Distribution and fluxes of total and methylmercury in Lake Superior

Despite the importance and size of Lake Superior, little is known regarding the biogeochemical cycling or distribution of mercury within its waters. We present the results from two research cruises on total Hg (HgT) and methylmercury (MeHg) distributions in aqueous and particulate phases, and in offshore sediments. Open waters of Lake Superior are similar in HgT content to Lakes Michigan and Ontario (subng L-1), whereas MeHg was only 1% of HgT. Seasonality in aqueous HgT distribution was observed, most likely from tributary inputs during Spring snowmelt. Suspended particles were enriched in MeHg relative to water and surficial sediments, suggesting enhanced particle partitioning followed by demethylation in the water column and in surface sediments. Distribution coefficients for mercury in surficial sediments were lower than those in suspended material, likely due to remineralization. Preliminary estimates of mass balance indicate that air-water exchange processes such as evasion and wet deposition dominate the HgT budget, due to the basin's relatively small watershed area relative to lake area. In contrast, methylmercury cycling within Lake Superior is influenced more strongly by watershed sources, as well as by sedimentary sources and photodemethylation. The Hg cycle in Lake Superior is unique in that it is more similar in many aspects to that in marine systems than in small lakes, where management data for freshwaters typically originates.