Determination of mercury methylation potentials in the water column of lakes across Canada

A stable isotope technique was used to trace the formation of methylmercury in lake water incubation assays at in situ conditions in five lakes across Canada. Methylation activity was only detected in the anoxic hypolimnia of lakes. The stable isotope was methylated at varying rates between lakes and depths within lakes ranging from 0.56%/day to 14.8%/day. A peak in methylation potential was typically observed just below the oxycline, which decreased with increasing depth. The depth and rates of methylation potential changed seasonally with no methylation activity occurring after fall turnover. A decrease in the sulfate concentration was concomitant with the zone of mercury methylation potential indicating the likely involvement of sulfate reducing bacteria in the methylation process. A simple correlation test between DOC concentrations and methylation rates indicated a positive relationship (r2 = 0.62; p = 0.006; n = 27). The demethylation rate constant in the anoxic hypolimnia was less than 0.12 d− 1.