SITES OF METHYL MERCURY PRODUCTION IN REMOTE CANADIAN SHIELD LAKES

Methylation and demethylation of mercury were studied in sediments and surface waters of several remote lakes on the Canadian Shield. Radioc hemical assays of mercury methylating activity, which peaked during su mmer, were 20-40 times faster in epilimnetic than in hypolimnetic sedi ments. Demethylation rates were usually highest during winter and in h ypolimnetic sediments. Epilimnetic sediments were capable of producing methyl mercury 20-40 times faster than hypolimnetic sediments sampled at the same time, with methylating activity peaking during the warm s ummer months. Because of the opposite pattern of methylating and demet hylating activity and because epilimnetic sediments often constitute m ost of the surface area of these lakes, most of the net methylation (M /D) occurred in the epilimnion of the lakes during summer. Mercury met hylation rates were not related to average sediment mercury concentrat ions, or directly to rates of microbial activity (decomposition) in se diments. It appeared that increasing temperature was an important cont rolling factor that stimulated methylation but retarded demethylation. Specific methylation rates, M/D, and the measured concentration of me thyl mercury in the surface waters of the lakes all peaked during mids ummer, suggesting that the radiochemical assays used reflected changes in the relative activities of natural methylating and demethylating m icroorganisms.