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. Radiochemical assays of mercury methylating activity, which peaked during summer, were 20-40 times faster in epilimnetic than in hypolimnetic sediments. Demethylation rates were usually highest during winter and in hypolimnetic 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 summer months. Because of the opposite pattern of methylating and demethylating activity and because epilimnetic sediments often constitute most of the surface area of these lakes, most of the net methylation (M/D) occurred in the epilimnion of the lakes during summer. Mercury methylation rates were not related to average sediment mercury concentrations, or directly to rates of microbial activity (decomposition) in sediments. It appeared that increasing temperature was an important controlling factor that stimulated methylation but retarded demethylation. Specific methylation rates, M/D, and the measured concentration of methyl mercury in the surface waters of the lakes all peaked during midsummer, suggesting that the radiochemical assays used reflected changes in the relative activities of natural methylating and demethylating microorganisms.