Mercury pollution: integration and synthesis
Atmospheric deposition is a source of mercury to freshwater fish that previously has not been adequately assessed, because past analytical and sampling procedures caused inaccurate estimates of deposition and of emissions from natural and anthropogenic sources. More accurate studies during the last decade have begun to fill this gap. However, global circulation and the relative importance of regional and local sources of mercury need further quantification. Recently developed techniques have led to better estimates of global sources and sinks, but considerable work is needed on the estimation of anthropogenic and terrestrial sources. At a smaller scale - a Wisconsin, U.S. seepage lake - the annual mercury atmospheric deposition (about 10 ug/m2/year) is greater than the mercury included in the total fish biomass of the lake. Atmospheric concentrations of gaseous mercury in Wisconsin are similar to those in Northern hemisphere oceanic air masses. However, mercury deposition appears to be principally on particles containing oxidized mercury (Hg(II), produced directly from emissions or oxidized in raindrops. The fraction of total atmospheric input that was elemental mercury oxidized near the deposition site and the fraction that was particulate inorganic mercury transported from regional sources is largely unknown. Best estimates suggest that about 5\% of the total mercury deposition within the lake system is transformed into methylmercury to be accumulated by fish. Estimates of methlmercury deposition from the atmosphere to the lake and net input to the sediments are both less than 1\% of total deposition. At least 95\% of mercury in whole, 1-year-old yellow perch from these seepage lakes is methylmercury. The Mercury Cycling Model (M\textlessCM, Version 1.0) simulates the cycling of environmental mercury, including fish bioaccumulation of mercury. The model can assess risk as well as test hypotheses and processes.
Ann Arbor, Michigan