 5 mg
    mercuric chloride/litre killed 100% of grass shrimps  Palaemonetes
     pugio, within 24 h, and 1 and 0.5 mg/litre over a period of 96 h. He
    calculated the 120 h LC50 to be 0.2 mg/litre, and found that
    0.05 mg/litre significantly impaired the conditioned avoidance
    response.

         Knapik (1969) studied the toxic effect of mercuric nitrate on
    four species of crustaceans, using concentrations of 10, 100, 200, and
    500 mg mercuric nitrate/litre. The most sensitive species was
     Neomysis vulgaris (only 10% survived for 2 h at 10 mg/litre),
    followed by  Palaemonetes varians and  Gammarus locusta.
     Rhithropanopeus harrisi tridentatus was unaffected by a 3-h exposure
    to 100 mg/litre and 23% of animals survived 1 h at 500 mg/litre.

         When Doyle et al. (1976) exposed crayfish  Orconectes limosus to
    mercuric chloride, they observed 100% survival at 0.25 mg/litre over a
    period of 96 h. Survivors of a 96-h exposure to 1 mg/litre
    (the LC60) showed a sluggish response to mechanical stimulation.
    Only occasional ventilative movements were observed in survivors of
    higher concentrations. All crayfish were dead within 96 h at
    5 mg/litre.

         Khayrallah (1985) studied the effect of both mercuric and
    methylmercuric chloride on the amphipod  Bathyporeia pilosa. The
    toxicity of both inorganic and organic mercury was directly related to
    both concentration (0.04-0.75 mg mercury/litre) and temperature
    (1, 10, and 20°C) and inversely related to salinity (10, 20, and 30%)
    and age (adult and juvenile).

         Meadows & Erdem (1982) calculated LT50s for  Corophium
     volutator in 1 µg mercuric chloride/litre of about 30 days and in
    1000 mg/litre of about 3 h. Krishnaja et al. (1987) studied the acute
    toxicity of phenylmercuric acetate to the intertidal crab  Scylla
     serrata and calculated the 24-h, 48-h, 72-h, and 96-h LC50s to be
    700, 580, 540, and 540 µg/litre, respectively. DeCoursey & Vernberg
    (1972) exposed larval stages (zoea I, III, and V) of the fiddler crab
     Uca pugilator to mercuric chloride at concentrations of 0.018, 1.8,

    or 180 µg mercury/litre. No stage V, and only a few of stages I and
    III, survived 180 µg/litre for longer than 24 h. Vernberg et al.
    (1974) found that the adult fiddler crab  Uca pugilator could survive
    prolonged periods of time in sea water (at 25°C and a salinity of 30%)
    and at a mercuric chloride concentration of 0.18 mg mercury/litre.
    However, under temperature and salinity stress, survival periods were
    reduced. At 5°C and 5%, LT50s were 20 and 7 days, for females and
    males, respectively, and these were further reduced to 8 and 6 days,
    respectively, by the addition of 0.18 mg mercury/litre. When the
    temperature was increased to 35°C, crabs survived to 28 days at low
    salinity, but the addition of mercury at 0.18 mg/litre again reduced
    survival, with LT50s of 17 days for males and 26 days for females.
    Exposure of larvae revealed that 0.18 mg/litre was fatal to stage I
    zoeae, the LT50 being < 24-h. At 1.8 and 0.0018 mg/litre the 50%
    survival times were 8 days (stage II) and 11 days (stage III),
    respectively, compared to a control value of 18 days (stage IV).

         McKenney & Costlow (1981) found that the survival of the
    megalopae stage of the blue crab  Callinectes sapidus was highest at
    a salinity of 30% and significantly reduced at 10%. Mercury at
    10 µg/litre significantly increased the number of deaths of megalopae
    developing at 10% but not those at salinities of 20-40%. At all
    salinities, fewer megalopae completed metamorphosis at 20 µg
    mercury/litre. Developmental times of the megalopae in the presence of
    20 µg mercury/litre were increased to 8 to 10 days when the salinity
    was reduced to 10%, and increased further, to nearly 13 days.
    Following metamorphosis, the crabs were found to be more resistant.
    There were no significant effects of salinity or mercury on survival
    or developmental duration at the first two adult crab stages.

         Depledge (1984a) found that exposure of the shore crab  Carcinus
     maenus to 0.05 mg mercuric sulfate/litre disrupted various
    endogenous rhythms. Locomotor activity increased and the mean heart
    rate rose from 32.1 beats/min to 44.7, although there was no change in
    the heart stroke volume (as indicated by a lack of change in the trace
    height of cardiograph readings). Exposure of crabs to 1 mg/litre
    suppressed cardiac activity and oxygen consumption. Alternating
    periods of bradycardia and tachycardia were observed together with
    marked changes in the heart stroke volume. There was an increase in
    the median perfusion index (volume of blood per unit volume of
    dissolved oxygen). All of the animals died within 24 to 48 h, this
    being associated with a loss of the ability to osmoregulate (Depledge,
    1984b).

         Weis (1980) exposed the fiddler crab  Uca pugilator to a mixture
    of methylmercuric chloride (0.5 mg mercury/litre) and as zinc chloride
    (3 mg zinc/litre) and found the effect of the combination of metals on
    the retardation of limb regeneration to be additive. The effect was
    also additive at a reduced salinity (7-8%).

    6.2.2  Behavioural effects

    Appraisal

          Mercury appears to increase the probability of prey organisms
     being eaten by predators (at least in a single study). Prior exposure
     of prey organisms leads to the selection of a resistant strain and
     the effect of mercury, at the same concentration, disappears. The
     development of tolerance in invertebrates in the field must be taken
     into account when evaluating laboratory studies on test animals that
     have not experienced exposure to mercury before.

         Kraus & Kraus (1986) tested predator avoidance in adult grass
    shrimps  (Palaemonetes pugio) collected from two sites, one polluted
    with mercury (sediment mercury levels "as high as 10.3 mg/kg") and the
    other relatively pollution-free (sediment levels of 0.05 mg/kg). The
    shrimps were maintained in water containing either mercuric chloride
    or methylmercuric chloride (both at 0.01 mg/litre), for 96 h prior to
    testing. Killifish, collected only from the nonpolluted area, were
    then added to the tanks and the time between first and second captures
    of shrimp were noted. This was significantly reduced by both inorganic
    and organic mercury in shrimp from the nonpolluted area. Control
    shrimp from the polluted area showed a reduced capture time compared
    to shrimp from the nonpolluted area, which was not reduced further by
    the mercury treatment. The overall survival of shrimps from the
    nonpolluted area, over 60 or 120 min of exposure to the predator, was
    not significantly affected by mercury treatment. In the shrimps from
    the polluted area, only the survival of shrimps in organic mercury,
    over the 60-min test period, showed a significant overall effect of
    the predator.

    6.3  Toxicity to Fish

    Appraisal

          Inorganic mercury is toxic to fish at low concentrations. The
     96-h LC50 s vary between 33 and 400 µg/litre for freshwater fish and
     are higher for sea water fish. Organic mercury compounds are more
     toxic. Toxicity is affected by temperature, salinity, dissolved
     oxygen, and water hardness. A wide variety of physiological and
     biochemical abnormalities have been reported after exposure of fish
     to sublethal concentrations of mercury. Reproduction is also
     adversely affected by mercury.

    6.3.1  Acute and short term toxicity to fish

         The acute toxicity of mercury to fish is summarized in Tables 5
    and 6. Schweiger (1957) investigated the effects of mercury ions on
    fish and their food organisms and suggests a concentration of 0.03 mg
    mercury/litre as the toxic threshold for the various species tested.

         Rodgers et al. (1951) investigated the toxicity of pyridyl
    mercuric acetate to three different species of trout. No deaths
    occurred in either brown trout or brook trout exposed to the compound
    at 10 mg/litre for 1 h. Rainbow trout were more susceptible with 99%
    mortality at 13°C and 33% mortality at 8.5°C. Deaths also occurred in
    rainbow trout exposed to 5 mg/litre (3% at 8.5°C; 36% at 13°C) but
    little mortality was noted at 2.5 mg/litre (0% at 8.5°C; 2% at 13°C).
    MacLeod & Pessah (1973) exposed rainbow trout  (Salmo gairdneri) to
    mercuric chloride concentrations between 0 and 2 mg mercury/litre and
    calculated 96-h LC50s of 0.4, 0.28, and 0.22 mg/litre at
    temperatures of 5, 10, and 20°C, respectively. At 10°C, the 24-h
    LC50 for mercuric chloride was approximately 30 times higher
    (in terms of mercury concentration) than for phenylmercuric acetate.
    Turnbull et al. (1954), using bluegill sunfish, calculated that the
    24-h and 48-h LC50s for pyridyl mercuric acetate were 12.5 and
    11.3 mg/litre, respectively. Rehwoldt et al. (1972) measured the acute
    toxicity of inorganic mercury to six species of fish (Table 5), and
    found that it was less when tests were conducted at 15°C than at 28°C.
    Amend et al. (1969) exposed  Salmo gairdneri to 125 µg ethylmercury
    phosphate/litre for 1 h, and found that increasing the temperature
    from 13°C to 15°C tended to increase the acute toxicity of the mercury
    solution. An increase in the water hardness from 23 to 120 mg
    CaCO3/litre also decreased the toxicity. But the dissolved oxygen
    content of the water had the most pronounced effect. At saturation, no
    deaths occurred, even at the highest water hardness, but at a
    dissolved oxygen level of < 6 mg/litre substantial losses occurred
    (72-76%) and even at the lowest temperature 37% of the trout died.

         Jones (1940) found that the mean survival time for the minnow
     Phoxinus phoxinus in mercuric chloride rose from 15 min for
    10-3mol/litre to 230 min at 5 × 10-6mol/litre. The addition of
    enough sodium chloride to convert the whole of the mercuric chloride
    into a double-chloride sodium mercuric chloride, and even the addition
    of ten times this amount, did not affect the toxicity of the solution.
    The addition of a considerable excess of sodium chloride caused a
    marked prolongation of the survival time, the maximum effect being
    attained when the solution was approximately isotonic.

    6.3.2  Reproductive effects and effects on early life stages

    Appraisal

          The data reveal an obvious difference between static and flow
     test concentrations, with LC50 values being up to 150 times lower
     under flow conditions. The increased LC50 in the static tests may
     be explained by a combination of adsorption of the compound to
     surfaces of the test vessels and to the gelatinous egg surface during
     embryo development. As a result, the larvae are exposed to much lower
     mercury concentrations at hatching time than are present at the
     beginning of the experiment. By contrast, the concentration is
     maintained throughout in a flow-through system.

