Investigating the Importance of the Coastal Zone as a Source of Methylmercury to the Ocean

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ABSTRACTOCE-0351050Mercury (Hg) is released to the environment from natural and anthropogenic sources and inputs have increased substantially this century, and have increased Hg deposition to the ocean, the dominant Hg input, by a factor of three. However, the evidence that the concentration of Hg, mostly as methylmercury (CH3Hg), in ocean fish has increased in concert is contradictory. Indeed, in most ecosystems studied to date, the relationship between Hg input and CH3Hg production is weak, suggesting that many factors influence the rate of Hg methylation. Researchers at the Chesapeake Biological Laboratory and the Benedict Estuarine Research Laboratory have been studying these interactions and relationships in freshwater and estuarine systems, and in this project they will extend investigations to the coastal ocean under this proposal.While prior ocean studies have focused on the air-water exchange, and on ocean distributions and biogeochemistry, the coastal zone has been relatively little studied even though it plays an integral role in the global Hg cycle. The researchers hypothesize that it is a potential source of methylated Hg to the ocean. A preliminary budget for CH3Hg in the ocean indicates that net in situ production is the most important source. It is generally accepted that sulfate- reducing bacteria are the primary Hg methylators in the environment and thus sediments under highly productive coastal waters, where sulfate-reduction is the main mineralization mechanism, should be important locations for Hg methylation. The researchers surmise that because of the interaction between Fe and S in these sediments, Hg bioavailability to methylating bacteria will not be effected by high sulfide concentration, which has been shown to inhibit Hg methylation. The primary working hypothesis is, therefore, that the coastal zone and the land-ocean margin are important sites of Hg methylation, are net sources of CH3Hg to the open ocean, and contribute an important fraction of the global ocean CH3Hg budget. Furthermore, the researchers propose that most of this CH3Hg is derived from in situ bacterial methylation within coastal and shelf sediments. The primary objectives are therefore to measure the concentration and speciation of total Hg and CH3Hg, and ancillary parameters (organic carbon, Fe, Mn, S) in the dissolved and solid phase, and with depth, in sediments collected from sites ranging from the upper Chesapeake Bay to the outer reaches of the shelf of the Mid-Atlantic Bight. In addition, researcher team will determine the relative rate of methylation and demethylation in these sediments, spatially and temporally, and investigate relationships between these measures and Hg speciation, sediment biogeochemistry and microbial activity. The distribution (speciation) of Hg in the solid phase will be examined using sequential extraction approaches. The effect of Hg speciation in the dissolved and solid phase on Hg methylation will also be examined.