Mercury methylation in the hypolimnetic waters of lakes with and without connection to wetlands in northern Wisconsin

Rates of Hg methylation and demethylation were measured in anoxic hypolimnetic waters of two pristine Wisconsin lakes using stable isotopes of Hg as tracers. One of the lakes is a clear-water seepage lake situated in sandy terrain with minimal wetland influence. The other is a dark-water lake receiving channelized inputs from a relatively large terrestrial wetland. Methyl mercury (MeHg) accumulated in the anoxic hypolimnia of both lakes during summer stratification, reaching concentrations of 0.8 ng·L^-1 in the clear-water lake and 5 ng·L^-1 in the dark-water lake. The stable isotopic assays indicated that rate constants of Hg^(II) methylation (K_m) ranged from 0.01 to 0.04·day^-1 in the clear-water lake and from 0.01 to 0.09·day^-1 in the dark-water lake, depending on the depth stratum. On average, K_m was threefold greater in the dark-water lake. Hypolimnetic demethylation rate constants (K_dm) averaged 0.03·day^-1 in the clear-water lake and 0.05·day^-1 in the dark-water lake. These methylation rates were sufficient to account for the observed accumulation of MeHg in hypolimnetic water during summer in both lakes. Despite substantial export of MeHg from the wetland to the dark-water lake, our study indicates that in-lake production and decomposition of MeHg dominated the MeHg cycle in both lakes.