        Table 5.  Toxicity of inorganic mercury to fishd

    Organism/            Stat/    Temperature    Alkalinityb   Hardnessb    pH       Dissolved     Parameter     Water           Reference
    weight (g)           flowa    (°C)                                               oxygen                      concentration
                                                                                     (mg/litre)                  (µg/litre)

    Tilapia
    (Tilapia
    mossambica)
       2.2-3.5           stat     28-30                                              > 4.8         48-h LC50     1000            Menezes &
                                                                                                                 (792-1261)      Qasim (1983)

       10-13             stat     28-32          7.7-11.7      32-38        7-7.3    5.4-6.2       24-h LC50     1256            Ramamurthi
                         stat     28-32          7.7-11.7      32-38        7-7.3    5.4-6.2       48-h LC50     1108            et al.
                         stat     28-32          7.7-11.7      32-38        7-7.3    5.4-6.2       72-h LC50     739             (1982)

    Catfish              stat     28                                                               96-h LC50     350             Das et al.
    (Heteropneustes                                                                                                              (1980)
    fossilis)  25

    Catfish              stat     28-32          7.7-11.7      32-38        7-7.3    5.4-6.2       24-h LC50     1700            Subbaiah
    (Sarotherodon        stat     28-32          7.7-11.7      32-38        7-7.3    5.4-6.2       48-h LC50     1500            et al.
    mossambicus)         stat     28-32          7.7-11.7      32-38        7-7.3    5.4-6.2       72-h LC50     1000            (1983)
      25                 stat     28                                                               96-h LC50     75              Das et al.
                                                                                                                                 (1980)

    Catfish                       24-27.5        165-190       245-285      7.1-7.7  5.5-8.2       24-h LC50     860             Khangarot
    (Channa marulius)                                                                                            (801-916)       (1981)
      3-4.5

                                  24-27.5        165-190       245-285      7.1-7.7  5.5-8.2       96-h LC50     314             Khangarot
                                                                                                                 (271-371)       (1981)

                                  24-27.5        165-190       245-285      7.1-7.7  5.5-8.2       240-h LC50    131             Khangarot
                                                                                                                 (103-158)       (1981)

    Table 5 (cont'd)

    Organism/            Stat/    Temperature    Alkalinityb   Hardnessb    pH       Dissolved     Parameter     Water           Reference
    weight (g)           flowa    (°C)                                               oxygen                      concentration
                                                                                     (mg/litre)                  (µg/litre)

    Rainbow trout
    (Salmo gairdneri)
      0.6-3.0            stat     9.3-10.7       70            101          8.55     > 8.0         24-h LC50     903             Wobeser
                                                                                                                 (783-1023)      (1975a)

      9.1-15.5           stat     5                            90           7.5-7.8                48-h LC50     650             MacLeod &
                         stat     5                            90           7.5-7.8                96-h LC50     400             Pessah
      13.2-21.3          stat     10                           90           7.5-7.8                48-h LC50     450             (1973)
                         stat     10                           90           7.5-7.8                96-h LC50     280             MacLeod &
      18.5-27.8          stat     20                           90           7.5-7.8                48-h LC50     300             Pessah
                         stat     20                           90           7.5-7.8                96-h LC50     220             (1973)
    length: 51-76mm      flow                    82-132                     6.4-8.3  4.8-9.0       96-h LC50     33d             Hale (1977)

    Banded killifish     stat     28                           55           8.0      6.9           24-h LC50     270             Rehwoldt
    (Fundulus            stat     28                           55           8.0      6.9           48-h LC50     160             et al.
    diaphanus)           stat     28                           55           8.0      6.9           96-h LC50     110             (1972)

    Striped bass         stat     28                           55           8.0      6.9           24-h LC50     220             Rehwoldt
    (Roccus              stat     28                           55           8.0      6.9           48-h LC50     140             et al.
    saxatilis)           stat     28                           55           8.0      6.9           96-h LC50     90              (1972)

    Pumpkinseed          stat     28                           55           8.0      6.9           24-h LC50     410             Rehwoldt
    (Lepomis             stat     28                           55           8.0      6.9           48-h LC50     390             et al.
    gibbosus)            stat     28                           55           8.0      6.9           96-h LC50     300             (1972)

    White perch          stat     28                           55           8.0      6.9           24-h LC50     420             Rehwoldt
    (Roccus americanus)  stat     28                           55           8.0      6.9           48-h LC50     340             et al.
                         stat     28                           55           8.0      6.9           96-h LC50     220             (1972)

    Table 5 (cont'd)

    Organism/            Stat/    Temperature    Alkalinityb   Hardnessb    pH       Dissolved     Parameter     Water           Reference
    weight (g)           flowa    (°C)                                               oxygen                      concentration
                                                                                     (mg/litre)                  (µg/litre)

    Carp                 stat     28                           55           8.0      6.9           24-h LC50     330             Rehwoldt
    (Cyprinus carpio)    stat     28                           55           8.0      6.9           48-h LC50     210             et al.
                         stat     28                           55           8.0      6.9           96-h LC50     180             (1972)

    American eel         stat     28                           55           8.0      6.9           24-h LC50     250             Rehwoldt
    (Anguilla            stat     28                           55           8.0      6.9           48-h LC50     190             et al.
    rostrata)            stat     28                           55           8.0      6.9           96-h LC50     140             (1972)

    Mummichog                     20             20c                        8.0                    96-h LC50     2000            Klaunig
    (Fundulus                                                                                                                    et al. (1975)
    heteroclitus)
      3.3-3.5
      2-6                stat     20             20c                        7.8      < 4           24-h LC50     23 000          Eisler &
                         stat     20             20c                        7.8      < 4           96-h LC50     800             Henneky
                         stat     20             20c                        7.8      < 4           168-h LC50    800             (1977)

    Flounder (adult)     stat     15                                                               48-h LC50     3300            Portmann &
    (Platichthys                                                                                                                 Wilson (1971)
    flesus)

    a  stat = static conditions (water unchanged for duration of test); flow = flow-through conditions (mercury concentration in water
       continuously maintained).
    b  alkalinity & hardness expressed as mg CaCO3/litre.
    c  These figures are values for salinity (expressed in %), not alkalinity.
    d  Mercuric chloride was used, except in the studies of Hale (1977), where the salt used was mercurous nitrate.

    Table 6.  Toxicity of organic mercury to fishc

    Organism/              Stat/   Temperature    Alkalinityb    Hardnessb     pH       Dissolved     Parameter    Water             Reference
    weight (g)             flowa   (°C)                                                 oxygen                     concentration
                                                                                        (mg/litre)                 (µg/litre)

    Blue gourami           stat    26-28                                       7.4      10            24-h LC50    123               Roales &
    (Trichogaster                                                                                                  (115.62-130.38)   Perlmutter
      trichopterus)        stat    26-28                                       7.4      10            48-h LC50    94.2              (1974)
      1.5-2                                                                                                        (85.5-102.9)      Roales &
                           stat    26-28                                       7.4      10            96-h LC50    89.5              Perlmutter
                                                                                                                   (85.38-93.62)     (1974)

    Rainbow trout (fry)    stat    9.3-10.7       70             101           8.55     > 8           24-h LC50    84 (81-87)        Wobeser
    (Salmo gairdneri)      stat    9.3-10.7       70             101           8.55     > 8           48-h LC50    45 (36-54)        (1975a)
                           stat    9.3-10.7       70             101           8.55     > 8           96-h LC50    24 (22-26)        Wobeser
    (fingerling)  0.6-3    stat    9.3-10.7       70             101           8.55     > 8           24-h LC50    125 (120-130)     (1975a)
                           stat    9.3-10.7       70             101           8.55     > 8           48-h LC50    66 (63-69)        Wobeser
                           stat    9.3-10.7       70             101           8.55     > 8           96-h LC50    42 (25-59)        (1975a)
    (juvenile)  22.9       flow    10                            90            7.5-7.8                24-h LC50    25c               MacLeod &
                                                                                                                                     Pessah
                                                                                                                                     (1973)
                           stat    18                            250                                  24-h LC50    5c                Alabaster
                           stat    18                            250                                  48-h LC50    4c                (1969)

    Table 6 (cont'd)

    Organism/              Stat/   Temperature    Alkalinityb    Hardnessb     pH       Dissolved     Parameter    Water             Reference
    weight (g)             flowa   (°C)                                                 oxygen                     concentration
                                                                                        (mg/litre)                 (µg/litre)

    Brook trout (juv.)     flow    11-13          41-44          45-46         6.9-7.6  7.7           96-h LC50    75                McKim et al.
    (Salvelinus                                                                                                                      (1976)
    fontinalis)
    Lamprey (larvae)       flow    12             150            146           8-8.5                  24-h LC50    > 166             Mallatt
    (Petromyzon marinus)   flow    12             150            146           8-8.5                  48-h LC50    88                et al.
      0.3-3                flow    12             150            146           8-8.5                  96-h LC50    48                (1986)

    a  stat = static conditions (water unchanged for duration of test); flow = flow-through conditions (mercury concentration in water
       continuously maintained).
    b  alkalinity & hardness expressed as mg CaCO3/litre.
    c  methylmercuric chloride was used, except in the studies of MacLeod & Pessah (1973) and Alabster (1969), where phenyl mercuric
       acetate was used.
    
          Selenium may increase the toxicity of mercury to fish eggs at
     higher concentrations of mercury. At low water concentrations
     selenium effects are additive.

         Table 7 summarizes the acute toxicity of mercury to embryolarval
    stages of fish.

         When Ram & Sathyanesan (1983) exposed adults of the freshwater
    teleost  Channa punctatus for 6 months to 0.01 mg mercuric
    chloride/litre, the mercury prevented oocytes development in the ovary
    and spermatogenesis in the testis. The number and activity of
    gonadotrophs in the pituitary were also reduced, giving the appearance
    of "resting phase" at a time when full reproductive development was
    expected. McIntyre (1973) exposed sperm from  Salmo gairdneri to
    concentrations of methylmercuric chloride between 1 µg/litre and
    10 mg/litre for 30 min. The sperm-containing solution was then added
    to eggs, and the percentage of fertilization was determined 17 days
    later. After exposure to 0.5 mg mercury/litre, there was an increase
    in the percentage of unfertilized eggs from 9.1% in controls to 12.5%
    in treated samples. This effect was enhanced with increasing mercury
    concentration, reaching 100% nonfertile eggs at 5 mg/litre or greater
    concentrations of mercury.

