Transactions on Ecology and the Environment vol 14, © 1997 WIT Press, www.witpress.com, ISSN 1743-3541 Mercury behaviour in estuarine and coastal environment M. Horvat Department of Environmental Sciences, Jozef Stefan Institute, Jamova 39, Ljubljana, Slovenia E-mail: [email protected] Abstract General facts about the global mercury cycle with particular emphasis on the coastal and ocean environment are summarized. In the coastal environment the largest source of mercury is river-born particulate bound species. This portion of mercury is unreactive and is quickly buried in nearshore sediments. Only a small fraction of reactive mercury (ionic mercury in solution that is immediately available for reaction) originates from river inputs. The most important source of reactive mercury in the coastal and oceanic environment is through atmospheric input and via upwelling. Biologically-mediated processes, mainly connected to primary production, are responsible for active redistribution of reactive mercury. In this process a large part of reactive Hg is reduced to elemental mercury which is returned to the atmosphere by evasion, while the rest is scavenged by particles and transported to deeper oceanic waters. Because of the active atmospheric mercury cycle oceans acts as a source and a sink of atmospheric mercury and the global oceanic evasion is balanced by the deposition. Current studies show that methylated species are primarily formed in the deeper ocean and the mam source of monomethylmercury (MMHg) compounds in coastal areas is through upwelling of oceanic waters and from in-situ methylation in coastal waters. All these environmental processes occur at extremely low concentration levels of mercury species; however MMHg in marine organisms accounts for a high proportion of this toxic compounds owing to its property for bioaccumulation and biomagnification. Coastal areas on the local scale may account for geochemical differences that significantly influence the conversion between various Hg species. In order to assess the impact of mercury in contaminated and non- contaminated coastal areas on man and his environment, it is of the greatest importance to understand these processes. The paper also identifies uncertainties and gaps in current knowledge of mercuy cycling 1 General facts Mercury and its compounds are extremely hazardous. The most toxic are monomethylmercury (MMHg) compounds which represent a health risk, particularly to the foetal neurosystem^. The risk to public health is evidenced in fish consumption regulations that have been issued in Canada, Scandinavia, by Transactions on Ecology and the Environment vol 14, © 1997 WIT Press, www.witpress.com, ISSN 1743-3541 78 Water Pollution more than 30 states, the US FDA, the World Health Organization (WHO) and numerous other governments. In the USA, the Clean Air Act Amendments require an assessment of health risk to humans and wildlife caused by Hg emissions. Also, the potential adverse effects of atmospheric Hg deposition on coastal waters is contained in a planned Protection Agency report to Congress^. In the environment mercury can exist in a number of different physical and chemical forms with a wide range of properties. Biogeochemical conversion between these different forms provides the basis for mercury's complex distribution pattern in local and global cycles and for its biological enrichment and effects. The most important chemical forms are: elemental mercury (Hg°), divalent inorganic mercury (Hg(II)), monomethylmercury (MMHg), and dimethylmercury (DMHg). There is a general biogeochemical cycle by which these different forms may interchange in the atmospheric, aquatic and terrestrial environments. Most studies of Hg cycling have, so far, been made in terrestrial systems, with the biogeochemistry of Hg in fresh water systems^, even though it is well known that the primary exposure of humans to MMHg is through the consumption of marine fish and fish products^. In principle, Hg cycles in terrestrial and marine aquatic systems are similar with some distinct differences. The most important feature in both systems is the in-situ bacterial conversion of inorganic Hg species to the more toxic MMHg, which concentrates in fish muscle?". The key feature that influences mercury distribution in aquatic environments is the high stability of its associations with sulphur and carbon, the stability of its volatile elemental form and its strong affinity to particles. Consequently, most inorganic and organic Hg appears to be bound to particles, colloids and high molecular weight organic matter, where it is probably coordinated with sulphur ligands on particles (distribution coefficients are in the 10^-10^ ml/g range). In turbid rivers most mercury, therefore, is transported by suspended matter. Only a small part of Hg in fresh, estuarine and sea water is likely to be present in dissolved form **"**. After entering the atmosphere mercury exchanges and cycles through the atmosphere to be deposited in the ecosystem, almost exclusively as Hg(II). When Hg enters a surface (soil or