Methyl mercury in Lake Superior: Offshore processes and bioaccumulation

Abstract

The effects of watershed type exert a strong influence on the speciation of mercury and the delivery of mercury to Lake Superior nearshore waters. As a consequence, tributary mixing zones are important locations for enhanced bioaccumulation in Lake Superior. Methyl Hg (MeHg) bioaccumulation, however, is also observed in regions of the lake that are remote from tributary influences. Three cruises aboard the USEPA vessel R/V Lake Guardian on Lake Superior revealed that offshore concentrations of total mercury (HgT) were low, similar to Lake Michigan and oceanic waters (0.21-1.0 ng/L HgT). During August 2000, MeHg averaged 3.0-12.6 pg/L, at least an order of magnitude lower than most tributaries during typical flow regimes. Despite these differences, initial comparisons of phytoplankton revealed only a two to threefold enrichment of MeHg in tributary mixing zones versus offshore regions. MeHg inputs to the open waters of the lake are dependent on three processes: mixing from nearshore zones, direct atmospheric inputs and MeHg diffusion from sediments. Direct sedimentary methylation rates are extremely low and modeling efforts suggest that photodegradation would eliminate tributary-derived MeHg. Therefore, we conclude that atmospheric sources strongly influence MeHg uptake in offshore zones. A detailed profile at a deep-lake station in August 2001 suggests enhanced bioaccumulation at a subsurface chlorophyll maximum, in a zone with close contact to atmospheric fluxes.