         Kihlstrom & Hulth (1972) transferred eggs laid by mature
    zebrafishes  (Brachydanio rerio) into solutions containing 10, 20, or
    50 µg phenylmercuric acetate (PMA)/kg. The frequency of hatching was
    significantly higher in the 10 µg/kg group than in the controls and
    the same as the controls in the 20 µg/kg group. None of the eggs
    transferred to the solution containing 50 µg/kg hatched. Most eggs
    hatched 3 days after fertilization, the frequency of eggs hatching up
    to and including the third day being significantly higher in water
    containing 10 or 20 µg PMA/kg when compared to the controls.

         Weis & Weis (1977) exposed early embryos of the killifish
     Fundulus heteroclitus to mercuric chloride at concentrations of
    0.01, 0.03, 0.1, or 1.0 mg mercury/litre. Mercury was added to the
    water at the start of the experiment and the solution was not
    replaced. The authors cite Jackim et al. (1970) to indicate that the
    loss of mercury from the solution would have amounted to about 26%
    over the course of the 96-h test period. Embryos treated at stage 12
    of development (the early blastula stage) showed a reduction in axis
    formation in solutions of 0.01 and 0.03 mg mercury/litre and a severe
    reduction at 0.1 mg/litre. There were no forebrain defects at 0.01 mg,
    but 20% of embryos showed defects after exposure to 0.03 and
    0.1 mg/litre. All the embryos exposed to 1 mg mercury/litre died
    before gastrulation. Embryos treated at stage 14, the late blastula
    stage of development, with concentrations of mercury of
    0.01-0.1 mg/litre), showed no reduction in axis formation. Negligible
    defects were noted at 0.01 and 0.03, but 20% of embryos were affected
    at 0.1 mg mercury/litre.

    Table 7.  Toxicity of inorganic mercury to the embryo-larval
              stages of fish

    Organism                  Stat/         LC50        95% confidence
                              flow       (µg/litre)         limits

    Rainbow trout             stata          4.7        4.2-5.3
    (Salmo gairdneri)         flowb        < 0.1

    Channel catfish           stata         30.0        26.9-33.2
    (Ictalurus punctatus)     flowb          0.3        0.2-0.4

    Bluegill sunfish          stata         88.7        73.5-106.3
    (Lepomis macrochirus)

    Goldfish                  stata        121.9        112.3-132.1
    (Carassius auratus)       flowb          0.7        0.6-0.8

    Redear sunfish            stata        137.2        115.0-162.8
    (Lepomis microlophus)

    Largemouth bass           stata        140.0        128.7-151.9
    (Micropterus salmoides)   flowb          5.3        5.0-5.6

    a  static conditions but water renewed every 12 h.
    b  flow-through conditions (mercury concentration in water
       continuously maintained).

       Exposure was initiated 30 min to 2 h after spawning and continued
       through to 4 days post-hatching. Hatching times were 24 days for
       rainbow trout, 6 days for channel catfish, and 3 to 4 days for
       the other fish. Therefore, total exposure was as follows: rainbow
       trout 28 days, channel catfish 10 days, and the other fish 7 to 8
       days. (Birge et al. 1979).

         When Sharp & Neff (1980) exposed embryos (4-8 cell stage) of
     Fundulus heteroclitus to mercuric chloride at concentrations of
    0-100 µg mercury/litre for 1 to 32 days, survival was reduced at all
    concentrations above 40 µg/litre. The hatching success of embryos
    exposed for 32 days was significantly reduced at concentrations above
    10 µg/litre. Reducing the duration of exposure from 5 days to 1 day
    significantly increased the total hatchability of the eleutheroembryos
    emerging after exposure for 32 days. Increases in the incidence of
    spinal curvature were also noted at concentrations exceeding
    20 µg/litre, which were significantly reduced if the exposure was

    reduced to 5 days or less. The 24-h LC50 for the embryos was
    89.6 µg/litre, the 24-h EC50 for spinal curvature was
    61.45 µg/litre, and the 24-h EC50 for hatching success was
    71.6 µg/litre.

         McKim et al. (1976) exposed three generations of brook trout
     (Salvelinus fontinalis) to methylmercuric chloride concentrations of
    0.03 to 2.93 µg/litre, over a 144-week period. At the highest dose,
    deformities were observed during the first 39 weeks and 88% of the
    first generation adults died. At 0.93 µg/litre, the second generation
    fish showed deformities and all but one female died during a 108-week
    exposure. No significant effects on survival, growth, or reproduction
    were observed in second generation trout at concentrations lower than
    0.93 µg/litre, and no toxic symptoms were found in the third
    generation below 0.29 µg/litre. The authors established that the
    maximum acceptable toxicant concentration (MATC) for brook trout
    exposed to methylmercuric chloride (hardness = 45 mg CaCO3/litre;
    pH = 7.5) was between 0.29 and 0.93 µg/litre.

         Weis & Weis (1984) measured the tolerance of eggs of the
    killifish  Fundulus heteroclitus to methylmercury in four successive
    years of sampling in the same pond. There was considerable variation
    in susceptibility between eggs from different females at the beginning
    of the sampling period, some females producing resistant and some
    susceptible eggs. After a period of heavy rainfall in the third year,
    when heavy metals and pesticides were washed into the ponds, the
    proportion of resistant eggs in the population increased. The authors
    noted an initial correlation between production of resistant eggs and
    numbers of fin rays in females. The same correlation indicated an
    increase in these females in the population after exposure to metals.
    Selection, rather than physiological adoption, had taken place.

         Birge et al. (1979) investigated the effects of combinations of
    mercury and selenium on the hatchability of eggs of the rainbow trout,
    catfish, goldfish, and bass. Mercury and selenium were added to the
    test medium in a 1:1 ratio over a wide range of concentrations (from
    1 to 2500 µg/litre). From separate tests with mercury and selenium
    alone, the author calculated additive values for the two materials and
    compared the results with those observed with the mixture. For each
    species, results were dependant on actual concentration. At lower
    concentrations the interaction between mercury and selenium was
    additive or antagonistic, whereas at higher concentrations interaction
    was synergistic, with the mixture leading to much greater inhibition
    of hatching than predicted. Calculated additive LC50s for mercury
    and selenium were 0.09 mg/litre for trout, 0.1 mg/litre for catfish,
    0.67 mg/litre for goldfish, and 0.35 mg/litre for bass. Actual LC50s
    for the mixture of 1:1 mercury:selenium were 0.01 mg/litre for trout,
    0.01 mg/litre for catfish, 0.16 mg/litre for goldfish, and
    0.35 mg/litre for bass, in all cases substantially greater toxicities
    than predicted. For the two most sensitive species, trout and catfish,
    synergism became evident at water concentrations of 5 µg/litre and

    increased in parallel with increasing concentration. At water
    concentrations of 75 µg/litre, the predicted hatchability of eggs
    (assuming mercury and selenium effects to be additive) was 44% for
    trout and 57% for catfish. Actual observed hatchability at this
    concentration was 0% for trout and 2% for catfish.

    6.3.3  Behavioural effects

         Weir & Hine (1970) pretrained goldfish  (Carassius auratus) to
    avoid electric shock with a light stimulus and then exposed them to
    solutions of mercuric chloride. The lowest concentration of mercuric
    chloride found to significantly impair the behavioural response was
    3 µg/litre. The lowest concentration causing deaths, under the same
    conditions, was 360 µg/litre. Hartman (1978) fed rainbow trout
     (Salmo gairdneri) for a year on a diet containing ethylmercury
    ( p-toluene sulfonanilide) "Ceresan" at 0.5-25 mg/kg diet each day,
    or 2.5 or 10 mg/kg delivered every fifth day of feeding. Fish
    receiving 10 mg/kg every 5 days or 5 mg/kg or more per day were
    unable, with few exceptions, to learn to avoid a shock preceded by a
    signal of light. However, there was no evidence of the impairment of
    general behaviour.

         When Sharma (1984) exposed  Channa punctatus to mercuric
    chloride (at concentrations of 0.034, 0.068, 0.102, or 0.136 mg/litre
    for 1, 7, 15, 30, or 45 days), hyperactive avoidance reaction was seen
    after exposure to the two highest doses within 24 h. Similar reactions
    occurred with the lower doses after 5 days (0.034 mg/litre) and 2 days
    (0.068 mg/litre). Acute distress symptoms were noted at the lowest two
    exposure levels during the last 5 days of the experiment. Feeding was
    normal up to 20 days of exposure at 0.034 mg mercury/litre, 12 days at
    0.068 mg/litre, 6 days at 0.102 mg/litre, and only 3 days at
    0.136 mg/litre. There were deaths in all treated groups within
    45 days, ranging from 16% at 0.034 mg/litre to 100% at 0.102 mg
    mercury/litre or more. Growth was inhibited by all treatments in
    proportion to the mercury dose. Blood glucose level showed an early
    elevation followed by a significant reduction, the timing of the
    effect varying according to dose. There was also a progressively
    significant depletion in liver and muscle glycogen which was similarly
    dose dependant.

    6.3.4  Physiological and biochemical effects

         Panigrahi & Misra (1978) found that concentrations of mercuric
    nitrate of 5 mg/litre or more killed all test fish  Anabas scandens
    within 24 h. At 3 mg/litre, the fish survived but showed pathological
    and biochemical disorders. The major clinical disorders (lack of
    movement and reduced food consumption) showed themselves within 5 days
    of exposure. After 3 weeks, 29% of the fish were blind and their
    respiratory rate was greatly reduced; 71% were blind within 4 weeks.
    When the fish were transferred to clean water, partial recovery to

    normal respiratory rate occurred. Considerable reductions in blood
    haemoglobin content, erythrocyte count, body weight, and body protein
    content were recorded.

         Lindahl & Hell (1970) exposed the roach  Leuciscus rutilus to
    phenylmercuric hydroxide at 1 mg/litre for 40 min, then killed the
    fish and measured the respiration rate of isolated gill filaments.
    Filament respiration was reduced by about 30%, the cause being damage
    to secondary lamellae, and the oxygen content of blood was reduced by
    82%. An  in vitro study with erythrocytes showed that half the cells
    haemolyzed after exposure for 55 min to 0.5 × 10-4mol phenylmercuric
    hydroxide/litre.

         Hara et al. (1976) studied the effect of mercuric chloride on the
    olfactory response of the rainbow trout  Salmo gairdneri. Mercury
    depressed the response, the lowest concentration to cause an
    appreciable effect within 2 h being 100 µg/litre. The depression
    increased with increases in mercury concentration and exposure time.

