Sediment methylation and demethylation rates were measured by short-term incubations of stable isotopes of inorganic and methylmercury in sediment cores. Methylmercury photodemethylation rates were performed, during which water samples were incubated in the field under different treatments. Some bottles were spiked with methylmercury or complexing agents and scavengers. Labile concentrations of methylmercury were measured in water and sediment using DGT (Diffusive Gradient in Thin Film) samplers. Characterization of the microbial community in sediment and quantification of targeted microbes potentially involved in MeHg cycling were performed using qRT-PCR approaches, cloning and library analyses combined with gel electrophoresis techniques. Methylmercury bioaccumulation at the base of the food web was examined by collecting filamentous benthic algae and chironomid larvae from nearshore and offshore areas in the study lakes.