David P. Krabbenhoft¹, William H. Orem², George R. Aiken³ and Cynthia C. Gilmour^4
1U.S. Geological Survey, Water Resources Discipline, Middleton, WI
²U.S. Geological Survey, National Center, Reston, VA
³U.S. Geological Survey, Water Resources Discipline, Boulder, CO
⁴Smithsonian Environmental Research Center, Edgewater, MD

The Florida Everglades has been home to many of the most significant advances in understanding of the factors controlling mercury (Hg) contamination of aquatic ecosystems over the past decade. In the early to mid 1990’s, Hg concentrations in game fish from the Everglades were some of the highest reported in the literature, and a consumption warning for humans was posted for the entire ecosystem. At that time, there was some speculation that elevated Hg levels in biota were a “natural condition” of the Everglades, owing to the fact it is a subtropical wetland, which are known to promote the genesis of methylmercury (MeHg), the most toxic and bioaccumulative form of Hg in the environment. In the case of south Florida, where the largest ecosystem restoration effort was being planned, additional questions were raised, including: (1) is this problem solely the result of atmospheric Hg inputs; (2) are there other factors, such as land management and land use practices that are also contributing to the problem; and (3) what role the restoration (present and future) play in mitigating or exacerbating this problem. For the past ten years, a multi-agency team of researchers on the Aquatic Cycling of Mercury in the Everglades (ACME) project have been evaluating these difficult questions, and is now at the point where definitive answers can be delivered to decision makers who are responsible for the Everglades restoration program.

Results from the ACME study have clearly shown links between MeHg abundance and several key ecosystem factors, including: atmospheric Hg loading, hydroperiod maintenance, sulfate loading from Everglades Agricultural Area (EAA) runoff, and dissolved organic carbon (DOC) levels in surface water. Of these factors, only atmospheric Hg loading will not be affected by the restoration effort, and our results clearly show that decisions regarding possible water delivery and land use changes in the Everglades are equally, if not more, important in controlling MeHg levels now and in the future. A long-term record of MeHg at a site in central Water Conservation Area 3A revealed a sharp decline in MeHg levels (>90%) since about 2000, which was concurrent with a commensurate decline in sulfate. These results challenge other recent studies concluding that declines in fish Hg levels are the result of reductions in Hg deposition. Controlled field dosing experiments have confirmed this observation, and challenge the assumption that this ecosystem is naturally high in MeHg, and suggest that changes to water quality and water flows from the restoration will have a great influence on MeHg exposure levels to indigenous wildlife and humans in south Florida.

Contact Information: David P. Krabbenhoft, USGS, 8505 Research Way, Middleton, WI 53562, Phone: 608-821-3843, Fax: 608-821-3817, Email: dpkrabbe@usgs.gov

(This abstract is from the 2006 Greater Everglades Ecosystem Restoration Conference.)

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