         Hilmy et al. (1982) exposed the cyprinodont  Aphanius dispar to
    acute concentrations of mercury of 1-12 mg/litre for 96 h or chronic
    concentrations of 1 mg/litre for up to 30 days. The acute treatment
    caused significant increases in plasma sodium, calcium, and potassium
    levels, which reached maxima of 3, 5, and 12 mg/litre, respectively.
    At the chronic exposure, the levels of sodium, calcium, and potassium
    initially rose, then fell to near normal levels by the end of the
    30-day experiment.

         Das et al. (1980) studied the acute and subacute toxicity of
    mercuric chloride to the air-breathing fish  Heteropneustes fossilis
    and the non-air-breathing fish  Sarotherodon mossambica. The air-
    breathing fish was more resistant to mercury, giving a 96-h LC50
    value of 350 µg mercury/litre compared to 75 µg/litre for
     Sarotherodon. The effect on several enzymes of mercury at
    50 µg/litre was also studied. Gill lysosomal acid phosphatase and
    liver microsomal glucose-6-phosphatase were significantly stimulated
    in both species, whereas liver acid phosphatase and intestinal
    alkaline phosphatase were significantly stimulated in  Heteropneustes
    and significantly inhibited in  Sarotherodon. In both species serum
    glucose levels were significantly increased and liver glycogen levels
    decreased, while muscle glycogen levels were unaffected.

         Gill & Pant (1985) exposed  Barbus conchonius to concentrations
    of mercuric chloride of 36, 60, or 181 µg/litre, the highest dose
    corresponding to the 96-h LC50 for the species. Acute exposure to
    181 µg/litre for 24 or 48 h led to deformities in the erythrocytes:
    vacuolation, nuclear deterioration, microcytosis, and collapsed
    cytoplasmic membranes. There was also significant thrombocytosis and
    neutropenia. Chronic exposure to 36 or 60 µg mercury/litre led to

    poikilocytosis, hypochromia, fragmentation and nuclear displacement of
    erythrocytes, thrombocytosis, lymphocytosis, neutropenia, and mild
    basophilia.

         When Ramalingam & Ramalingam (1982) exposed the catfish
     Sarotherodon mossambicus to a concentration of mercuric chloride of
    0.09 mg mercury/litre, they found no effect on the liver or muscle
    total protein content over 24 h. There were, however, significant
    decreases after both 7 and 15 days.

         Verma et al. (1984) dosed the lungfish  Notopterus notopterus
    with mercuric chloride concentrations of 0.017-0.088 mg/litre for up
    to 60 days. Concentrations of 0.022 or more caused significant
    increases in serum glutamic oxaloacetic transaminase and serum
    glutamic pyruvic transaminase activities within 15 days. The lowest
    dose took at least 30 days to significantly increase the activity of
    the same enzymes.

         O'Connor & Fromm (1975) exposed rainbow trout  Salmo gairdneri
    to methylmercuric chloride, at 10 µg mercury/litre, in a flow-through
    system. The fish were killed and assayed at 4, 8, and 12 weeks. There
    was no significant difference in plasma electrolyte concentrations
    (Na+, K+, Cl-, Mg2+, and Ca2+) or between the  in vitro
    oxygen consumption of excised gill filaments from control and mercury-
    treated fish determined in 10% or 100% phosphate-buffered saline.

         In studies by Sastry et al. (1982), the freshwater murrel  Channa
     punctatus was exposed to mercury either directly once into the
    intestinal sac (0.001-10 mmol/litre) or in the water at 3 µg/litre for
    15 or 30 days. A significant decrease in the rate of intestinal
    absorption of glucose, fructose, glycine, and tryptophan occurred at
    the higher concentrations of 0.1, 1.0, and 10 mmol/litre. At 0.01 and
    0.001 mmol/litre there was a reduction in absorption but this was not
    significant except at 0.01 mmol/litre in the case of tryptophan. There
    was a significant decrease of the absorption rate of all four
    nutrients in the mercury solution, but only after a 30-day exposure.

         Dawson et al. (1977) exposed juvenile striped bass  Morone
     saxatilis to 1.0, 5.0, or 10 µg mercuric chloride/litre for between
    30 and 120 days. The fish were then allowed to recover for a further
    30 days in clean running sea water. Fish exposed to the lowest dose
    did not differ significantly from controls with regard to respiration
    rate. Exposure to 5 µg/litre for 30 days significantly lowered the
    respiration rate but the effect had disappeared after 60-days
    exposure. Fish exposed to 10 µg mercury/litre showed a decreased
    respiratory rate after 30 days, which was reversed until a significant
    increase in rate was observed after 120 days of exposure. Mercury
    exposure did not significantly affect liver activities of aspartate
    aminotransferase, glucose-6-phosphatase, malic dehydrogenase, or
    magnesium activation of aspartate aminotransferase.

         Christensen (1975) examined a range of biochemical parameters in
    brook trout  (Salvelinus fontinalis) embryos and alevins exposed to
    methylmercuric chloride at concentrations from 0.01 to 1.03 µg
    mercury/litre. The fish were exposed as eggs for 16-17 days and then
    for a further 21 days as alevins. There was a significant decrease in
    glutamic oxaloacetic transaminase activity in embryos after exposure
    to 1.03 µg/litre, and a significant increase in its activity in
    alevins at 0.93 µg/litre. The alevin effect was accompanied by a
    significant decrease in weight. Christensen et al. (1977) exposed
    brook trout to methylmercuric chloride concentrations of 0.01 or
    0.03 µg/litre and 2.93 µg/litre, for either 2 or 8 weeks. After
    8 weeks, they found no significant effects on body weight, body
    length, blood plasma glucose, chloride or sodium, or plasma lactic
    dehydrogenase, and glutamic oxaloacetic transaminase activities. There
    were, however, significant increases in haemoglobin and blood plasma
    sodium and chloride after 2 weeks, but no effect on the other
    parameters measured.

         Varanasi et al. (1975) noted structural alterations in the
    epidermal mucus of rainbow trout exposed to 1 mg of mercuric
    chloride/litre. Mercury accumulated in the mucus and altered the
    physical characteristics of the layer, which is important for
    locomotion and protection of the fish. Lock & Overbeeke (1981) studied
    the effects of methylmercuric chloride and mercuric chloride on mucus
    production in rainbow trout. Of three measurements made, density of
    mucus cells, mucus in the tissue, and release of mucus into water,
    only the latter was affected by mercury. The effect was less with
    organic than with inorganic mercury, where mucus production was
    increased significantly. Exposure to 10 µg inorganic mercury/litre for
    4 h increased mucus production, and greater exposure concentrations
    and times enhanced the effect. Opercular movements increased with
    increased mucus production, suggesting mucus-induced hypoxia. Lock et
    al. (1981) attributed the osmoregulatory effect of mercury on fish as
    an effect on the permeability of the gill to water, rather than as an
    effect on active ionic transport.

         Roales & Perlmutter (1977) found that methylmercury (9 µg/litre),
    or methylmercury and copper combined, resulted in a decrease in the
    immune response of blue gourami  (Trichogaster trichopterus) to both
    infectious pancreatic necrosis (IPN) virus and  Proteus vulgaris. The
    two toxicants jointly produced no greater or lesser effect than when
    each was added alone.

    6.4  Toxicity to Amphibia

          Mercury has a toxicity for amphibian tadpoles similar to that
     for fish. There is considerable species variability in susceptibility
     to the metal. Sublethal effects and developmental effects have been
     reported. There is no information on effects on adult amphibians.

         Acute toxicity of mercury to amphibian tadpoles is summarized in
    Table 8.

         Birge et al. (1979) conducted embryo-larval bioassays on 14
    species of amphibia. Exposure to inorganic mercury was maintained from
    fertilization to 4 days after hatching, using static renewal
    procedures (Table 9). Gastrophryne and five species of  Hyla were the
    most sensitive, with LC50 values ranging from 1.3 to 2.8 µg/litre,
    compared to an LC50 value of 4.7 µg/litre for rainbow trout (the
    exposure period was shorter than for the trout; 6.6 to 7.4 days
    compared to 28 days.

         Chang et al. (1974) dosed leopard frog  (Rana pipiens) tadpoles
    with methylmercuric chloride, either via the water at concentrations
    of 0-1.0 mg mercury/litre or via injections of 0.025 mg mercury/day
    for 10 days. There was 100% mortality after 48 h at a water
    concentration of 50 µg/litre or more. At 1-10 µg/litre there was total
    arrest of development and differentiation after 48 h, which continued
    for 3 to 4 months. Mercury-injected tadpoles showed extensive
    deposition of blood pigment in their livers. The authors suggest that
    this was due to haemolysis of red blood cells caused by mercury,
    followed by severe peripheral oedema and haemopoietic reactions in the
    kidneys of the tadpoles. Dial (1976) exposed  Rana pipiens embryos
    (at the cleavage, blastula, gastrula, and neural-plate stages of
    development) to concentrations of methylmercuric chloride of
    0.5-200 µg/litre. Concentrations of 40 µg/litre or more were lethal to
    embryos treated at the cleavage stage. Embryos at the blastula,
    gastrula, and neural-plate stages were treated for 5 days at
    concentrations of 5-30 µg/litre. Tadpoles treated with 5 µg/litre
    showed only minor effects, whereas 10, 15, or 20 µg/litre caused
    various effects, including exogastrulae, poor tail development, and
    poor general development. Death rates increased with exposure time and
    concentration. At 30 µg/litre many defects were observed after 24 h
    and all tadpoles had died within 3 days.

    6.5  Toxicity to Aquatic Mammals

         There appears to be only a single experimental study on the
    effects of methylmercury on aquatic mammals. Ronald et al. (1977) fed
    harp seals on herring dosed with methylmercuric chloride. Two animals
    were used as controls, two were fed 0.25 mg/kg body weight per day and
    two fed 25.0 mg/kg body weight per day. Various blood parameters were
    monitored and found to be unaffected by the lower dose. The two
    animals on the higher dose died after 20 and 26 days of dosing. Prior
    to death these animals exhibited toxic hepatitis, uremia, and renal
    failure.

