Increased Methylmercury Contamination in Fish in Newly Formed Freshwater Reservoirs
The formation of major new hydroelectric reservoirs in northern Canada in the last two decades was invariably followed by increased methylmercury concentrations in fish. Concentrations in piscivorous fish exceeded marketing limits and often approached mercury concentrations (>5 μg g−1 wet weight) in muscle formerly associated only with industrial mercury pollution. Experimental manipulations of large enclosures demonstrated that terrestrial vegetation and organic soils caused increased net methylation of mercury and bioaccumulation of methylmercury at low total mercury concentrations in water (1–2 ng L−1) and sediment (0.1–1.0 μg g−1 dry weight). Total mercury concentrations per se in water or sediments did not predict mercury concentrations in fish. Enhancement of microbial methylation relative to demethylation can be demonstrated in these new reservoirs and in reservoirs up to 60 years of age. Disruption of the natural microbially mediated mercury cycle accounts for the elevated Hg concentrations in fish, and indications are that it will be a persistent problem in boreal reservoirs. The reservoir experience emphasizes the critical role of microbial activity in mercury cycling. In natural lakes of the boreal forest, water temperaturc seems to be a critical variable controlling net mercury methylation by microbial activity.
KeywordsMercury Concentration Yellow Perch Mercury Methylation Total Mercury Concentration Mercury Uptake
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