Project Summary Sheet

Project: Integrated Biogeochemical Studies in the Everglades: Task 2 - Mercury Cycling and Bioaccumulation

Web Sites: http://sofis.usgs.gov; http://wi.usgs.er.gov

Location (Subregion & County): Total Everglades Ecosystem; Palm Beach, Broward, and Miami-Dade Counties; Florida Bay.

Funding (Source): USGS Place-Based Studies

Principal Investigator(s): David P. Krabbenhoft (dpkrabbe@usgs.gov, 608-821-3843)

Project Personnel: David P. Krabbenhoft (dpkrabbe@usgs.gov, 608-821-3843), Mark L. Olson (mlolson@usgs.gov, 608-821-3878), John F. DeWild (jfdewild@usgs.gov, 608-821-3843), Shane D. Olund (sdolund@usgs.gov, 608-821-3844)

Supporting Organizations: USGS, FLDEP, USFWS-Loxahatchee NWR, SFWMD, ENP, USEPA

Associated / Linked Projects: Integrated Biogeochemical Studies in the Everglades: Task 1 - Nutrients, Sulfur, and Organic Matter, William H Orem (borem@usgs.gov, 703.648.6273); TIMES Project, Raymond Shafranek (rws@usgs.gov); Groundwater Hydrology of the Everglades (Judson Harvey, jwharvey@usgs.gov, 703-648-5876)

Overview & Status: This project seeks to provide a more general understanding of water quality issues and driving forces (chemical, physical, hydrological, and biological) that control water quality in both the spatial and temporal dimensions in the south Florida ecosystem. Mercury cycling is an extremely complex issue that results from the intersections of an air-source pollutant (Hg) and biochemical transformations (methylmercury formation) that are regulated by natural microbiological communities but that can be exacerbated by other co-contaminates (e.g., sulfate, carbon, and phosphate). Previous work by these investigators under the ACME project demonstrated that factors control the relative amount of net methylmercury production in the Everglades, including changes in external loading and internal recycling of mercury, sulfate, and carbon in time and space. In addition, we demonstrated hydroperiod maintenance and natural wetting and drying cycles have a substantial influence on methylmercury production and bioaccumulation. All of these findings have direct implications for the Everglades Restoration effort by drawing lines connecting manageable (at least partially) exogenous factors (e.g., sulfate, phosphate, and dissolved organic carbon routing) to engineering plans (water depth, duration and flushing rates) leading to the production of a potent toxic chemical (methylmercury) and exposure to wildlife. Our ACME project studies also demonstrated many previously undiscovered aspects of the mercury cycle and the biogeochemistry of the Everglades more generally. For example, we showed that the central region of Everglades (north central WCA-3A) has the optimal conditions for methylmercury formation with moderate levels of sulfate and dissolved organic carbon, yet still retaining the oligotrophic conditions that appear to maximize bioaccumulation rates. Other major findings were: (1) almost all aspects of the mercury cycle, as well as many other biogeochemical processes in the Everglades, demonstrate very strong diel cycles; (2) photochemical processes play a central role in determining net standing pools of methyl and inorganic mercury; (3) natural wetting and drying cycles (or those regulated by man) can greatly exacerbate net methylmercury formation and bioaccumulation; and (4) mercury levels in native fish are strongly tied to the levels of methylmercury observed in sediments, which is contrary to most other studies where water borne methylmercury has been observed to be the strongest co-variant. Our next phases of investigations include the use of wetland enclosures (mesocosms), and laboratory studies (microcosms) to provide direct quantitative estimates of the relative importance of each of our previously determined co-factors in methylmercury production and bioaccumulation.

Needs & Products: ACME provided much of the essential information for the development of the Everglades Mercury Cycling Model, which has already been used for providing estimates of ecosystem response to possible management scenarios for the Everglades, and the first TMDL ever attempted for mercury by the USEPA. State and Federal agency managers currently need more detailed and quantitative estimates of the relative importance of the various cofactors contributing to methylmercury formation, such that responsible management actions can be taken to mitigate the mercury problem in the Everglades. Our research efforts have provided essential information that has lead to new fundamental understandings of the Everglades including those directly relating to restoration plans. Products include numerous refereed journal articles, USGS reports, synopsis reports, graduate Ph.D. thesis, database, geochemical maps, and presentations of results.

Application to Everglades Restoration: Our results have provided direct connections between a significant contamination issue (mercury bioaccumulation) and other issues prominent to the Restoration process, including effects of agricultural runoff, exacerbative and mitigative effects of water-treament wetland (STA’s) construction, the need to consider airshed management, and the direct impact of hydrological manipulations and/or changes will potentially have on mercury exposure to native wildlife and humans. The project findings provide CERP and GEER information that is fundamental to our understanding of how the Everglades operate biogeochemically, and how it responds to external stresses, and relate directly to management decisions. In addition, with the use of in situ mesocoms and dosing studies and minicosm incubation experiments, we are currently deriving direct quantitative information relating ecosystem stressors and methylmercury production and bioaccumulation that can be used for decision-making on water quality and competing issues, and estimate ecological recovery times.