        Table 8.  Toxicity of mercuric chloride to amphibians

    Organism              Lifestage   Stat/    Temperature   Alkalinitye   Hardnesse   pH         Parameter     Water            Reference
                                      flowa    (°C)                                                             concentration
                                                                                                                (µg/litre)

    Frogc                 tadpole     stat     13-16         24-40         13-80       6.2-6.7    24-h LC50     762 (677-837)    Khangarot
    (Rana hexadactyla)    tadpole     statb    13-16         24-40         13-80       6.2-6.7    48-h LC50     121 (93-151)     et al. (1985)
                          tadpole     statb    13-16         24-40         13-80       6.2-6.7    72-h LC50     68 (57-85)       Khangarot
                          tadpole     statb    13-16         24-40         13-80       6.2-6.7    96-h LC50     51 (33-53)       et al. (1985)

    Clawed toad           3- to       stat     19-21                                              48-h LC50     100              de Zwart &
    (Xenopus laevis)      4-week                                                                                                 Slooff (1987)
                          larva

    Toadd                 tadpole     stat     29-34         120-160       165-215     7.1-7.6    12-h LC50     69.8             Khangarot &
    (Bufo melanostictus)  tadpole     stat     29-34         120-160       165-215     7.1-7.6    24-h LC50     52.8             Ray (1987)
                                                                                                                (43.6-61.5)
                          tadpole     stat     29-34         120-160       165-215     7.1-7.6    48-h LC50     45.6             Khangarot &
                                                                                                                (40.9-56.7)
                          tadpole     stat     29-34         120-160       165-215     7.1-7.6    96-h LC50     43.6             Ray (1987)
                                                                                                                (36.8-58.5)

    a  stat = static conditions (water unchanged for duration of test).
    b  static conditions but test water renewed every 24 h.
    c  tadpole length 15-25 mm, weight 350-800 mg (wet weight).
    d  tadpole length 18-22 mm, weight 90-120 mg (wet weight).
    e  alkalinity & hardness expressed as mg CaCO3/litre.

    Table 9.  Toxicity of inorganic mercury to the embryo-larval stage
              of amphibians

    Organism                            LC50         95% confidence
                                     (µg/litre)         limits

    Narrow-mouthed toad                 1.3             0.9-1.9
    (Gastrophryne carolinensis)
    Southern grey tree frog             2.4             1.5-3.4
    (Hyla chrysoscelis)
    Squirrel tree frog                  2.4             1.5-3.8
    (Hyla squirrella)
    Barking tree frog                   2.5             1.7-3.4
    (Hyla gratiosa)
    Grey tree frog                      2.6             1.2-4.2
    (Hyla versicolor)
    Spring peeper                       2.8             1.9-3.9
    (Hyla crucifer)
    Leopard frog                        7.3             4.8-10.0
    (Rana pipiens)
    Cricket frog                       10.4             8.5-12.6
    (Acris crepitans blanchardi)
    Red-spotted toad                   36.8            18.3-51.1
    (Bufo punctatus)
    Green toad                         40.0            25.6-52.2
    (Bufo debilis debilis)
    River frog                         59.9            53.8-65.9
    (Rana heckscheri)
    Fowlers toad                       65.9            44.0-84.0
    (Bufo fowleri)
    Pig frog                           67.2            54.3-79.5
    (Rana grylis)
    Marbled salamander                107.5            72.5-153.5
    (Ambystoma opacum)

        Exposure was under static conditions (but water renewed every
        12 h), and was initiated 30 min to 2 h after spawning and
        continued to 4 days post-hatching. Hatching times varied from 2.6
        to 3.4 days, therefore total exposure was between 6.6 and 7.4
        days. (Birge et al., 1979).

    7.  TOXICITY TO TERRESTRIAL ORGANISMS

    7.1  Toxicity to Terrestrial Plants

    Appraisal

          The main problem with studies on the effects of mercury on
     terrestrial plants is their relevance to the natural situation.
     Mercury normally binds to soil particles, which may reduce its
     availability to plants. In most studies, mercury has been
     administered as a solution in hydroponic culture. Most of the
     experiments have been on crop plants; wild plants might behave
     differently.

         Oberlander & Roth (1978) measured the uptake and translocation of
    potassium and phosphate, into the roots and shoots of 7-day-old barley
    plants, from doubly labelled (42K, 32P) nutrient solutions
    containing mercuric chloride. Uptake and translocation was monitored
    over 5 h during exposure to mercury at 10-4mol/litre. Potassium and
    phosphate uptake was significantly reduced to 21% and 31%,
    respectively, of the control level. Potassium and phosphate
    translocation was also significantly reduced to 6% and 8%,
    respectively, of the control level.

         Barker (1972) exposed explants of cauliflower inflorescence stem,
    lettuce stem, secondary phloem of carrot root, and tubers of potato
    for 20 days to mercuric chloride at concentrations between 0.005 and
    50 mg mercury/litre of medium. There was a significant reduction in
    growth (measured as mean fresh weight) after exposure to 0.5 mg/litre
    or more, although carrot and potato showed significant increases in
    growth at low levels (0.005 mg/litre) of mercury.

         Mhatre & Chaphekar (1984) exposed young plants of three species
    (a cereal  Pennisetum typhoideum, a forage crop  Medicago sativa, and
    a vegetable  Abelmoschus esculentus) to solutions containing mercuric
    chloride at 1-1000 µg mercury/litre for 24 h. They then estimated the
    percentages of leaf area injured and number of leaves injured.
     Abelmoschus was found to be the least sensitive of the plants,
    showing no damage at 10 µg/litre, whereas the other two species showed
    injury at this concentration. All species showed increasing
    percentages of leaf area injury and number of leaves injured with
    increasing mercury exposure. At the highest dose, 1000 µg/litre, all
    leaves were injured in  Pennisetum and  Abelmoschus and 50% of the
    leaves of  Medicago.

    7.2  Toxicity to Terrestrial Animals

    7.2.1  Toxicity to terrestrial invertebrates

    Appraisal

          The experimental information available on the effects of mercury
     on terrestrial invertebrates is insufficient to make any proper
     appraisal.

         Marigomez et al. (1986) fed the terrestrial slug  Arion ater for
    27 days on a diet containing mercuric chloride at 0, 10, 25, 50, 100,
    300, or 1000 mg/kg. The number of slugs dying was low in all
    treatments (a maximum of three deaths out of 24 animals per treatment)
    and unrelated to the dose. The results indicated that exposure of
    slugs to mercury at levels likely to be found in the environment will
    not kill them. A significant reduction in food consumption was noted
    at mercury exposures > 10 mg/kg diet, the effect being dose-related.
    A significant dose-related reduction in growth rate also occurred.
    Only at the highest dose (1000 mg/kg diet) did mercury severely
    disrupt growth.

         Abbasi & Soni (1983) kept the earthworm  Octochaetus pattoni in
    cement tanks at a density of 120 animals/m3, the average density of
    the species in the wild, and mixed mercuric chloride, into the soil
    and animal dung mixture in the tanks to dose levels of 0, 0.5, 1.0,
    2.0, or 5.0 mg mercury/kg. The experiment ran for 60 days and
    estimates of mortality were used to give LC50 values. There was less
    than 50% mortality within 5 days. The LC50 was 2.39 mg/kg at 10 days
    and had fallen to 0.79 mg/kg over a 60-day exposure period. As the
    mortality of adult earthworms progressed throughout the experimental
    period, so the earthworms still alive reproduced more than the
    controls. The reason for this effect is unclear; that the animals were
    stressed by the metal is evidenced by the continuing deaths. Beyer et
    al. (1985) exposed the earthworm  Eisenia foetida to soil containing
    methylmercuric chloride at 0, 1, 5, 25, or 125 mg/kg. All worms dosed
    at 25 or 125 mg/kg died within 12 weeks. Survival at 12 weeks was 97%,
    92%, and 79%, respectively, for doses of 0, 1, and 5 mg/kg.
    Regeneration of amputated segments was normal after treatment with
    methylmercuric chloride at 1 mg/kg soil, but reduced or eliminated by
    5 mg/kg.

    7.2.2  Effects of mercury on birds

    Appraisal

          Interpretation of the results of laboratory experiments on birds
     should take into account that practically all studies have been
     carried out using gallinaceous birds, which are unrepresentative of
     bird species as a whole.

          Birds fed inorganic mercury show a reduction in food intake and
     consequently in growth. Many other sublethal effects have been
     reported. Organomercury compounds are more toxic to birds and cause
     reproductive impairment.

         Acute toxicity to birds is summarized in Table 10. The majority
    of tests have been carried out using organic mercury compounds, which
    are generally much more toxic than inorganic salts. The 5-day dietary
    toxicity of mercuric chloride was in excess of 3000 mg/kg diet for
    those species tested. The organic mercury fungicidal preparations were
    the most toxic, with 5-day dietary LC50s as low as 50 mg/kg diet.

    7.2.2.1  Inorganic and metallic mercury

         When Beliles et al. (1967) exposed male Carneaux pigeons to
    mercury vapour (0.1 mg/m3) for 6 h per day over 20 weeks, no
    behavioural, histological, or gross signs of mercury toxicity were
    noted. Armstrong et al. (1963) trained pigeons to respond to coloured
    lights to obtain food. The birds were then exposed to mercury vapour
    (17 mg/m3) for 2 h daily (5 days/week) for 30 weeks. Marked changes
    in behaviour were observed, as measured by a decrease in the averaged
    response rate. A return to normal response was found when exposure to
    mercury ceased.

         Ridgway & Karnofsky (1952) injected chicken eggs, after 4 and 8
    days of development, with mercuric chloride solutions into the yolk
    sac and, after 8 days of development, into the chorio-allantoic
    membrane, and estimated LD50s. These were 0.3, at day 4, and 3.1, at
    day 8, expressed as molar equivalents of mercury, for the yolk sac
    route, and 0.21 Meq, at day 8, for the chorio-allantoic route. The
    result on day 4 is equivalent to a dose of 0.08 mg mercuric
    chloride/egg. Birge & Roberts (1976) injected chicken eggs (into the
    yolk sac), immediately prior to incubation, with mercuric chloride and
    obtained an EC50 for hatchability of 1.0 mg/litre yolk.

         Grissom & Thaxton (1985) exposed 4-week-old male chickens to
    mercuric chloride (0 or 500 mg mercury/litre) in their drinking water
    for up to 15 days. Rates of growth, together with feed and water
    consumption, decreased significantly within 3 days of the beginning of
    mercury treatment and remained depressed throughout the study.
    Mortality was greater in the mercury-treated group. Red blood cell
    numbers, haematocrit, mean corpuscular volume, and haemoglobin level
    increased within 3 days of the start of treatment. Mean corpuscular
    haemoglobin concentration (as pg/cell) was unchanged, but mean
    corpuscular haemoglobin (as % of cell) decreased.

