In 1995, the U.S. Geological Survey, in cooperation with several State, Federal and private agencies, initiated the project Aquatic Cycling of Mercury in the Everglades (ACME) to evaluate mercury (Hg) cycling processes. Anthropogenic activities have affected hydrology, nutrient status and the quality of atmospheric deposition in the Everglades. In the late 1980's, an analysis of game fish from across Florida revealed elevated levels of Hg in fish statewide, with some of the highest levels (> 1.5 mg g-1) in the Everglades. In response, a fish consumption advisory was issued for almost the entire Everglades system. The ACME project was designed to evaluate the factors responsible for Hg bioaccumulation in this unique ecosystem. The specific goals of the ACME project are to evaluate fluxes, rates and factors affecting Hg transport through the Everglades wetland system. The project has two phases which run simultaneously. The first phase evaluates seasonal distributions of Hg species in the aqueous phase at fixed canal and marsh sites in the northern Everglades. Our sampling frequency precludes a detailed analysis of season cycling at a given site, however, data from water column sampling at these sites aids in identifying contrasting sites for detailed process-level studies. Our initial goals were to evaluate the forms and species of Hg entering Water Conservation Area 2A (WCA-2A) from canal inputs and to identify trends in Hg distribution across the eutrophication gradient in WCA-2A. We further compare the eutrophied northern canal system to a less impacted southern canal system. Next, we evaluate aqueous Hg dynamics in the marsh system by comparing nutrient-impacted to unimpacted sites (including sites in WCA 2A and WCA-3A). Finally, we examine the relationship of Hg with dissolved organic carbon of this subtropical wetland to that of a northern temperate wetland system.

Our observations of aqueous Hg speciation in the Northern Everglades canals and marshes reveal strong spatial and seasonal patterns. These observed trends in aqueous speciation of Hg are a result of numerous formative, degradative and bioaccumulative processes. In canals feeding WCA-2A, HgT associated with soil-derived particles from EAA presumably settle from the system. Reduced flows and stagnation of waters within canals lead to increases in both MeHgU and particle-associated MeHg. Unfiltered HgT in surface water entering the marsh of WCA-2A ranged from about 1 to 1.5 ng L-1 while MeHgF exhibited a sixfold increase from winter to summer.

Transects across the marshes of WCA-2A, coupled with measurements in WCA-2B and WCA-3A reveal no apparent north to south trends in HgTU. Unfiltered MeHg, however, does appear to show enrichment at southern sites for individual sampling periods. Additional analyses of HgR and DGM also suggest greater reactivity of aqueous Hg species at the southern sites. The strong relationship of HgTF and MeHgF observed in northern Wisconsin is not apparent, suggesting differences in DOC type between regions. These general observations on aqueous trends in Hg speciation in the Northern Everglades form the basis for process-designed studies. Our future efforts are directed at obtaining a better understanding of speciation, partitioning, methylation and the temporal extent of biotransformation and bioaccumulation of Hg in this unique environmentally-sensitive ecosystem.

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