Mercury in freshwater and marine water

​​​Toxicant default guideline values for protecting aquatic ecosystems

October 2000

Description of chemical

Mercury in the aquatic environment exists mainly as complexes of mercury (II) and as organomercurials (Hart 1982). Of particular concern to the aquatic environment is the fact that inorganic forms of mercury (of relatively low toxicity and availability to bioconcentrate) may be converted by bacteria in situ into organomercury complexes (particularly methylmercury), which are more toxic and tend to bioaccumulate. High concentrations of methylmercury can result from flooding of new impoundments, anthopogenic discharges and atmospheric deposition (Wiener & Spry 1996). Bioconcentration factors for methylmercury for fish are consequently very high, ranging from 10⁶ to 10⁸. The fraction of total mercury that exists as methylmercury in aquatic organisms increases progressively from primary producers to fish, which can contain up to 99% methylmercury (Wiener & Spry 1996). The highest concentrations of mercury are reported in aquatic and marine mammals such as otters, seals and whales, particularly in the livers of these animals, although the proportion of methylmercury is generally very low.

Summary of factors affecting mercury toxicity

• Mercury occurs in the environment as mercury (II) and organomercurial compounds. The latter are particularly toxic and can bioconcentrate with the potential for secondary poisoning. Inorganic mercury can be converted into organomercurials by bacteria in situ.
• Mercury toxicity is hardness-dependent but no hardness algorithm is currently available. The uptake rate in organisms increases with decreasing water hardness and pH.
• Mercury is strongly adsorbed by particles and is more often associated with sediments.
• Mercury has a strong affinity for chlorine and sulfur-containing ligands, particularly sulfide. The neutral HgCl₂ in seawater rapidly permeates biological membranes.
• Toxicity of inorganic mercury in marine environments usually increases with decreasing salinity.

The ultratrace concentrations of mercury species in natural waters are a major obstacle to determining mercury speciation. There are only a few analytical techniques with sufficient sensitivity to measure inorganic mercury and methylmercury species. These include chromatography coupled with atomic fluorescence spectrometry or cold vapour atomic absorption spectrometry [see review by Clevenger et al. (1997)]. Geochemical speciation modelling may be used to predict the concentrations of the various inorganic mercury species in solution (Florence & Batley 1980), however, this approach is incapable of predicting the proportion of mercury present as methylated mercury.

Bioassays are typically used to determine metal-organism interactions. These can be used in conjunction with the measured and/or predicted speciation of mercury to define bioavailable mercury species. The current analytical practical quantitation limit (PQL) for mercury is 0.02 µg/L in both fresh and marine water (NSW EPA 2000).

Factors that affect availability and toxicity of mercury

Sorption onto suspended matter or bottom sediments is the most important process controlling the concentration of mercury in natural waters (CCREM 1987). Only a small proportion of total mercury is found in the dissolved phase. Dissolved mercury concentrations rarely exceed 12 ng/L in freshwaters (Gill & Bruland 1990), and in estuarine waters typically range from 1-18 ng/L (Nelson 1981, Cossa & Noel 1987). Mercury concentrations in seawater range from 0.08-2.0 ng/L (Gill & Fitzgerald 1988, Cossa et al. 1992). Some background figures are given in Table 8.3.2 of the ANZECC & ARMCANZ (2000) guidelines.

Mercury (II) shows a strong affinity for chlorine and sulfur-containing ligands, particularly, sulfide. However, in waters containing natural DOM, the majority of mercury will be bound in organic complexes, particularly in freshwaters (Fergusson 1990).

The proportion of dissolved mercury present as methylmercury is of critical importance, as this is the most bioavailable and toxic form of mercury (Fitzgerald & Clarkson 1991). In most cases, methylmercury concentrations comprise 1 to 20% of total mercury. The typical background concentration of methylmercury in lake water is believed to be 0.05 ng/L (Bloom 1989, Bloom & Effler 1990). Much higher concentrations (0.5 to 2.0 ng/L) are found in waters systems polluted with mercury.