water) Hg(II) can be methylated to MMHg. This is the first step in aquatic and terrestrial bioaccumulation processes. The mechanism of synthesis of MMHg is not very well understood. The main factors that affect the levels of MMHg in fish are the dietary trophic level of the species, the age of the fish, microbial activity and the mercury concentration in the upper layer of the local sediment, its dissolved organic carbon content, salinity, pH, and redox potential^. Recent work suggest that sulphato-reducing bacteria are the most important methylating agents along with environmental conditions existing in transition regions between oxygenated and anoxic conditions^. Methylation-demethylation reactions are assumed to be widespread in the environment and each ecosystem attains its own steady state equilibrium with respect to the individual species of mercury. However, owing to Transactions on Ecology and the Environment vol 14, © 1997 WIT Press, www.witpress.com, ISSN 1743-3541 Water Pollution 79 bio accumulation of MMHg, methylation is more prevalent in the aquatic environment than demethylation. Once MMHg is formed, it enters the food chain by rapid diffusion and tight binding to proteins in aquatic biota, and attains its highest concentrations in the tissues of fish at the top of the aquatic food chain due to biomagnification through the trophic levels. For example, most predatory fish species show MMHg values > 1 ppm, while concentrations in water are commonly < Ippt (e.g. amplification factor of about 10^'^^. Although MMHg is the dominant form of mercury in higher organisms, it represents only a very small amount of the total mercury in aquatic ecosystems and in the atmosphere. 2 Global mercury cycle Mercury vapour is released into the atmosphere from a number of natural sources (e.g. the ocean surface and other water surfaces, soils, minerals, and vegetation located on land, forest fires and volcanoes) and through anthropogenic emissions (metal extraction processes, agricultural uses, paints, waste disposal, burning of fossil fuels, smelting of ores and other industrial minerals, and from power plants burning fossil fuels for electricity generation). Recent studies ^^ show that human activity contributes about 50 to 75% (e.g. 3500 to 4500 tons) of the total yearly input from all sources (7000 tons/y) (Figure 1). About half of the anthropogenic emissions (approx. 2000 tons/y) appear to enter the global Hg cycle, while the other half is deposited locally. As a consequence human activities have tripled the concentrations of Hg in the atmosphere and in the surface ocean. It is estimated that 60% (approx. 5000 tons) of the total Hg is deposited on terrestrial environments (30% of the surface of the Earth) and the remainder to the ocean. This is due to the oxidation of Hg in the abundant terrestrial aerosols. The ocean receives about 90% of its Hg through wet and dry deposition as Hg(II) and the remaining (200 tons/y) from river inflows. The particulate scavenging and removal to the deep ocean is equal to the riverine Hg flux (200 tons/y). Due to biological reduction of deposited Hg(II) in the mixed layer of the ocean and its evasion most Hg° deposited (2000 ton/y) is re-emitted to the atmosphere. This active process and the minimal removal of Hg to the deep ocean makes terrestrial systems the dominant sink. The model presented in Figure 1 is based on rather limited data and uncertainties may account for a factor of two or more. The main reasons for this uncertainty are the limited data for Hg concentrations and speciation in precipitation, both temporally and spatially. Almost no data are available for the Southern Hemisphere, Asia and the Arctic coastlines. Tropical regions also need to be assessed, as the impact of current activities (e.g. biomass burning and gold extraction^) on the global Hg cycle and budget may be significant. The flux of MMHg to the oceans is also unknown. In particular, coastal rain could be an important source of MeHg. Another reason for uncertainty is the limited data available on evasion of Hg from the ocean surface. This process is controlled by the ability of the system to Transactions on Ecology and the Environment vol 14, © 1997 WIT Press, www.witpress.com, ISSN 1743-3541 80 Water Pollution reduce reactive Hg. Current studies show that this process is directly connected to primary productivity. It has also been shown that there are ocean areas where's atmospheric deposition exceed the evasion ( e.g. the Siberian Arctic) and other where evasion exceeds the deposition (e.g. the productive zones at lower latitudes). Probably, net atmospheric transfer of mercury to higher latitudes is occurring. More studies need to be done in productive and non- productive ocean areas in order to understand better global Hg transport and cycling. The current model also does not take into account some other potential sources of Hg in the ocean. These are hydrothermal inputs and oil and gas deposits. It is assumed that most inorganic mercury from hydrothermal vents would be trapped on particles and precipitated.