Biogeochemistry of methylmercury in sediments of Long Island Sound

Cycling of toxic monomethylmercury (MMHg) in biologically productive coastal marine systems is affected by geochemical and macro/microbiological factors that control its production and mobilization from sediment. We examined the biogeochemistry of MMHg in sediment at three sites in Long Island Sound (LIS) in March, June, and August. Bioturbation homogenizes profiles of inorganic Hg (Hg(II)=total Hg−MMHg) in the upper 10 cm of sediment, whereas MMHg is enriched in surface material, indicating rapid and dynamic cycling of the toxic species. Sediment–water partitioning of MMHg and Hg(II) is controlled largely by sedimentary organic matter. Distribution coefficients (KD; l kg−1) of MMHg and Hg(II) are correlated with those for organic carbon, which suggests that adsorption/desorption of Hg species with sediment particles is mediated by partitioning of associated organic ligands. A fraction of MMHg and Hg(II) on sediment particles is associated with Fe complexes, presumably oxyhydroxides. Bioturbation enhances Hg methylation; subsurface peaks in 200Hg methylation potentials coincide with anomalies in a bioturbation index (d[organic matter]/dz) that indicate non-local disturbance of sediment. The estimated diffusive sediment–water flux of MMHg in LIS (55±20 mol year−1) exceeds inputs from external sources. This indicates that in situ production is the major source of MMHg to LIS, and by extension, other comparable near-shore systems. Furthermore, sedimentary methylation can account for most of the MMHg in primary producers of LIS. Hence, through dietary bioaccumulation, much of the MMHg in higher trophic levels within the Sound may be attributed to MMHg production in the sediments.