| Methylmercury, an organic form of mercury, is a deadly neurotoxin that causes serious health problems in humans and other organisms. Because it is much more toxic and more likely to bioaccumulate throughout food webs than its inorganic form, processes controlling the conversion of inorganic mercury to methylmercury are of interest to ecosystem and public health managers.;Wetlands are known to produce and export methylmercury to watersheds, primarily as a result of mercury methylation by sulfate- and iron-reducing bacteria. In order to be methylated mercury must first be taken up by the bacteria, and the factors controlling its bioavailability are not well understood. In this study, I investigated the biogeochemical controls on mercury bioavailability to methylating bacteria in the Allequash Creek Wetland, in northern Wisconsin.;Mercury methylation was found to occur in the wetland sediments, but with a high degree of spatial and seasonal variation. Dissolved sulfide and organic matter concentrations were shown to be the strongest controls on mercury bioavailability and methylation. Sulfide concentrations were shown to be controlled by the concentration of aqueous iron(II) in many instances. Partitioning of mercury between the solid and aqueous phase, which has been shown to be an important factor controlling bioavailability at other sites, was not shown to be important in this study.;Strong, mercury-binding organic ligands, which are a subset of dissolved organic matter, are thought to bind mercury more strongly than most other aqueous compounds and to render it unavailable to methylating bacteria. These ligands were measured in porewaters collected from Allequash Creek, but were not shown to have a significant negative effect on mercury methylation, most likely due to their low concentrations.;This research adds to a growing body of knowledge on a variety of factors controlling mercury methylation in diverse environments. The findings of this study support the dominant theory that mercury methylation is controlled primarily by sulfide and organic matter concentrations. |