        Table 10.  Toxicity of mercury to birds

    Species                  Age            Compounda                       Parameterb        Concentration       Reference
                                                                                              (mg/kg)

    Japanese quail           14 days        methyl mercuric chloride        acute LD50c       18 (14-24)          Hill & Soares (1984)
    (Coturnix coturnix       14 days        mercuric chloride               acute LD50c       42 (33-54)          Hill & Soares (1984)
    japonica)                2 months       ceresan M                       acute LD50c       668 (530-842)       Hudson et al. (1984)
                             4 months       ceresan L                       acute LD50c       1498 (1190-1888)    Hudson et al. (1984)
                             14 days        methyl mercuric chloride        5-day LC50        47 (36-60)          Hill & Soares (1984)
                             14 days        mercuric chloride               5-day LC50        5086 (3743-6912)    Hill & Soares (1984)
                             14 days        methoxyethylmercury chloride    5-day LC50        approx. 1750        Hill et al. (1975)
                             14 days        phenyl mercuric acetate         5-day LC50        614 (496-761)       Hill & Camardese (1986)
                             14 days        morsodren                       5-day LC50        45 (40-52)          Hill & Camardese (1986)
                             14 days        ceresan M                       5-day LC50        147 (120-180)       Hill & Camardese (1986)

    Pheasant                 12 months      ceresan M                       acute LD50c       360                 Hudson et al. (1984)
    (Phasianus               3-4 months     ceresan L                       acute LD50c       1190                Hudson et al. (1984)
    colchicus)               3-4 months     phenyl mercuric acetate         acute LD50c       169 (101-283)       Hudson et al. (1984)
                             10 days        mercuric chloride               5-day LC50        3790 (2768-5541)    Hill et al. (1975)
                             10 days        methoxyethylmercury chloride    5-day LC50        1102 (957-1263)     Hill et al. (1975)
                             10 days        phenyl mercuric acetate         5-day LC50        approx. 2350        Hill et al. (1975)
                             10 days        morsodren                       5-day LC50        64 (55-73)          Hill et al. (1975)
                             10 days        ceresan M                       5-day LC50        146 (127-167)       Hill et al. (1975)

    Mallard duck             6-8 days       ceresan M                       acute LD50c       > 2262              Hudson et al. (1984)
    (Anas platyrhynchos)     3 months       ceresan M                       acute LD50c       > 2262              Hudson et al. (1984)
                             3 months       ceresan L                       acute LD50c       > 2000              Hudson et al. (1954)
                             3-4 months     phenyl mercuric acetate         acute LD50c       878 (169-4558)      Hudson et al. (1984)
                             10 days        mercuric chloride               5-day LC50        > 5000              Hill et al. (1975)
                             10 days        methoxyethylmercury chloride    5-day LC50        approx. 280         Hill et al. (1975)
                             10 days        phenyl mercuric acetate         5-day LC50        approx. 1175        Hill et al. (1975)
                             5 days         morsodren                       5-day LC50        51 (43-60)          Hill et al. (1975)

    Table 10 (cont'd)

    Species                  Age            Compounda                       Parameterb        Concentration       Reference
                                                                                              (mg/kg)

                             10 days        morsodren                       5-day LC50        60 (47-76)          Hill et al. (1975)
                             5 days         ceresan M                       5-day LC50        approx. 54          Hill et al. (1975)
                             10 days        ceresan M                       5-day LC50        approx. 50          Hill et al. (1975)

    Bobwhite quail           2-3 months     ceresan L                       acute LD50c       1060 (841-1330)     Hudson et al. (1984)
    (Colinus                 14 days        ceresan M                       5-day LC50        approx. 70          Hill et al. (1975)
    virginianus)

    Prairie chicken                         ceresan M                       acute LD50c       360 (233-566)       Hudson et al. (1984)
    (Tympanuchus cupido)

    Chukar partridge         4 months       ceresan M                       acute LD50c       841                 Hudson et al. (1984)
    (Alectoris chukar)

    Grey partridge           9-20 months    ceresan M                       acute LD50c       550 (385-786)       Hudson et al. (1984)
    (Perdix perdix)

    Rock dove                               ceresan M                       acute LD50c       714 (437-1164)      Hudson et al. (1984)
    (Columba livia)

    Fulvous whistling        3-6 months     ceresan L                       acute LD50c       1680                Hudson et al. (1984)
         duck
    (Dendrocygna bicolor)

    a  morsodren = cyano methylmercury guanidine (1.51% mercury);
       ceresan M = N(ethylmercury)-p-toluenesulfonalide (3.2% mercury);
       ceresan L = methylmercury 2,3-di-hydroxyl propyl mercaptide + methylmercury acetate (2.25% mercury).
    b  concentrations expressed as mg/kg food, unless stated otherwise.
    c  concentrations expressed as mg compound per kg body weight in a single oral dosage (i.e., birds were fed with a dosed diet for
       5 days followed by a 'clean' diet for 3 days).
    
         Grissom & Thaxton (1984) investigated the interaction of mercury
    treatment (as mercuric chloride in the drinking water) and water
    deprivation in chickens. Birds (3-weeks-old) were treated at a rate of
    500 mg/litre water over 15 days. One group had water  ad libitum,
    while a second group were given limited water by intubation. Water
    consumption increased as the birds grew during the experiment.
    Monitored water intake was 25, 55, 70, 50, and 80 ml/kg body weight at
    0-3, 3-6, 6-9, 9-12, and 12-15 days into the experiment for the
    mercury-treated birds. Birds on water by intubation were given 20, 35,
    60, 70, and 70 ml/kg water for the same periods of the experiment.
    Water limitation resulted in a significant inhibition of the growth
    rate of untreated birds within the first 3 days of the experiment and
    this inhibition continued throughout the experiment. Mercury did not
    cause a significant inhibition of growth until between 12 and 15 days
    after the beginning of treatment. The only significant interaction
    between the effects of mercury and water deprivation occurred at 15
    days. Food consumption was significantly reduced in water-deprived
    birds. Mercury caused a significant reduction in food intake during
    the 9-12 and 12-15 day periods. Dehydration increased mortality of the
    groups to 10% compared with 3.75% for controls on water  ad libitum.
    Mercury results in birds refusing to take water or food contaminated
    with the metal. Therefore, the effects of mercury can be direct or
    indirect. Direct mercury effects appear to need more than 2 weeks of
    exposure to develop. Examination of the birds during a 14-day recovery
    period on clean water showed incomplete restoration of normal patterns
    of food and water consumption over this time.

         Brake et al. (1977) treated juvenile chickens with mercuric
    chloride in the drinking water (300 mg/litre) or by injection
    (5 consecutive days at 3 or 12 mg/kg body weight). Growth was retarded
    by the chronic treatment in drinking water and by the higher of the
    two injection rates. Relative heart weights (the ratio of heart weight
    to body weight) were increased by mercury in drinking water, decreased
    by the higher injected dose, and unchanged by the lower injected dose.
    Similar results were reported for relative aorta weights.
    Electrocardiograms showed a consistent decrease in the amplitude of
    R-S and T waves, with the greatest effect in the injected birds (both
    doses). Histological examination of the hearts of treated birds showed
    myocardial histopathological changes described as a myocarditis with
    polymorphonuclear and lymphocytic infiltration and fatty degeneration.
    The authors concluded that mercury causes cardiovascular disturbance
    in chickens even when administered at doses which do not inhibit
    growth.

         Hill & Shafner (1975) fed Japanese quail from hatching to one
    year of age on a diet containing mercuric chloride (0, 2, 4, 8, 16, or
    32 mg mercury/kg). Food consumption, growth rate, weight maintenance,
    hatchability, and eggshell thickness were unaffected. As dietary
    mercuric chloride increased, so initial oviposition occurred at a
    younger age. The average rate of egg production was also positively

    related to the concentration of mercuric chloride. The rate of egg
    fertilization, however, was generally depressed for all mercury
    treatments above 4 mg/kg.

         Kosba et al. (1982) dosed 8-month-old hens with mercuric chloride
    in drinking water at 0, 150 or 250 mg mercury/litre. Dosing at
    250 mg/litre caused a slight, but insignificant, decrease in body
    weight and egg numbers. Birds given the maximum dose consumed less
    food than controls, but birds on 150 mg/litre consumed more food than
    controls. All treated birds laid significantly smaller eggs than
    controls. Fertility and hatchability were adversely affected by
    mercury, and chicks hatched from eggs laid by treated birds were
    lighter.

         Hill & Spares (1984) studied the sublethal effects of feeding
    9-week-old Japanese quail with mercuric chloride in the diet. They
    calculated EC50s (a reduction to 50% of the activity of controls)
    for the activities of aspartate aminotransferase,
    alpha-hydroxybutyrate dehydrogenase, lactate dehydrogenase, and
    ornithine carbamoyl-transferase, in blood plasma, of 8.6, 11.2, 3.0,
    and 62.8 mg/kg diet, respectively.

         Dieter (1974) fed male Japanese quail for 12 weeks on a diet
    containing mercuric chloride at concentrations of 2, 4, and 8 mg/kg.
    The dosed diets did not significantly effect the carcass or liver
    weights or the blood haematocrit, and, although there was a
    significant decrease in haemoglobin at the 4 mg/kg treatment, this was
    not reflected in the other treatment groups. The treatments had no
    significant effect on the activity of the plasma enzymes creatine
    kinase, asparate aminotransferase, or fructose-diphosphate aldolase,
    but cholinesterase and lactate dehydrogenase activities were altered.
    The maximum decrease in cholinesterase activity amounted to 25% below
    that in controls, and showed almost a linear relationship with the
    logarithm of the dose. Irrespective of the mercuric chloride dose,
    lactate dehydrogenase activity increased 3-fold above control values.

         In studies by Scott (1977), Japanese quail were fed diets
    containing mercuric sulfate (0, 100, or 200 mg mercury/kg). With the
    highest dose, there was a significant reduction in the hatchability of
    fertile eggs and the strength of the eggshells. There were no
    significant effects on daily food intake, egg production, average egg
    weight, or percentage of fertile eggs.

         Nicholson & Osborn (1984) found kidney lesions in juvenile
    starlings  (Sturnus vulgaris) fed on a commercial diet contaminated
    by mercury. Analysis of the food showed mercury levels at 1.1 mg/kg.
    No signs of overt toxicity were seen in the birds. Damage to the
    kidney was mainly confined to the proximal tubules, and was similar to
    that found in mercury-contaminated sea birds in the field.

         Bridger & Thaxton (1983) demonstrated the effects of mercuric
    chloride on the humoral immune response of chickens. Three treatments
    were employed: chronic treatment with mercuric chloride at
    300 mg/litre of drinking water; acute low dose with five consecutive
    daily injections of 3 mg mercury/kg body weight; and acute high dose
    with five daily injections of 12 mg/kg. The drinking-water treatment
    was inhibitory to growth, while the acute treatments were not.
    Chronically treated birds also showed suppressed primary and secondary
    responses to a challenge with sheep red blood cells. Immunoglobulin M
    levels were reduced to a greater extent than immunoglobulin G in
    chronically treated birds. The primary response to  Brucellus abortus
    was also suppressed in chronically treated birds, but the secondary
    response was enhanced, with a greater titre of circulating antibodies.
    Bridger & Thaxton (1982) exposed chicks to either mercuric chloride in
    drinking water (300 mg/litre) or five consecutive daily injections of
    mercuric chloride into pectoral muscle (3 or 12 mg mercury/kg body
    weight). The authors found that these treatments did not significantly
    affect cell-mediated immune responses, in contrast to the effects on
    humoral immune responses.