The uptake and toxicity of mercury in aquatic organisms is often attributed to the lipid-solubility of organic mercury. The accumulation of inorganic mercury is generally regarded as being of secondary importance. The assimilation of methylmercury by zooplankton feeding on the marine diatom Thallassiosira weissflogii was four times more efficient than that for inorganic mercury (Mason et al. 1996). Higher concentrations of methylmercury in organisms higher up the food chain, therefore, reflect the higher trophic transfer efficiency of methylmercury compared with inorganic mercury.

The uptake rate of mercury in biota in freshwater lakes increases with decreasing water hardness and pH (Jensen 1988). In temperate waters, the bioaccumulation of mercury is greatest in summer, when microbial methylation and fish metabolic rates are at their maximum. Increasing the selenium concentration in waters also reduces the bioaccumulation of methylmercury in fish (Paulsson & Lundbergh 1991). Mercury is more toxic at higher temperatures (CCREM 1987).

High phosphate concentrations reduced the toxicity of mercury to the freshwater alga Selenastrum capricornutum (Chen 1994). Organic carbon sources, particularly glucose, glutamate and 2-oxoglutarate, also reduced mercury toxicity to the freshwater alga Chlorella in culture medium (Mohanty et al. 1993). The significance of these ameliorative effects in natural phytoplankton populations is unknown.

In seawater, dissolved inorganic species of mercury (II) include HgCl₄^2- and the neutral HgCl₂, which, due to its high lipid solubility, penetrates cell membranes 10⁷ times faster than the free metal ion, Hg²⁺. Inorganic mercury (as HgCl₂) was found to be toxic to the locally-isolated marine alga, Nitzschia closterium, with a 72-hour EC50 of 9 µg/L (Florence & Stauber 1991). Salinity has also been shown to influence the toxicity of mercury. Toxicity data for several species of annelid worms and crustaceans indicate an increase in inorganic mercury toxicity with decreasing salinity. However, a study using fish and mud crabs has shown that highest survival rates occur in intermediate salinities (Hall & Anderson 1995).

Mercury levels in muscle tissue of freshwater fish between 6 and 20 mg/kg are associated with toxicity (Wiener & Spry 1996). Whole body concentrations between 5 and 10 mg/kg are associated with lethal or sublethal effects. Fish embryos are more at risk from maternal exposure to mercury than from waterborne exposure.

Jarvinen and Ankley (1999) report data on tissue residues and effects for inorganic mercury for 15 freshwater species and 10 marine species. It is not possible to summarise the data here but readers are referred to that publication for more information. There are also data on eight freshwater and one marine species for methylmercury.

Aquatic toxicology

Acute toxicity of mercury (II) to freshwater invertebrate species ranged from 2.2 µg/L for Daphnia pulex (Canton & Adema 1978) to 2000 µg/L for a mayfly (Warnick & Bell 1969). Acute values for fish ranged from 30 µg/L for a guppy to 1000 µg/L for Tilapia (Deshmukh & Marathe 1980, Quereshi & Saksena 1980). Inorganic mercury is particularly toxic to marine microalgae with EC50 values ranging from 0.1 to 10.0 µg/L. Inorganic mercury (as HgCl₂) was found to be toxic to the locally-isolated marine alga, Nitzschia closterium, with a 72-hour EC50 of 9 µg/L (Florence & Stauber 1991).

Few data are available regarding acute toxicity of organomercury compounds, but they all appear to be 4 to 31 times more toxic than inorganic mercury (II) (USEPA 1986). Methylmercury appears to have the highest chronic toxicity of the tested mercury compounds, with chronic toxicity occurring at less than 0.04 µg/L for Daphnia magna and 0.52 µg/L for brook trout (McKim et al. 1976, Biesinger et al. 1982). The most sensitive plant species generally appear to be less sensitive than sensitive animal species to both mercury (II) and methylmercury (CCREM 1987). The freshwater alga Scenedesmus dimorphus was strongly inhibited by 10 µg/L mercury (as methylmercury), with similar inhibition requiring 50 µg/L inorganic mercury (as HgCl₂). In mixed phytoplankton populations, concentrations of methylmercury as low as 0.1 µg/L inhibited primary productivity by 30%.

