Little is known about the sources and cycling of monomethylmercury (MMHg) in the marine environment. Continental shelf and slope sediments appear to be a potential source of MMHg. For this reason, investigators from the University of Connecticut and Wright State University, plan to further our understanding of the biogeochemical processes and reactions affecting MMHg production and mobilization from sediments on the continental shelf and slope of the northwestern Atlantic Ocean. To attain their goal, the scientists propose to carry out field and laboratory experiments during three cruises to this region, one in late summer when sedimentary mercury (Hg) methylation is optimal and the water column is stratified and two cruises in late winter/early spring when the water column in well mixed and MMHg production is reduced. Results would be used to test the following hypotheses: (1) MMHg production in sediments is dependent on the availability of Hg(II) substrate, which is a function of Hg loadings, organic matter, sulfur chemistry, and activity of methylating bacteria; (2) vertical and horizontal distributions of MMHg in the water column would be indicative of in situ sedimentary production in the summer, whereas limited vertical and horizontal segregation would occur in the winter; (3) sulfate reducing bacteria are the principal functional group of microorganisms producing MMHg in sediments on the continental shelf/slope; (4) water column methylation is not a major source of MMHg in continental shelf waters; and (5) MMHg bioaccumulates and biomagnifies in planktonic food webs on the shelf.
As regards broader impacts, knowledge on the role of continental shelf/slope sediments in the production of methyl mercury fills an important gap in our current understanding of the biogeochemistry of mercury. One graduate and one undergraduate student from the University of Connecticut and one graduate and one undergraduate student from Wright State University would be supported and trained as part of this project.
Methylmercury accumulating in wildlife is the major public health concern associated with mercury in the environment. Not only is methylmercury quite toxic but it can be produced and bioaccumulate within aqueous systems, especially marine. Indeed, consumption of seafood has resulted in about 5% of American women of childbearing age having blood methylmercury levels that pose an increased risk for fetal neurodevelopment problems. Knowledge about the behavior and fate of mercury and methylmercury in the oceans is limited. The major sources of methylmercury to the ocean and its biota are neither well quantified nor are the vital marine biogeochemical reactions leading to its synthesis well understood. Key questions include whether internal or external sources are more important and what is the impact of human-related mercury contamination. Co-investigators William Fitzgerald and Chad Hammerschmidt and their students have been studying the production, distribution, and fate of mercury and its methylated chemical forms in sediment, water, and plankton on the continental margin of the northwest Atlantic Ocean. Their prior research and that of others have pointed to mercury methylation in coastal sediments and oceanic water column production at depths shallower than 1000m as the major sources of methylmercury to marine ecosystems. Methylmercury formation in biologically productive coastal areas is likely the leading source for its presence in marine fish. This NSF-Chemical Oceanography Program supported study of the marine biogeochemical cycling of mercury is the most comprehensive of its kind and included three major oceanographic expeditions. Collaborative efforts at sea and in the lab have provided new information and insights as to how methylmercury is produced in sediments, the rates at which it is transferred to overlying water, and how it is transformed by photochemical and microbiological processes before its accumulation into the base of the marine food web. This research shows that mercury deposited to sediments on the continental shelf and slope is being transformed to methylmercury by sulfate-reducing bacteria. Moreover, much of the methylmercury escapes the sediments to overlying ocean waters where it is transported both vertically and horizontally to be either destroyed or taken in by plankton and passed throughout the food web to include humans who consume seafood. Discoveries and well constrained measurements by these research teams also show pathways for the synthesis and mobilization of dimethylmercury in sediments and the water column. Not only are methyl and dimethylmercury produced in sediments on the continental margin and mobilized through benthic transport processes, but they are produced in the water column in zones coinciding with lower oxygen levels (e.g., minimum regions). These scientists have reported that dimethylmercury generally follows the distributional patterns for methylmercury in sediments and seawater. This fundamental finding suggests a causal link and potential important precursor role for dimethylmercury in the production of methylmercury. Increased knowledge of biogeochemical processes affecting the production, destruction, and uptake of methylmercury is required to manage and diminish the impact of mercury contamination. This project has shown quantitatively that methylmercury and dimethylmercury are formed on the continental margin and in the upper ocean in regions affected by pollution-borne mercury. This implies that reductions of mercury inputs to the continental margin and open ocean, mostly from human sources via atmospheric deposition, would result is less methylmercury production and accumulation in fish. Comparable sedimentary production and mobilization of methyl and dimethylmercury is likely in other shelf and slope regions and can represent an important source of methylated Hg to the oceans. Indeed, and regardless of the sources, Hg substrates and the methylated species at depths shallower than 1000 meters will have an anthropogenic component.
