Sediment Methylmercury Concentrations and Production Rates in Contrasting Coastal Wetlands of Chequamegon Bay (WI), Lake Superior

Methylmercury (MeHg) is an extremely toxic form of mercury that bioaccumulates and biomagnifies in aquatic food webs, threatening humans and other upper trophic level organisms. Methylmercury is formed primarily by sulfate reducing bacteria in surficial sediments through the conversion of inorganic divalent mercury to the organic (methylated) form, dramatically increasing potential toxicity. The methylation process is complex, and substantially influenced by multiple biogeochemical factors. In wetlands, these factors are typically the most favorable for sulfate reducing bacteria activity, resulting in relatively high MeHg concentrations. Overall conditions of oligotrophic Lake Superior and its watershed are relatively unfavorable for MeHg formation, however, MeHg bioaccumulation is evident in certain upper trophic level fish. Numerous coastal wetlands, which are heavily utilized by biota, are present on Lake Superior shores and likely important areas of MeHg formation and subsequent transfer to the biota.

In this study we examined sedimentary MeHg concentrations, MeHg formation rates, and other geochemical factors in three contrasting coastal wetlands and one pelagic site in Chequamegon Bay (Lake Superior, WI). One lagoon wetland (without riverine influence) and two drowned-river wetlands (located near the mouth of a Lake Superior tributary) were sampled. Sediment cores and porewater were collected in the summer of 2005, with the upper strata of cores (5-20 cm) analyzed for ambient MeHg concentrations. Rates of MeHg formation were also estimated in sediment cores from each site using stable mercury isotopes as tracers. Ancillary variables such as porewater sulfide and sulfate, as well as sediment organic carbon content and total mercury concentration were also measured. Methylmercury concentrations were greatest near drowned-river coastal wetlands, intermediate in the coastal lagoon wetland, and lowest near the center of the bay. The fraction of inorganic mercury isotope tracer methylated was 0.14 - 0.037 day^-1 and was highest at two sites within a drowned-river wetland.