USEPA (1985d) summarised data on the acute toxicity of mercuric chloride in marine water, with values ranging from 3.5 µg/L to 1700 µg/L. Generally, fish tend to be more resistant than molluscs and crustaceans. Mercury (II) concentrations ranging from 10 µg/L to 160 µg/L inhibited growth and photosynthetic activity of saltwater plants. Mercury acetate at 1 µg/L was toxic to marine dinoflagellates, causing theca to burst to release naked motile cells, which formed vegetative resting spores. The Australian marine amphipod Allorchestes compressa was sensitive to mercury, with a 96-hour LC50 of 80 µg/L. In general, freshwater and marine molluscs are less sensitive to inorganic mercury, with acute LC50 values ranging from 3 to 10,000 µg/L (Florence & Stauber 1991).

Mercury is not as toxic to fish as some other metals, such as Cu, Pb, Cd or Zn. The concentrations of mercury in most surface waters are generally much too low to cause any direct toxic effects to either adult fish or the more sensitive early life stages. The main danger is diet-derived methylmercury, which accumulates in internal organs and exerts its effects by disruption of the central nervous system. Bioaccumulation of mercury from water may also be an issue. Bioconcentration factors of 5000 have been reported for mercury (II); factors for methylmercury ranged from 4000 to 85,000 (USEPA 1986). Bioconcentration factors 10,000 to 40,000 were found for mercuric chloride and methylmercury with an oyster (USEPA 1986).

The primary effect of mercury on fish populations is most likely to be reduced reproductive success resulting from maternally derived mercury to embryonic and larval stages. Lethal effects on rainbow trout embryos were associated with mercury levels in eggs of 0.07 to 0.10 mg/kg, less than 1% of the levels (10 to 30 mg/kg FW) associated with lethal effects in adult fish (Wiener & Spry 1996).

Freshwater guideline

Chronic freshwater data for mercury were screened to 4 taxonomic groups, as follows (pH range 7 to 8.7):

Fish: seven species, 7 to 91-day LC/EC50, 0.7 µg/L (Carassius auratus) to 6355 µg/L, which converted to no observed effect concentration (NOEC) values of 0.14 to 1271 µg/L.

Crustacean: one species, Hyalella azteca, 42 to 70-day NOEC, 1.12 µg/L.

Mollusc: one species, 7-day LC50, 60 to 95 µg/L, converting to NOEC of 12 to 19 µg/L.

Algae: three species, 14-day NOEC, growth, 33 to 85 µg/L.

Blue–green algae: NOEC, 253 µg/L.

Macrophyte: one species, Myriophyllum spicatum, 32-day EC50, growth, 1200 to 3200, converting to NOEC of 240 to 640 µg/L.

Marine guideline

Chronic data for mercury in marine environments was available for six taxonomic groups, covering 43 data points, as follows (pH range of 7 to 8.5 from 19 of 45 data points):

Fish: one species, Fundulus heteroclitus, 5 to 32-day EC50, hatching, of 37 to 49 µg/L, and for mortality of 800 µg/L, overall converting to NOEC equivalent of 7.4 to 160 µg/L.

Crustaceans: three species, 7 to 35-day LC50, 1.8 µg/L (Mysidiopsis bahia) to 50 µg/L; 7 to 11-day lowest observed effect concentration (LOEC), mortality, of 10 µg/L, overall converting to NOEC of 0.8 to 10 µg/L.

Echinoderm: one species, Asterias forbesi, 7-day LC50, 20 µg/L, equivalent to NOEC of 4 µg/L.

Molluscs: seven species, 5 to 12-day LC50, 4 µg/L (Mya arenaria) to 5071 µg/L; 5-day LOEC growth, Mytilus edulis, 0.3 µg/L, overall equivalent to NOECs of 0.12 to 1014 µg/L.

Annelids: two species, 7 to 28-day LC50, 17 to 90 µg/L, equivalent to NOEC of 3.4 to 18 µg/L.

Algae: seven species, NOEC growth of 0.9 to 88 µg/L.

References

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