Wetlands Ecology and Management 9: 295–310, 2001. © 2001 Kluwer Academic Publishers. Printed in the Netherlands. 295 Biogeochemical cycling of methylmercury at Barn Island Salt Marsh, Stonington, CT, USA C.S. Langer, W.F. Fitzgerald ∗ , P.T. Visscher & G.M. Vandal The University of Connecticut, Department of Marine Sciences, Groton, CT 06340, U.S.A.; ∗ author for correspondence: E-mail: wfitzger@uconnvm.uconn.edu Key words: methylation, methylmercury fluxes, pore waters, radiotracers, salt marshes, sediments Abstract Monomethylmercury (MMHg) is toxic, and is the primary form of Hg that bioaccumulates in the food web. An understanding of its distribution, production, and transport is needed. Prior investigations indicate that methylation is mediated by sulfate-reducing bacteria, yet limited in high sulfate environments. High rates of microbial respir- ation and strong oxygen gradients 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 survey of Barn Island Salt Marsh in October 1996 showed that ca. 61 pmol m −2 d −1 of dissolved MMHg were discharged to adjacent coastal waters, while 16 pmol m −2 d −1 of particulate MMHg were entrained in the marsh, implying an in situ source. In-sediment MMHg production rates were determined by 203 Hg radiotracer studies. At the surface, methylation rates varied over both long (i.e., 100’s m; 11–1120 pmol m −2 d −1 ) and short (i.e., 10 cm; 11–108 pmol m −2 d −1 ) spatial scales. Methylation rate profiles from both low and high MMHg production sites exhibited an exponential decrease below the redox transition zone. Pore water was collected with multi-chambered in situ dialysis (30 kDa) samplers [Peepers] and analyzed for MMHg. Temporal differences in pore water MMHg accumulation (i.e., May > September > November) were found. Results from May showed a significant gradient at the sediment water interface. The transport out of the sediments estimated by Fick’s Law (ca. 390 pmol MMHg m-2 d −1 ) suggests that MMHg entered the marsh water by diffusion. This work demonstrates the potential for elevated in situ Hg methylation in high sulfate environments. Introduction The partitioning of Hg into various species (e.g., MMHg, Hg 0 , Hg 2+ , or dimethylmercury) is important because the relative distribution of the different forms controls the bioavailability and affects the transport of Hg in the environment (Jackson, 1997). Mercury species are involved in a wide range of inorganic and organic transformations (e.g., methylation, demethyl- ation, oxidation, reduction, adsorption, precipitation, etc.) that may occur under varying chemical (e.g., pH, dissolved oxygen, organic carbon), physical (e.g., temperature), and biological (e.g., bacterial activity) conditions. Figure 1 depicts the hypothetical cyc- ling of MMHg in salt marsh sediments. The main source of Hg to the sediments is through settlement of Hg associated with particulate matter. Mercury as a dissolved species and in particulate association can be transformed by a variety of abiotic and bi- otic reactions. The particulate Hg may be methylated directly, sequestered by organic or sulfidic complexa- tion agents, or made more labile. Labile inorganic Hg species (Hg R , operationally defined as the fraction of Hg that is easily reducible upon addition of SnCl 2 ), for example, may be reduced to Hg 0 , sequestered in humic acid complexes, precipitated as insoluble sulf- ides, or methylated. The methylated Hg species can be taken up by organisms; demethylated (Oremland et al., 1991) with Hg(II) converted to Hg 0 (Xiao et al., 1991; Mason et al., 1994) or scavenged onto particulates (Hg P ). The reactions and transformations affiliated with labile Hg and methylated Hg species will be re-