geer > 2000 > poster > aquatic cycling of mercury in the everglades (acme) project: synopsis of phase I studies and plans for phase II studies
Aquatic Cycling of Mercury in the Everglades (ACME) Project: Synopsis of Phase I Studies and Plans for Phase II StudiesPoster presented December 2000, at the Greater Everglades Ecosystem Restoration Conference David P. Krabbenhoft, Mark L. Olson, John DeWild (U.S. Geological Survey, Middleton, WI), Cynthia C. Gilmour (Academy of Natural Sciences, Benedict Estuarine Laboratory, St. Leonard, MD), William H. Orem (U.S. Geological Survey, Reston, VA), George R. Aiken (U.S. Geological Survey, Boulder, CO), Carol Kendall (U.S. Geological Survey, Menlo Park, CA) [ Disclaimer ] Abstract
Synopsis Phase I Studies (1995-1998)The precise underpinnings of why wetlands, such as the Everglades, are susceptible to methylmercury production and bioaccumulation are not completely understood. High rates of microbial MeHg production in anaerobic, organic-rich sediments and advective flows of nutrient bearing water are probable causes. Previous work by the ACME project has revealed that mercury (Hg) and methylmercury (MeHg) distributions in water, sediment and biota show complex seasonal and spatial trends, and that the cycling rates of Hg and MeHg are so rapid that many measurements need to be conducted on a diel basis. In addition, in situ microbial processes and photochemical processes control MeHg levels at the ecosystem level. Mercury loads to the Everglades are dominantly derived from atmospheric sources, but toxicity is largely controlled by the relative rates of conversion to methylmercury, which in turn appears to be intimately associated with other complex biogeochemical cycles, such as the sulfate/sulfide cycle. Specific key finding of the ACME project are the following:
Plans Phase II Studies (1999-2003)In order to provide predictive capabilities of the potential effects of Everglades restoration efforts on Hg cycling, the complex relationships between loadings of Hg, sulfur, and nutrients on MeHg production and bioaccumulation need to be better understood. In the next phase of this research, we propose better quantify these individual relationships, and the interactions among these three key parameters, through amendments to in situ mesocosms. Short-term addition experiments are useful in examining processes, but may not predict long-term responses, for a number of reasons. Response of plant growth to nutrients is the obvious example, but other changes, like changes in Hg speciation and bioavailability over time, or development of microbial communities, are also important. An understanding of the relationship between Hg, S or nutrient loading and MeHg production and bioaccumulation requires a long-term, large-scale approach because there are many steps between the entry of "new" Hg to the ecosystem, its conversion to MeHg, and bioaccumulation in the foodweb. We propose to use stable Hg isotope amendments to examine the relationship between Hg loading and MeHg production and bioaccumulation. This new approach will allow us to track the fate of newly deposited Hg separately from the larger existing pools, and to track the bioavailability of new Hg over time. The use of individual stable Hg isotopes will allow us to follow the cycle of new Hg added to the system, from initial partitioning, accumulation in vegetation, MeHg production and accumulation in sediments, fluxes and accumulation in the food web. We will also be able to trace burial, post-depositional reworking of Hg through sediments and plants, and Hg0 formation. Important unknowns that stable isotopes will allow us to address are the availability of Hg in decaying plant material relative to newly deposited Hg adsorbed to sediments for methylation; and the recycling of buried Hg to the sediment surface through plant growth and decay. We also propose to make sulfur amendments as either a stable or radioisotope for the same reason. Sulfur isotope additions will allow us to find out how fast sulfide is turned over is sediments and made available again for sulfate reduction (through photosynthetic re-oxidation of sulfide), a key variable in modeling the relationship between sulfate load, sulfate reduction rate and sulfide accumulation in the oligotrophic Everglades. Isotopes will also allow us to track what fraction of newly added sulfate is retained in sediments.
Averages for Primary ACME Sites
Everglades Transect: July '97 Surface (0.4 cm) Sediment
Mercury and methylmercury have one of, if not the, most complicated environmental cycles of any known compound or element. This complex story is intensified by the fact that they are both "photo sensitive", or react with sunlight.
MeHg Uptake Pathways for Gambusia
Predicted Ecosystem Response to Hg loading
Click here for a printable version of this poster (note: document will open in a new browser window) For more information contact: David Krabbenhoft, U.S. Geological Survey, 8505 Research Way, Middleton, WI, 53562, Phone: 608-821-3843, Fax: 608-821-3817, dpkrabbe@usgs.gov Related information: SOFIA Project: Aquatic Cycling of Mercury in the Everglades [ Disclaimer ] |
U.S. Department of the Interior, U.S. Geological Survey
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Last updated: 22 December, 2004 @ 07:41 AM (KP)