Sources of methylmercury to a wetland-dominated lake in northern Wisconsin

Several lines of evidence suggest that wetlands may be a major source of methylmercury (MeHg) to receiving waters, perhaps explaining the strong correlation between concentrations of waterborne MeHg and dissolved organic carbon (DOC) in regions such as northern Wisconsin. We evaluated the relative importance of wetland export in the MeHg budget of a wetland-dominated lake in northern Wisconsin using mass balance. Channelized runoff from a large headwater wetland was the major source of water and total mercury (Hg_T) to the lake during the study period. The wetland also exported MeHg in high concentrations (0.2-0.8 ng L^-1), resulting in an export rate similar to those reported for other northern wetlands (ca. 0.3 μg MeHg m ^-2y^-1). Yet, based on intensive sampling during 2002, the mass of MeHg that accumulated in the lake during summer was an order of magnitude greater than the export of MeHg from the wetland to the lake. Hence, a large inlake source of MeHg is inferred from the mass balance. Most of the accumulated MeHg built-upin anoxic hypolimnetic waters; and the build-up was roughly balanced by losses of inorganic Hg (Hg^(II)) implying a chemical transformation within the anoxic water column. An abundance of sulfate-reducing bacteria (SRB) in hypolimnetic waters, established by DNA analysis of the pelagic microbial community, along with a previous report documenting high methylation rates in the hypolimnion of this lake (ca. 10% d^-1), suggest that this transformation was microbially mediated. These findings indicate that the direct effect of wetland runoff may be outweighed by indirect effects on the lacustrine MeHg cycle, enhancing the load of Hg^(II), the activity of SRB, and the retention of MeHg, especially in northern lakes with flushing times longer than six months.