    7.2.2.2  Effect of organic mercury on birds

         When Birge & Roberts (1976) injected chicken eggs, immediately
    prior to incubation, with methylmercuric chloride, into the yolk sac
    the EC50 for hatchability was 0.1-0.5 mg/litre yolk.

         Haegele et al. (1974) dosed female mallard ducks with 200 mg/kg
    diet of Ceresan M (3.1% ethylmercury) and measured eggshell thickness
    on days 76 and 85 of treatment. No significant effects were found.
    When mercury was added to the diet along with DDE at 40 mg/kg, mercury
    did not increase the effect of the organochlorine on shell thickness.

         Mullins et al. (1977) dosed captive hen pheasants with
    phenylmercuric acetate (PMA) either in capsules (20 mg/kg body weight)
    or added to the diet (at the normal fungicidal treatment rate of
    14.18 g/bushel of seed wheat). Birds given mercury by capsule showed
    significant decreases in egg hatchability, eggshell thickness, and
    chick weight and survival, but no effect on egg production, egg
    volume, fertility, or chick behaviour. The mercury-dosed diet had no
    effect on any of these reproductive parameters.

         Hill & Soares (1984) studied the sublethal effects of feeding
    9-week-old Japanese quail with methylmercuric chloride in the diet,
    and calculated EC50s for the activity of asparate aminotransferase,
    alpha-hydroxybutyrate dehydrogenase, lactate dehydrogenase, and
    ornithine carbamoyltransferase, in blood plasma, of 4.8, 6.1, 1.2, and
    3.5 mg/kg diet, respectively.

         In studies by Scott (1977), Japanese quail were fed diets
    containing methylmercuric chloride (0, 10, or 20 mg mercury/kg). The
    daily food intake, egg production, average egg weight, percentage of

    fertile eggs, and the hatchability of fertile eggs were all
    significantly reduced at 10 mg/kg. There were greater effects on all
    these parameters with 20 mg/kg, but the difference was not significant
    relative to the lower dose in terms of percentage fertility or
    hatchability of fertile eggs. The strength of the eggshell was
    significantly reduced by the 10 mg/kg dose after 3 weeks of dosing.
    Insufficient eggs were laid by the group dosed at the higher rate to
    monitor this factor.

         Tejning (1967) studied the effects on domestic fowl of
    methylmercuric-dicyandiamide (MMD)-treated grain (0-18.4 mg mercury/kg
    diet). Food consumption was unaffected in birds treated with 0 or
    4.4 mg mercury/kg, but fell gradually over 50 days, in birds treated
    with 9.2 or 18.4 mg/kg. Food consumption returned to normal after
    about 60-65 days, but then fell below control levels again later. Egg
    production (eggs/hen per day) was unaffected by 4.4 mg mercury/kg or
    by 8.8 or 9.2 mg/kg for the first 40 days of exposure. After treatment
    at 17.6 or 18.4 mg/kg diet, egg production gradually fell over the
    period of exposure. There was no effect on body weight of any of the
    treated birds. Some birds on the highest doses showed ataxia with
    difficulty in walking. In a study comparing three treatment levels of
    MMD (0, 9.2, and 18.4 mg mercury/kg diet, various reproductive
    parameters were monitored. There was an increase, relative to
    controls, in the number of soft-shelled eggs of 17.1% at the highest
    dose and 1.4% in the 9.2 mg/kg group. Percentages of deaths of embryos
    in shell during the first 5 days of incubation were also increased
    (values were 10.5% in controls, 43.7% in the birds dosed with
    9.2 mg/kg, and 62.1% in the 18.4 mg/kg group). Mortality later in the
    incubation period was similar in all groups. Overall hatchability was
    reduced from 60% in the controls to 16% in the 9.2 mg/kg group and 10%
    in the 18.4 mg/kg group.

         Fimreite (1970) exposed leghorn cockerels to a diet dosed with
    Panogen 15 (2.5% MMD) at concentrations of 6, 12, and 18 mg MMD/kg for
    3 weeks, from 2 weeks of age. The total intake of mercury, based on
    monitoring food consumption, was calculated to be 1.7, 3.4, and
    5.1 mg/chick, respectively, for the three dosing levels. All treated
    birds showed significant reductions in weight, but only at the highest
    dose was there a significant increase in deaths. Fimreite (1971) fed
    penned pheasant  (Phasianus colchicus) breeder ration and treated
    grain containing MMD at 2.25, 4.5, or 9 mg mercury/kg, for 2, 4, or 12
    weeks. There was no weight reduction amongst adults, and food
    consumption was only adversely affected by the highest dose. Some hens
    fed the highest dose showed extensive demyelination of the spinal
    cord. All treated birds showed reduced hatchability and egg
    production, with a large number of shell-less eggs. There was a
    significant reduction in the weight of eggs laid by mercury-treated
    birds. The highest dose group laid eggs of an abnormal colour.

         Spann et al. (1986) fed 12-day-old bobwhite quail on diets
    containing methylmercuric chloride at 0, 5.4, or 20 mg/kg (equivalent
    to 0, 4.3, or 16 mg mercury/kg). Birds dosed at the lower rate showed
    low mortality, not significantly different from controls, whereas
    birds dosed at the higher rate showed high mortality after 6 weeks (at
    between 55% and 80% for three different vehicles: no solvent; corn
    oil; and propylene glycol). When acetone was used as carrier, deaths
    were significantly reduced (to about 30%), deaths in the control group
    being < 10%.

         When Mykkanen & Ganther (1974) fed 1-day-old Japanese quail a
    diet containing 0-30 mg mercury/kg (as methylmercury hydroxide) for up
    to 32 days, no effect on erythrocyte glutathione reductase activity
    was found.

         Fimreite & Karstad (1971) dosed chicks with MMD and then fed them
    to red-tailed hawks for up to 12 weeks. Mercury levels in the liver of
    the chicks were between 3.9 and 10 mg/kg. Three of the six birds,
    given chicks with mercury in the liver at 10 mg/kg and died, one bird
    out of six, given chicks with mercury in the liver at 7.2 mg/kg, died.
    All the poisoned birds showed neurological symptoms, weakness in
    extremities, and impaired coordination of muscular movement, and,
    although the hawks did not lose their appetite, they had difficulty
    feeding. There was no effect on food consumption, even poisoned birds
    maintaining appetites until in an advanced stage of poisoning. Only
    birds with overt signs of poisoning showed substantial body weight
    loss. Borg et al. (1970) fed chickens with 8 mg MMD/kg diet for 5 to 6
    weeks, and muscle and liver from the contaminated chickens were fed to
    goshawks  (Accipiter gentilis gentilis). Three goshawks receiving
    muscle and liver averaging 13 mg mercury/kg died within 30, 38, and 47
    days. One goshawk receiving muscle only (10 mg mercury/kg) died within
    39 days. The major clinical symptoms, appearing after about two weeks,
    were inappetance, muscular weakness, ataxia, and loss of body weight.
    Autopsy revealed that the dominating effect was muscular atrophy,
    which was presumably the main cause of weight loss. Pronounced
    histological changes included demyelination and nerve cell
    degeneration of the cerebellum and medulla oblongata and demyelination
    of peripheral nerves. No lesions were found in the cerebrum.

         When Heinz (1974) fed mallard ducks a dry mash diet containing
    MMD (0.5 or 3.0 mg mercury/kg) for 21 weeks, the lower of the two dose
    levels had no effects on reproduction but the higher reduced egg
    laying and increased embryonic and duckling mortality. Eggs laid by
    controls tended to be heavier than eggs laid by treated birds, but
    there was no effect on eggshell thickness. Heinz (1976a) fed mallard
    for 2 consecutive years on the same doses of MMD as above. There was
    no significant effect on egg production or hatching success or on
    approach behaviour of ducklings. Ducklings from females fed 3.0 mg/kg
    were less likely to survive to 1 week than those from other groups.
    Ducklings from parents fed the highest dose were hyper-responsive in
    avoidance behaviour. Heinz (1976b) fed ducklings (from 9 days of age)

    whose parents had been fed MMD at 0.5 mg/kg diet, on the same dosed
    diet. Dosed second generation females laid a greater proportion of
    their eggs on open ground outside the nest boxes. They also produced
    fewer ducklings surviving to 1 week. In ducklings from second
    generation females, there were no significant differences in behaviour
    patterns such as approach response to maternal calls, avoidance
    response to frightening stimuli, and open-field behaviour. There was a
    reduction in growth of third generation ducklings. Heinz (1979) dosed
    three generations of mallard with 0.5 mg MMD/kg diet. As in the second
    generation, females laid a greater number of eggs outside the nest
    box. They also laid fewer eggs and produced fewer ducklings. There was
    some eggshell thinning in the third generation and a reduced response
    of ducklings to maternal calls.

         Prince (1981) tested mallard ducks through four generations in an
    attempt to establish if resistance to the reproductive effects of
    methylmercury was developed. The parental generation was exposed to
    two doses of 8 mg methylmercuric chloride within a 2-week period. The
    parents were split into two groups on the basis of the survival of
    ducklings after exposure to mercury. Three further generations of each
    line were produced. The percentage survival of ducklings exposed, via
    the parent, to mercury tended to increase in the "resistant" strain in
    successive generations. This suggested an ability of the birds to
    adapt to mercury exposure over time.

         Ganther et al. (1972) fed Japanese quail with diets containing up
    to 20 mg methylmercury/kg. Some groups of quail were given tuna fish
    as 17% of the total diet, while other groups had corn-soya instead of
    the tuna. Mortality in the group fed corn-soya with 20 mg
    methylmercury/kg diet was 61% over 6 weeks, the majority (52%) of
    deaths occurring between 4 and 6 weeks of dosing. The same amount of
    methylmercury added to the tuna diet led to only 14% mortality over
    the 6-week-period of dosing. The authors ascribe the protective effect
    of the fish diet to the high selenium level in the tuna. Selenium,
    which in these diets amounted to 0.3-0.6 mg/kg, becomes toxic to birds
    only at dietary concentrations more than 10 times higher than this.

