METHYLMERCURY IN CHESAPEAKE BAY

Over the last 5 years, we have examined a number of the potential sources of methylmercury (MeHg) in and to the Chesapeake, and studied the biogeochemical controls on net MeHg production. Sources of MeHg to the Bay include riverine and marine inputs, plus de novo production in bottom sediments, tidal marshes, anoxic bottom waters, and shallow groundwater. Controls on MeHg production in the estuary were similar to controls observed in freshwater ecosystems – a balance between anaerobic microbial activity, mercury bioavailability and sulfide accumulation. We observed significant MeHg production and accumulation in zones of low to moderate sulfide concentration (generally <50 uM) throughout the Bay. In bottom sediments, a zone of low sulfide can often be found in a shallow depth horizon just below the surface. The anoxic “dead zones” of the Bay supported microbial sulfate reduction and Hg methylation in the water column. We observed the highest methylation rate constants in tidal marshes, where the large supply of organic matter supports high rates of bacterial metabolism, but oxygen pumping by tide and plants roots can minimize sulfide accumulation. Across the sites examined, total Hg and sulfide were the strongest correlates of MeHg concentration and production. Organic matter character, and the bulk phase concentrations of Fe and S minerals were also significant correlates. In tidal marsh soils, methylation occurred in zones of Fe(III) and sulfate reduction. In bottom sediments, microbial iron reduction did not appear to be an important process. Shallow groundwaters/vadose zones can support significant MeHg production if flow paths become anoxic and support iron and sulfate reduction. A revised MeHg budget for the Chesapeake is being constructed from these data. Efflux from bottom sediments to overlying water is the largest source of MeHg to Chesapeake surface waters. Although MeHg production rates are rapid in marsh soils, efflux from marshes to the Bay appears to be a less important source to surface waters than MeHg efflux from bottom sediments. However, MeHg accumulation in marshes has a potentially significant negative impact on marsh-utilizing organisms. The magnitude of MeHg production in anoxic bottom waters may rival MeHg efflux from bottom sediments, and probably exceeds fluvial fluxes of MeHg.