MERCURY CONCENTRATIONS IN FISH RELATED TO SIZE OF REMOTE CANADIAN SHIELD LAKES

Mercury concentrations in planktivorous, omnivorous, and piscivorous fishes were inversely related to lake size in six lakes in northwestern Ontario. The lakes were remote from direct anthropogenic influences and ranged in surface area from 89 to 35 000 ha. Fish mercury concentrations were not related to ratios of drainage basin area to lake size, to ratios of epilimnetic area to lake size, to lake alkalinity, or to the concentration of mercury in lake sediments. Rates of mercury methylation (M) were positively dependent on water temperature whereas rates of methyl mercury demethylation (D) were inversely related to temperature. Thus, M/D was strongly temperature dependent. Mercury concentrations in four fish species were significantly positively correlated with mean epilimnetic water temperatures (r2's ranged from 0.66 to 0.88). This suggested that higher water temperatures in smaller lakes during the open-water season influenced M/D ratios and were the cause of higher fish mercury levels. No lake size related variation was observed in the mercury concentrations in benthivorous fishes. Our hypothesis that epilimnetic temperature affects mercury concentrations in fish deserves further attention, given the possible effects of climate warming on mercury contamination of fishery resources.