Assessing methylmercury formation in lake water using a stable isotope technique.

摘要

The objective of this study is to determine where and under what conditions mercury is methylated in the water column of lakes. A stable isotope technique was used to trace the formation of methylmercury in lake water incubation assays at in situ conditions. Samples were collected from 7 lakes across Canada and the northern United States. Methylation activity was only detected in the anoxic hypolimma. A peak in methylation activity was typically observed just below the oxycline. The depth and rates of methylation activity changed seasonally with no methylation activity occurring after fall turnover. Mercury methylation activity was concomitant with the zone of sulfate reduction (as measured by the decrease in sulfate concentration in lakes), indicating the likely involvement of sulfate reducing bacteria. A simple correlation test between DOC concentrations and methylation rate constants indicated a positive relationship. The demethylation rate constant in anoxic the hypolimma was less than 0.13 d-1. Modeling results indicate that water column methylation could account for the buildup of ambient MeHg in the hypolimmon of a seepage lake.