Factors that influence methylmercury flux rates from wetland sediments

Sediments are thought to be an important source of methylmercury (MeHg) to the water column of wetlands. We measured sediment MeHg pore water concentrations as a function of depth in four wetlands to determine the concentration gradient and used it determine sediment–water flux of MeHg. Fluxes of MeHg ranged from −1.60 to 10.02 ng m−2 day−1 and were shown to be a function of 1) redox conditions at the sediment–water interface, 2) oxygen gradient above the sediment surface, 3) water temperature, and 4) pore water and water column-dissolved sulphide. MeHg water column concentration in each of the four wetlands was positively correlated with MeHg concentrations present in surface sediment and pore water, and with the calculated sediment–water MeHg flux rate. In addition to MeHg, ethylmercury (EtHg) was detected in the sediment in all four wetlands, but not in the pore water or the water column. EtHg levels in sediment exceeded MeHg concentrations in two of the wetlands. This demonstrates that Hg ethylation is a significant part of the Hg cycle in some aquatic environments.