Biogeochemical cycling of methylmercury at Barn Island Salt Marsh,Stonington, CT,USA

Monomethylmercury (MMHg) is toxic, and is the primary form of Hg thatbioaccumulates in the food web. An understanding of its distribution,production, and transport is needed. Prior investigations indicate thatmethylation is mediated by sulfate-reducing bacteria, yet limited in highsulfate environments. High rates of microbial respiration and strong oxygengradients are found in salt marshes. It is hypothesized that significant in situ methylation takes place in the redox transition zone of sulfate rich( 28 mM) salt marsh sediment. Results from a water column surveyof Barn Island Salt Marsh in October 1996 showed that ca. 61pmol m^-2 d^-1 of dissolved MMHg were discharged toadjacent coastal waters, while 16 pmol m^-2 d^-1 ofparticulate MMHg were entrained in the marsh, implying an in situsource. In-sediment MMHg production rates were determined by²⁰³Hg radiotracer studies. At the surface, methylation rates variedover both long (i.e., 100's m; 11–1120 pmol m^-2 d^-1) andshort (i.e., 10 cm; 11–108 pmol m^-2 d^-1) spatial scales. Methylation rate profiles from both low and high MMHg production sitesexhibited an exponential decrease below the redox transition zone. Porewater was collected with multi-chambered in situ dialysis (30 kDa)samplers [Peepers] and analyzed for MMHg. Temporal differences in porewater MMHg accumulation (i.e., May > September > November)were found. Results from May showed a significant gradient at thesediment water interface. The transport out of the sediments estimated byFick's Law (ca. 390 pmol MMHg m-2 d^-1) suggeststhat MMHg entered the marsh water by diffusion. This workdemonstrates the potential for elevated in situ Hg methylation in highsulfate environments.