Mercury (Hg) contamination of the Everglades ecosystem is one of the most severe cases in the published literature. Currently, no human consumption of any Everglades sport fish is recommended. Although it is widely recognized that mercury contamination of aquatic ecosystems is largely the result of atmospheric mercury emissions, long-range transport and subsequent deposition, providing a scientific understanding of this problem with the necessary detail to prescribe appropriate corrective measures has remained unclear. From 1995 to 1999, the Aquatic Cycling of Mercury in the Everglades (ACME) project studied the biogeochemical cycling of Hg in detail at a series of sites spanning most of the north-to-south extent of the Everglades, and many of the major sub-ecosystem types. The ACME project revealed that mercury 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 must be conducted on a diel basis in order to provide an adequate understanding of the controlling factors. These studies revealed relations between biogeochemical factors and the production rate of MeHg, which is the most bioaccumulative and toxic form of mercury, and thus is central to understanding the mercury problem in the Everglades and elsewhere. Specifically, through field experiments and laboratory studies, we established links between MeHg production and several key ecosystem factors, including: hydroperiod, atmospheric Hg loading, sulfate loading from Everglades Agricultural Area (EAA) runoff, and dissolved organic carbon (DOC) levels in surface water. However, because all these driving factors of MeHg production co-vary spatially across our study sites, definitive quantitative assessments of which parameters are most important for limiting future methylmercury production remained unclear. In addition, because all of these factors are likely to be altered by the Everglades Restoration Project, a more “controlled” experimental approach was developed to estimate how this ambitious project might affect mercury toxicity for the Everglades in the future.

One approach for sorting out the complex responses of MeHg formation to alterations in critical water quality constituents (Hg, sulfate, phosphate and DOC) is through the use of in situ mesocosms (or wetland enclosures), in which dosing studies can be conducted. Such an approach has the advantage of maintaining many of the qualities of natural ecosystems that are very difficult to replicate in a laboratory setting, such as soil structure, redox, rainfall inputs, diel temperature and light cycles, as well as indigenous flora and fauna. On the other hand, mesocosms do potentially suffer from so called “wall” effects, and thus strict monitoring of the enclosed and native environment must be maintained to validate results. Starting in May 2000, the ACME Phase II project initiated mesocosm experiments at four of our long-term study sites (F1, U3, 2BS, and 3A15) located in Water Conservation Areas (WCA) 2A, 2B and 3A. Initial experiments were limited only to mercury dosing to quantify the ecosystem response to changes in mercury loading. At each site, we added mercury to mesocosms at 0.5, 1.0, and 2.0 times the current ambient loading rate (about 22 µg/m²/y), and dedicated two mesocosms as experimental controls. At each site we also constructed clear plastic “roofs” for two mesocosms, which excluded mercury deposition from contemporary rainfall, but allowed for exchange of air and light to promote algal and plant productivity.

To distinguish between “new” and “old” (previously existing) mercury, mesocosm dosing was conducted using stable isotopes of mercury (e.g., ²⁰¹Hg, ²⁰²Hg), which can be distinguished from ambient, mercury using a mass spectrometer. The examination of new versus old mercury was also tested by adding multiple Hg doses in the same mesocosms but, at different times, and using different isotopes for each addition. It had been hypothesized that the mercury contamination problem may continue for very long periods of time if existing mercury pollution in soils and sediment sustained the mercury cycle. Results of these types of experiments, therefore, are significant for assessing the potential effectiveness of mercury emissions reductions on mercury cycling in the Everglades.

Results from the mercury dosing experiment revealed several important findings. First, there is a positive and linear relation between mercury added and the production of MeHg in the Everglades. Second, there is an exceptionally close tie between mercury added and bioaccumulation of the added mercury in fish (Gambusia); the relation has a coefficient of determination greater than 0.9. Last, there is an “aging effect” for new mercury added to the ecosystem, such that more recent doses of mercury isotopes are more likely to be bioaccumulated than older mercury. All these results support the conclusion that recent mercury additions are proportionally more responsible for sustaining mercury exposure to wildlife and humans in south Florida, and any attempts to reduce current loading rates would likely have rapid and positive effects.

In 2001 we expanded the experimental design of our mesocosm experiments to examine the effects of sulfate, phosphate, and DOC dosing on mercury cycling. In addition, a new experimental site in Loxahatchee National Wildlife Refuge (WCA1) was added. Because DOC and sulfate levels in WCA2 and WCA2B are elevated due to runoff from the EAA, sulfate and DOC dosing could only be performed at our sites in WCA 1 and 3, where more pristine conditions exist. Also, because ecosystems can require extended periods of time to achieve a new equilibrium after the addition of phosphate, phosphate-dosing mesocosms previously established by researchers from the South Florida Water Management District were accessed and sampled. Lastly, in some mesocosms, we conducted mixed addition experiments (mercury and sulfate, and mercury and DOC) to test for possible synergistic or antagonist effects of co-dosing.

The mercury, sulfate, DOC, and phosphate addition experiments revealed several novel observations. Surprisingly, the addition of DOC alone stimulated the production of additional MeHg from “old” mercury in sediments. In the mixed DOC plus mercury addition experiments we observed greatly elevated methylation of the added mercury isotope, about 4 to 8 times greater than when mercury alone was added. These results suggest DOC is directly involved in the methylation process, rather than the common assumption that DOC is simply an attractive ligand for mercury in aqueous solution. Finally, when compared against the mercury-only dosed mesocosms, the mixed sulfate plus mercury mesocosms, yielded significant additional isotopically-labeled MeHg. However, high sulfate dosing levels gave rise to lower overall MeHg formation, which is consistent with our previous field observations in WCA2 where very high levels of sulfate contamination exist and these high levels reduce mercury methylation rates by sulfide inhibition. Lastly, no clear trend could be observed between phosphate dosing level and MeHg formation and accumulation.

Contact: David P. Krabbenhoft, U.S. Geological Survey, 8505 Research Way, Middleton, WI 53562 Phone: 608-821-3843, FAX: 608-821-3843, dpkrabbe@usgs.gov

(This abstract was taken from the Greater Everglades Ecosystem Restoration (GEER) Open File Report 03-54)

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