Mercury in freshwater and marine water
Toxicant default guideline values for protecting aquatic ecosystems
Extracted from Section 8.3.7 ‘Detailed descriptions of chemicals’ of the ANZECC & ARMCANZ (2000) guidelines.
The default guideline values (previously known as ‘trigger values’) and associated information in this technical brief should be used in accordance with the detailed guidance provided in the Australian and New Zealand Guidelines for Fresh and Marine Water Quality.
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 HgCl42- 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.
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).
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.
A freshwater high reliability trigger value of 0.6 µg/L was calculated for inorganic mercury using the statistical distribution method with 95% protection. This has not specifically considered bioaccumulation. The 99% protection level is 0.06 µg/L, and is the figure recommended for slightly to moderately disturbed systems for three reasons: a) as a precaution for bioaccumulation—see under ‘marine’ below; b) the 95% figure is close to the chronic LC50 figure for Carassius auratus; and c) the 95% figure is only 3 to 4-fold lower than the lowest acute LC50 for D. magna. There were insufficient data to derive a trigger value for methyl mercury.
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.
A marine high reliability trigger value of 0.4 µg/L was calculated for inorganic mercury using the statistical distribution method with 95% protection. This has not specifically considered bioaccumulation. The 99% protection level is 0.1 µg/L and is recommended for slightly to moderately disturbed systems if there are no data to allow for adjustment for bioaccumulation at the specific site (Section 126.96.36.199 of the ANZECC & ARMCANZ 2000 guidelines). The 99% figure (0.1 µg/L) is the same as that recommended by Canada (CCREM 1987) to protect human consumers of fish. There were insufficient data to derive a trigger value for methyl mercury.
ANZECC & ARMCANZ 2000. Australian and New Zealand Guidelines for Fresh and Marine Water Quality, Australian and New Zealand Environment and Conservation Council and Agriculture and Resource Management Council of Australia and New Zealand, Canberra.
Biesinger KE, Anderson LE & Eaton JG 1982. Chronic effects of inorganic and organic mercury of Daphnia magna: Toxicity, accumulation and loss. Archives of Environmental Contamination and Toxicology 11, 469-774.
Bloom NS 1989. Determination of picogram levels of methylmercury by aqueous phase ethylation, followed by cryogenic gas chromatography with cold vapour atomic fluorescence detection. Canadian Journal of Fisheries and Aquatic Science 46, 1131–1140.
Bloom NS & Effler SW 1990. Seasonal variability in the mercury speciation of Onondaga lake (New York). Water Air and Soil Pollution 53, 251–265.
Canton H & Adema DMM 1978. Reproducibility of short-term and reproduction toxicity experiments with Daphnia magna and comparison of the sensitivity of Daphnia magna with Daphnia pulex and Daphnia cucullata in short-term experiments. Hydrobiologia 59, 135-140.
CCREM 1987. Canadian water quality guidelines. Canadian Council of Resource and Environment Ministers, Ontario.
Chen CY 1994. Theoretical evaluation of the inhibitory effects of mercury on algal growth at various orthophosphate levels. Water Research 28, 931–937.
Clevenger WL, Smith BW & Winefordner JD 1997. Trace determination of mercury: A review. Critical Reviews in Analytical Chemistry 27, 1–26.
Cossa D & Noel J 1987. Concentrations of mercury in the nearshore surface waters of the Bay of Biscay and in the Gironde estuary. Marine Chemistry 20, 389–396.
Cossa D, Noel J & Auger D 1992. Vertical mercury profile in relation to arsenic, cadmium and copper at the eastern North Atlantic ICES reference station. Oceanologica Acta 15, 603–608.
Deshmukh SS & Marathe VB 1980. Size related toxicity of copper and mercury to Lebistes reticulatus (Peter), Labeo rohita (Ham), and Cyprinus carpio (Linn). Indian Journal of Experimental Biology 18, 421-423.
Fergusson JE 1990. The heavy elements: Chemistry, environmental impact and health effects. Pergamon Press, Oxford.
Fitzgerald WF & Clarkson TW 1991. Mercury and monomethylmercury: Present and future concerns. Environmental Health Perspectives 96, 159–166.
Florence TM & Stauber JL 1991. The toxicity of heavy metals to aquatic organisms. In Proceedings of IIR Conference on environmental monitoring, Sydney [9 pp].
Florence TM & Batley GE 1980. Chemical speciation in natural waters. Critical Reviews in Analytical Chemistry 9, 219–296.
Gill GA & Bruland KW 1990. Mercury speciation in surface freshwater systems in California and other areas. Environmental Science and Technology 24, 1392–1400.
Gill GA & Fitzgerald WF 1988. Vertical mercury distributions in the oceans. Geochimica et Cosmochimica Acta 52, 1719–1728.
Hall LW & Anderson RD 1995. The influence of salinity on the toxicity of various classes of chemicals to aquatic biota. Critical Reviews in Toxicology 25, 281–346.
Jarvinen A W & Ankley G T 1999. Linkage of effects to tissue residues: Development of a comprehensive database for aquatic organisms exposed to inorganic and organic chemicals. SETAC Technical Publication Series, SETAC Press, Pensacola FL.
Jensen AL 1988. Modelling the effect of acidity on mercury by Walleye in acidic and circumneutral lakes. Environmental Pollution 50, 285–294.
Mason RP, Reinfelder JR & Morel FMM 1996. Uptake, toxicity and transfer of mercury in a coastal diatom. Environmental Science and Technology 30, 1835-1845.
McKim JM, Olson GF, Holcombe GW & Hunt EP 1976. Long term effect of methylmercuric chloride on three generations of brook trout (Salvelinus fontinalis): Toxicity, accumulation, distribution and elimination. Journal of Fisheries Research Board of Canada 33, 2726-2739.
Mohanty RC, Mohanty L & Mohapatra PK 1993. Change in toxicity effect of mercury at static concentrations to Chlorella vulgaris with addition of organic carbon sources. Acta Biologica Hungarica 44, 211–222.
Nelson LA 1981. Mercury in the Thames estuary. Environmental Technology Letters 2, 225–232.
NSW EPA 2000. Analytical Chemistry Section, Table of Trigger Values 20 March 2000, LD33/11, Lidcombe, NSW.
Paulsson K & Lundbergh K 1991. Treatment of mercury contaminated fish by selenium addition. Water Air and Soil Pollution 56, 833–841.
Quereshi SA & Saksena AB 1980. The acute toxicity of some heavy metals to Tilapia mossambica (Peters). Aqua (London) 1, 19-20.
USEPA 1986. Quality criteria for water. US Department of Commerce, National Technical Information Service, US Environmental Protection Agency, Springfield, Virginia. PB87-226759, EPA 440/5 86-001.
USEPA 1985d. Ambient water quality criteria for mercury — 1984. Criteria and Standards Division, US Environmental Protection Agency, Washington DC. EPA-440/5-85-026.
Warnick SL & Bell HL 1969. The acute toxicity of some heavy metals to different species of aquatic insects. Journal of the Water Pollution Control Federation 4, 280-284.
Wiener JG & Spry DJ 1996. Toxicological significance of mercury in freshwater fish. Chapter 13 in Environmental contaminants in wildlife: Interpreting tissue concentrations, eds WN Beyer, GH Heinz & AW Redmon-Norwood, CRC Press, Boca Raton, FL, 297-339.