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. Mercury also forms organometallic compounds, some of which have found industrial and agricultural use. These organometallic compounds are stable, although some are readily broken down by living organisms, while others are not readily biodegraded.

A number of reviews (e.g. WHO 1989 and 1991 and CCME 1992, US EPA 1984) reviewed the environmental fate and behaviour and aquatic toxicity of mercury. These are discussed below. The reader is referred to the above reports for a more comprehensive assessment.

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 l^-1). 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 decreasing, 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.

WHO (1989) stated that for the open ocean and for coastal sea-water, concentrations of dissolved mercury in the range of 0.5 - 3 ng l^-1 and 2 - 15 ng l^-1 respectively could be considered to be representative.

However, 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 or where near to anthropogenic sources, e.g. in the vicinity of mining sites and chloralkali plants for the industrial extraction of mercury. However, the majority of mercury in the environment can be considered to be natural rather than the result of human activities.

WHO (1989 ) estimated concentrations in ocean sediments probably lie in the range between 20 and 100 µg kg^-1.

Concentrations of mercury have been measured in water, sediments and biota as part of the National Monitoring Programme at sites throughout the UK in estuaries and coastal waters (MPMMG 1998). The results of the National Monitoring Programme are summarised in Appendix D. MPMMG 1998 should be consulted for further details.

These show elevated levels in some sediments. However, with regard to the water column, the available data suggest that concentrations of mercury in UK coastal and estuarine waters appear unlikely to exceed relevant quality standards derived for the protection of saltwater life.

As an example of the recorded levels of dissolved mercury in the marine environment, the following concentrations have been reported by DETR (1998) for some English estuaries (See tables below).

Dissolved mercury has a strong affinity for organic matter and suspended sediment and so can be expected to be bound to these particles in the water column and subsequently to accumulate in sediments. Campbell et al 1986 reported a well defined increase in dissolved mercury concentrations in a seaward direction from the Mersey estuary. Within the estuary where suspended material concentrations are greater, concentrations of dissolved mercury decrease away from a point source but increase again towards Liverpool Bay.

Once deposited in sediments, mercury can undergo methylation to produce methylmercury. This form of mercury is bioavailable and is a hazard to aquatic life.

An exhaustive literature review on the toxicity of mercury to marine organisms has not been carried out for the purposes of this profile. The information provided in this section is taken from existing review documents (WHO 1989, 1991, CCME 1992 and US EPA 1994)). The most sensitive groups of organisms have been identified.

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 l^-1 for inorganic mercury but at much lower concentrations of organic mercury.

Aquatic invertebrates vary greatly in their susceptibility to mercury, with the concentration and species of mercury, the developmental stage of the organisms, and the temperature, salinity, water hardness, and flow rate all affecting the sensitivity. 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.

Levels of 1 to 10 µg l^-1 normally cause acute toxicity for the most sensitive developmental stage of many different species of aquatic invertebrates.

Inorganic mercury is toxic to fish at low concentrations. 96-h LC50s as low as 30 µg l^-1 have been reported. 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 sub-lethal concentrations of mercury, although the environmental significance of these effects is difficult to assess. Reproduction is also adversely affected by mercury.

Fatalities and severe poisonings in birds have been reported in association with outbreaks of human poisoning. 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 and scavenging birds were also killed (Harada, 1978). Birds found dead in the area showed characteristic pathological changes in the central nervous system of Minamata disease, but no measurement of mercury content was made (Takeuchi et al. 1957).

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.

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^-1. Productivity (i.e. 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 and 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.

There is some limited information on the effects marine 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^-1 body weight per day and two fed 25.0 mg kg^-1 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.

Although environmental levels can be considered to be low, the high capacity of organisms to accumulate mercury means that the metal is found widely in aquatic animals and plants.

Inorganic mercury can be methylated in the environment and the resultant methylmercury is taken up into organisms more readily than inorganic mercury. The 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 is strongly bound to sediment or organic material and is unavailable to organisms.

Aquatic invertebrates 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, there is a strong indication that bacterial action leads to methylation in aquatic systems. 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.

Langston et al (1996) reported levels of methylmercury bioaccumulated by a range of estuarine algae and invertebrates in the Mersey estuary in 1995 to be between 10 (Fucus vesiculosus) and 100 (Mytilus edulis) times higher than sediment concentrations at the sites they were collected from. Correlations between sediment and tissue levels of methylmercury in the invertebrates suggest that sediments are the prime source of the contaminant for these animals.

Seabirds and those feeding in estuaries have also been found to be contaminated. The form of retained mercury in birds is more variable and depends on species, organ, and geographical site.

Potential effects include:

• acute toxicity in the water column to interest features comprising macrophytes, invertebrates and fish at concentrations of dissolved mercury above the EQS of 0.3µg l^-1 (annual average). Toxic effects below this concentration may occur if methylmercury is present;
• inorganic mercury accumulates in sediments and may be a hazard to sediment-dwelling organisms at concentrations above 0.13 mg kg^-1, according to Canadian interim marine sediment quality guidelines. Toxic effects below this concentration may occur if methylmercury is present;
• methylmercury bioaccumulates in the food chain and poses a hazard to fish, birds and Annex II sea mammals.

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