The seasonal and spatial variations of mercury methylation and methylmercury decomposition in an oligotrophic northern Wisconsin lake

Abstract

The formation of methylmercury represents a potentially serious problem in aquatic ecosystems because of its toxicity and accumulation in aquatic biota. Elevated levels of mercury in edible fish have recently been reported for several lakes in Northern Wisconsin. The majority of mercury in fish tissue is in the methylated form, suggesting that biological mercury methylation is a key process regulating mercury levels in fish. Numerous studies have examined mercury methylation in aquatic ecosystems but most have addressed only net rates of methylmercury production. The net amount of methylmercury produced in the aquatic environment is dependent on both methylation and demethylation. Recent studies have examined the effects of pH on mercury methylation and demethylation in several lakes in Canada and Northern Wisconsin. However, little else is known about the relative rates of these simultaneous processes in aquatic ecosystems.

The purpose of this study was to examine where and when the potential for methylation was the greatest in Lake Clara; to examine the products of microbial demethylation; and to examine the effects of acidification on methylation and demethylation. Microbial mercury methylation and methylmercury decomposition were examined in Lake Clara, an oligotrophic Northern Wisconsin seepage lake, using 203Hg(N03)2 and 14cH3Hgi, respectively. The primary sites for mercury methylation in Lake Clara were in the surficial sediments of the backwater area and the littoral-profundal transitionary zone. Mercury methylation in surficial sediments was greatest from mid-July through September. The potential for methylation in the water column and attached microbial communities was low; whereas, demethylation activity was substantially greater. Thus, the net rate of methylmercury production may be significantly affected by demethylation activity.