Biogeochemical Controls on MeHg Production in the Mid-Atlantic Coastal Region

An increasing awareness of methylmercury (MeHg) contamination of coastal food webs, and the general lack of information on sources of MeHg to marine fish, led us to study MeHg production in mid-Atlantic coastal sediments. Sediment cores were collected from seven stations in the Chesapeake Bay and mid-Atlantic continental shelf and slope during the spring, summer and fall of 2005 and 2006, and analyzed for Hg, MeHg, and ancillary parameters in sediment and porewater. In addition, mercury methylation, sulfate reduction and carbon dioxide and methane production rates were determined. Organic matter strongly controls inorganic Hg concentration and partitioning between the solid-phase and porewater, with the sandy sediment of the lower Bay and shelf having lower Hg solid-phase concentrations and partition coefficients and relatively similar Hg porewater concentrations. MeHg concentrations and production rates in estuarine, shelf and slope sediments were comparable with those found in many freshwater systems, and controlled by similar mechanisms. Results suggest organic matter quantity and quality, sulfide and bioturbation strongly affect MeHg production in these sediments. Zones of relatively low dissolved sulfide concentration, but high microbial sulfate reduction rates, supported high rates of net MeHg production. These zones were most common in spring, and in sandy sediments of the lower Bay and shelf and heavily bioturbated sediments of the slope. Preliminary diffusive flux calculations based on porewater data provide evidence that estuarine, shelf and slope sediments are all potential sources of MeHg to coastal waters, and reinforce the idea that offshore coastal sediments may be important sources of MeHg to coastal and open-water food webs.