The biogeochemical cycling of methylmercury in coastal marine sediments

Humans are exposed to toxic monomethylmercury (MMHg) principally by consumption of marine fish; however, little is known about the in situ production, biogeochemistry, and bioaccumulation of MMHg in biologically productive coastal marine ecosystems. This dissertation is focused on the mechanisms and processes affecting the cycling of MMHg in near-shore sediments. It is hypothesized that in situ sedimentary production and mobilization is the primary source of MMHg in coastal marine systems and biota, and that the synthesis and distribution of MMHg in sediments is controlled by five interrelated biogeochemical factors; inorganic Hg, organic matter, sulfide, bioturbation, and activity of methylating bacteria. Long Island Sound (LIS), the continental shelf of southern New England, and New York/New Jersey Harbor were selected as the primary study locations because they have diverse and contrasting ranges of these sedimentary constituents and are representative of other near-shore deposits. Biogeochemical investigations at these locations are complemented by an examination of MMHg bioaccumulation and its abiotic/artifactual production in LIS, and by related mechanistic studies of MMHg cycling in Arctic lakes. There is excess methylating potential in near-shore sediments, and MMHg production is limited by the availability of inorganic Hg (Hg(II) = total Hg-MMHg) to methylating bacteria. Potential rates of Hg methylation are related positively to the level of Hg(II), mostly as HgS0, in pore waters. Sulfide controls the speciation of dissolved Hg(II) complexes, and sedimentary organic matter influences the sediment-water partitioning of Hg(II). Bioturbation enhances Hg methylation. Mobilization from sediments is a major source of MMHg to coastal marine systems, and possibly the open ocean through hydrological and/or biological transport. Bioaccumulation is the principal sink for MMHg. These results suggest that anthropogenic loadings of Hg to the coastal zone during the past 150 y should have enhanced MMHg production. However, coincident increases in sedimentary loadings of organic matter, mostly associated with eutrophication, may have attenuated its synthesis. It is evident, however, that a large potential for MMHg production exists in near-shore and shelf sediments, and that these deposits may be a major source of MMHg in marine fish, including those consumed by humans.