IMPORTANCE OF WETLANDS AS SOURCES OF METHYL MERCURY TO BOREAL FOREST ECOSYSTEMS

Wetlands were found to be important sources of methyl mercury to the b oreal forest ecosystem. Yields of methyl mercury were about 26-79 time s higher from wetland portions of catchments (1.84-5.55 mg.ha-1.yr-1) than from purely upland areas (0.07 mg.ha-1.yr-1). Mass-balance estima tes using methyl mercury inputs in wet deposition and outputs in runof f water indicated that purely upland catchments and lakes were sites o f methyl mercury retention or demethylation, while catchments with wet land areas were sites of net methyl mercury production. These observat ions may explain the high concentrations of mercury in fish taken from lakes that are high in colour because they receive water from wetland s. There was no relationship between the concentration of total mercur y and the concentration of methyl mercury in runoff water. Total mercu ry yields were low from a wetland-dominated catchment, higher from a c ombination upland/riparian wetland catchment, and highest from a purel y upland catchment. The opposite was true for methyl mercury yields fr om these same catchments. This indicates that environmental factors ot her than total mercury concentration are controlling the production an d loss of methyl mercury from catchments.