HR: 17:00h
AN: B14B-04    [Abstracts]
TI: Direct and Indirect Effects of Vegetation on Methylmercury Production in Wetlands as Assessed by Experimental Plant Removal
AU: * Windham-Myers, L
EM: lwindham@usgs.gov
AF: United States Geological Survey, 345 Middlefield Road, MS480, Menlo Park, CA 94025, United States
AU: Marvin-DiPasquale, M
EM: mmarvin@usgs.gov
AF: United States Geological Survey, 345 Middlefield Road, MS480, Menlo Park, CA 94025, United States
AB: Although vegetated wetlands are among the most active habitats for microbial methylmercury (MeHg) production, the relative influence of wetland vegetation itself is poorly understood. Plant physiology and biomass (live and dead) can modify both microbial populations and inorganic mercury (Hg(II)) bioavailability through a number of soil, water and atmospheric interactions. Alternatively, plant activity and structure can be simply a response to geochemical conditions that also favor Hg(II)-methylation. Linked studies within the San Francisco Bay watershed have demonstrated that habitat-specific biogeochemical characteristics are the dominant factors controlling MeHg production, and that differences in wetland plant biomass, root density, decomposition rates, can directly influence sediment mercury cycling. A vegetated:de-vegetated paired plot approach was used to directly assess the influence of live plant activities on surface sediment mercury dynamics and associated biogeochemistry in differing wetland settings: salt marshes, permanent and seasonal freshwater wetlands, a freshwater floodplain, and agricultural rice fields. Although results from several of these subhabitats are pending, the data thus far have illustrated linkages between wetland plants and microbial Hg(II)-methylation. De- vegetation strongly influenced sediment biogeochemistry (e.g. redox, dissolved organic content, and reduced sulfur pools) in high interior pickleweed (Sarcocornia pacifica) dominated saltmarshes, where the high rates of MeHg production (up to 1 ng g^-1_{dry sed} d^-1) observed in vegetated plots were reduced to <10 pg g^-1_{dry sed} d^-1 in de-vegetated plots. Further, plant root densities were positively correlated with the activity of Hg(II)-methylating bacteria in these interior saltmarsh settings. The pool size of mercury available for methylation ("reactive mercury") was not measurably influenced by this short-term de-vegetation experiment, but across field studies, rhizosphere biomass was often negatively correlated with reactive mercury concentration due to a corresponding increase in solid-phase reduced-sulfur compounds associated with this zone. Because mercury methylation is controlled by both the reactive mercury pool size and the microbial Hg(II)-methylation activity, the direct influence of wetland plants on both of these terms can be profound and reflect multiple, and potentially contrasting, mercury cycling pathways. Experimental field manipulations, in conjunction with comparative habitat and process studies, represent essential tools to elucidate the influence of wetland plant communities on Hg cycling.
DE: 0422 Bio-optics
DE: 0461 Metals
DE: 0476 Plant ecology (1851)
DE: 0488 Sulfur cycling
DE: 0497 Wetlands (1890)
SC: Biogeosciences [B]
MN: 2007 Fall Meeting