    7.2.3  Effects of mercury on non-laboratory mammals

    Appraisal

          Few studies have been published on truly wild, non-laboratory
     mammals. Work is of most value when done not on mammal species that
     have been changed by generations in captivity but on those that are
     still found in the wild, or are genetically close to wild forms. The
     only work in this last category is on mink and prairie vole. The
     available evidence indicates that toxic effects, including
     reproductive changes, can be produced. Methylmercury has been found
     to be more toxic than inorganic mercury.

         Aulerich et al. (1974) dosed the diet of mink with either 5 mg
    methylmercury/kg (as contained in Ceresan L, which contains 2.25%
    mercury) or 10 mg mercuric chloride/kg. No adverse effects attributed
    to these diets were observed for 3 weeks. After 25 days, the mink
    dosed with organic mercury showed signs of lack of coordination, loss
    of balance, anorexia, and loss of weight. Within 4 days, ataxia,
    paralysis, tremors, and, finally, death were observed. Attempts to
    arrest these symptoms in the least affected mink by reverting to a
    control diet, with either EDTA or methionine injections, had no effect
    and the mink still died. Mink dosed with inorganic mercury showed no
    clinical signs. The mercuric chloride treatment did not affect
    reproductive performance and no teratological effect was noted. There
    was a significant reduction in the weight of the kits from treated
    parents, at birth, but this had recovered by 4 weeks of age.

         Wren et al. (1987a) fed adult mink a daily diet containing 1 mg
    methylmercury/kg for 3 months. Later, because of mortality, the dosed
    diet was administered every other day for a further 3 months. The
    initial, daily-dosed diet resulted in the death of 8 out of 12 females
    and 1 out of 4 males. There were no observed effects of the treatment
    on the thyroid, pituitary, or adrenal glands or on serum
    triiodothyronine (T3) or thyroxine (T4) levels during the experimental
    period. Mortality was thought to be caused by a combination of mercury
    poisoning and cold stress (the animals were kept outside during the
    winter). Under laboratory conditions, 1 mg/kg would not be considered
    fatal to mink (Wobeser et al., 1976). Under the same experimental
    conditions, Wren et al. (1987b) found that the fertility of adult male
    mink, percentage of females whelped, and number of kits born per
    female were not affected by the mercury treatment.

         Hartke et al. (1976) calculated an acute LD50 of 10 mg/kg body
    weight for phenylmercuric acetate (PMA) in female prairie voles
     (Microtus ochrogaster), after intraperitoneal injection. Female
    voles were also injected on days 8, 9, and 10 of gestation with
    0.06-5.0 mg PMA/kg body weight. Some normal foetuses and some
    resorption sites (where implantation had occurred but the foetal
    material had been reabsorbed) were found in voles injected with
    0.5 mg/kg or less on days 8 and 9 of gestation. Animals treated with
    > 1.0 mg/kg had no live foetuses, but all had resorption sites in
    the uterus. Similar results were found for voles treated on day 10.
    No resorption sites were found in voles treated with < 0.25 mg/kg.
    To study the effects of dose and the stage of gestation when dosing
    occurred, the authors further injected voles with 0.5 mg/kg on days
    7, 11, and 12 of gestation. Normal embryos and some resorption and
    abortion sites were found after dosing on days 7 and 11. Dosing on
    day 12 of gestation produced no resorption or abortion and the
    numbers of live foetuses accounted for all  corpora lutea in the
    ovary. 

    8.  EFFECTS OF MERCURY IN THE FIELD

    Appraisal

          Fatalities and severe poisonings in birds have been reported in
     association with outbreaks of human poisoning. In addition, the
     agricultural use of organomercury fungicides has caused poisoning in
     birds. A statistical association has been reported between the
     mercury content of birds' eggs and reproductive failure. These eggs
     also contained organochlorine residues, but these residues did not
     correlate with the observed reproductive effects.

         Methylmercury levels in fish in Japan have caused a major problem
    for human health. During these incidents, there were also reports of
    direct effects of mercury on wildlife in the area. Fish carrying
    methylmercury were found dead or showed symptoms of mercury poisoning.
    Fish-eating birds and scavenging birds were also killed (Harada,
    1978). Birds found dead in the area showed the characteristic
    pathological changes in the central nervous system of Minamata
    disease, but no measurement of mercury content was made (Takeuchi et
    al., 1957).

         The use of organic mercury compounds as a fungicidal seed
    dressing has led to deaths in the field of birds, mostly grain-eating
    species. Some raptors, feeding on the poisoned birds, were also
    casualties (Borg et al., 1969). Koeman et al. (1969) reported large
    numbers of birds of prey killed by indirect poisoning with
    organomercury fungicides in the Netherlands.

         Mercury contamination has been implicated in the breeding failure
    of some raptor species both in Europe and North America, where
    residues have equalled those found to cause reproductive impairment in
    laboratory species. These birds also contained organochlorine
    insecticide residues and the separation of effects is difficult
    (Newton, 1979). More recent work suggests more strongly that mercury
    affects the breeding of birds of prey in the field. Merlins sampled in
    Scotland contained organochlorines along with mercury in their eggs.
    Statistical analysis of the data showed a clear inverse relationship
    between mercury content of eggs and brood size; the higher the mercury
    content, the less likelihood of successful breeding. Productivity fell
    markedly when mercury residues in eggs exceeded 3 mg/kg. Productivity,
    that is the number of young successfully reared, showed no
    statistically significant relationship with residues of other
    chemicals present in the eggs. Levels of mercury were highest in birds
    sampled in Orkney and Shetland, but the relationship between mercury
    residue and productivity remained when these, particularly high,
    residue levels were excluded from the analysis (Newton & Haas, 1988).
    The merlins were feeding on wading birds in estuaries and this was
    presumed to be the source of the mercury. A similar, but not quite
    significant, relationship was found in peregrine falcons breeding near
    the coast.

         Jefferies et al. (1973) sampled small mammals from fields sown
    with mercury-treated grain. They express the view that residues were
    sufficiently high to have caused deaths in small mammals feeding on
    the grain. Some mammals were found dead and deemed to have been killed
    by mercury poisoning.

    9.  EVALUATION

         In evaluating the environmental hazard of mercury it is necessary
    to extrapolate from laboratory experiments to ecosystems. This must be
    done with extreme caution for the following reasons.

    (i)    Speciation of mercury and its adsorption to environmental
           components such as soil, sediment, organic matter, and biota
           limit its availability to organisms in the environment.

    (ii)   Environmental variables such as temperature, pH and chemical
           composition of water, soil type, and geology have been shown
           in limited studies on a narrow range of species to affect
           both uptake and effect of mercury. There is insufficient
           information to fully assess the probable affects of, for
           example, tropical conditions and acid precipitation.

    (iii)  There are few data measuring mercury availability to
           organisms. Most data represent nominal or total metal
           concentration, rather than that component which could be
           taken up by organisms. True exposure is, therefore,
           difficult to assess.

    (iv)   There are limited data on the behaviour of mixtures of
           metals from controlled experimental work; organisms in the
           environment are exposed to mixtures.

    (v)    Experimental work seldom, if ever, is conducted on species
           or communities that are either representative or key
           components of natural communities and ecosystems. Studies do
           not consider all of the interactions between populations and
           all of the environmental factors affecting these
           populations.

         It is probable that subtle disturbances to the community occur at
    much lower concentrations than those suggested in laboratory studies
    on acute effect, perhaps as much as one order of magnitude lower.

    9.1  The Marine Environment

         Marine aquatic organisms at all levels accumulate mercury into
    tissues. This mercury is retained for long periods if it is in an
    organic form. A number of factors affect the susceptibility of aquatic
    organisms to mercury. These include the life-cycle stage (the larval
    stage being particularly sensitive), the development of tolerance,
    water temperature, and salinity. Some incidents of severe pollution
    have resulted in the death of fish at that time. Few follow-up studies
    have been reported so that it is impossible to assess the long-term
    hazards. Toxic effects have been produced experimentally only at
    concentrations much higher than those found in the non-polluted marine
    environment. Furthermore, most of the studies have been on acute

    lethality and have used inorganic mercury compounds in the main.
    Birds, particularly coastal species or those eating prey that feed in
    estuaries, have been affected by mercury contamination. It has
    adversely affected breeding and may have influenced population
    stability.

    9.2  The Freshwater Environment

         Mercury compounds are acutely toxic to freshwater microorganisms.
    Using photosynthesis and/or growth as parameters, the NOTEL
    (No-observed-toxic-effect-level) for inorganic mercury lies between 1
    and 50 µg/litre, depending on the organism, density of cells in
    culture, and experimental conditions. Diversity of species in mixed
    culture may be affected by 40 µg mercuric chloride/litre. For
    organomercury compounds, the NOTEL is 10-100 times lower.

         Aquatic plants sustain damage after exposure to inorganic mercury
    at concentrations of 800 to 1200 µg/litre. Organomercury produces
    toxic effects at concentrations 10-100 times lower.

         Many aquatic invertebrates are sensitive to mercury toxicity,
    particularly as larvae. Organic mercury compounds are toxic at
    concentrations 10 to 100 times less than inorganic mercury. For the
    most sensitive species,  Daphnia magna, the NOTEL for reproductive
    impairment is 3 µg/litre for inorganic mercury and < 0.04 µg/litre
    for methylmercury.

         Freshwater fish show lethal responses to mercury in acute nominal
    concentrations from approximately 30 µg/litre. Larvae under the same
    static conditions are 10 times more sensitive. In flow-through tests,
    fish are up to 100 times more sensitive. In both static and flow-
    through tests, organomercury compounds are approximately 10 times more
    toxic than inorganic compounds. The NOTEL for the most sensitive
    parameters may be well below 0.01 µg/litre.

         Aquatic developmental stages of amphibia show sensitivity to
    mercuric compounds similar to that of fish.

    9.3  The Terrestrial Environment

         Based on the current state of knowledge, it is not possible to
    determine the true exposure or concentration of mercury available to
    terrestrial organisms. It can, however, be stated that exposure via
    soil, soil water, and food is most important; exposure via open water
    and air is less important.

         Mercury has been shown, in laboratory studies, to be toxic to
    terrestrial organisms over a broad range of concentrations. However,
    most of these studies are at high exposure levels (birds) or
    environmentally unrealistic exposure routes (hydroponic culture of
    plants).

         It can be stated that acute effects would not be seen in
    terrestrial plants growing in natural soils, nor in terrestrial birds
    or mammals, other than by exposure to mercurials used as fungicidal
    seed-dressings. Other effects seen in birds derive from mercury in the
    marine environment.

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    See Also:
       Toxicological Abbreviations