Project Summary Sheet

Study Title: Integrated Biogeochemical Studies in the Everglades: Task 2 - Mercury Cycling and Bioaccumulation
Study Start Date: 10/01/2000 Study End Date: 9/30/2007
Web Sites: http://sofia.usgs.gov; http://infotrek.er.usgs.gov/mercury
Location (Subregions, Counties, Park or Refuge): Total Everglades Ecosystem
Funding Source: USGS Greater Everglades Priority Ecosystems Science (GE PES)
Other Complementary Funding Source(s): USACE CERP/ASR, to conduct an assessment of the potential implications of the Aquifer Storage and Recovery program on mercury toxicity in the south Florida environment. This project is the result of information gained by the ACME project, which showed the multiple biogeochemical factors that can affect mercury cycling, bioaccumulation and toxicity in south Florida.
Principal Investigator(s): David P. Krabbenhoft, dpkrabbe@usgs.gov
Study Personnel: D. Krabbenhoft, M. Olson, J. DeWild, and S. Olund
Supporting Organizations: SFWMD, FLDEP, ENP, USFWS, USEPA
Associated / Linked Studies: (1) Integrated Biogeochemical Studies in the Everglades: Task 1 - Nutrients, Sulfur, and Organics (W.H. Orem, borem@usgs.gov), (2) Interactions of Mercury with Dissolved Organic Carbon in the Everglades (George Aiken, graiken@usgs.gov), (3) Ecological Risk Assessment of Toxic Substances in the South Florida Ecosystem: Wildlife Effects and Exposure Assessment (Tim Gross, tim_s_gross@usgs.gov), (4) The Effects of Water Flow on the Transport of Suspended Particles and Particle-Associated Nutrients in the Everglades (Judson Harvey, jwharvey@usgs.gov)

Overview & Objective(s): This project focuses on mercury contamination of the south Florida ecosystem. Mercury is a sparingly soluble trace metal that is principally derived from atmospheric deposition, and thus affects the entire south Florida ecosystem, as well as aquatic ecosystems worldwide. Unlike most other contaminants, the overall net toxicity of mercury on any ecosystem upon which it deposits is greatly affected by native biogeochemical conditions, which in turn are often affected by land-management activities. Especially important factors include: water chemistry (sulfate, dissolved organic carbon, and pH), hydrology (wetting and drying cycles, flushing rates, sediment-water exchange), and food-web characteristics (trophic position, food-chain length, and introduced exotics). In south Florida, and the Everglades in particular, the two most relevant land-management factors that affect mercury toxicity are surface water chemistry and hydroperiod. Surface water chemistry is greatly affected by contributions from agricultural runoff, and the native hydroperiod of the Everglades has been altered by past compartmentalization and present decompartmentalization. Although this study is being conducted in the south Florida environment, most of the findings and approaches will have general applicability to the broader mercury contamination problem, which is of global extent. Presently, we are addressing several major questions that are confronting mercury researchers globally, and the Everglades Restoration program: (1) What ecological benefit to the Everglades would be realized if reductions in mercury deposition were achieved, and over what time scales? (2) In the present condition, is controlling sulfur or mercury inputs more important for reducing the mercury problem in the Everglades? (3) Does sulfur loading have any additional ecological impacts that have not been realized previously (e.g., toxicity to plant and animals)? The centerpiece of our research continues to be the use of dual approaches that involve detailed natural ecosystem measurements that are paired with in situ experiments conducted in environmental chambers (enclosures or mesocosms). The goal of the mesocosm experiments is to quantify ecological response to our chemical dosing (sulfate, dissolved organic carbon and mercury isotopic tracers), that will be critical for estimating ecosystem recovery times to proposed emission reductions, and for anticipating ecosystem-wide changes in methylmercury toxicity as a result of restoration changes.

Status: The Aquatic Cycling of Mercury in the Everglades (ACME) project, started in 1995, set a new standard worldwide in the depth and breadth of field-based mercury research. Many fundamental discoveries that are now applied across the globe, including the need to focus on understanding sulfur and carbon cycling, in addition to mercury, is a direct outcome of the ACME project. Researchers from the ACME project demonstrate that sulfate can have a dual effect on the production of methylmercury. At low levels, sulfate becomes limiting to the methylation process, and at high levels there is an inhibitory effect. This break-through observation proved to be pivotal in providing a general (ecosystem wide) understanding of what controls the overall levels of methylmercury across the Everglades, and was applied widely to ecosystems globally. In addition, these observations led ACME researchers to hypothesize that sulfate contamination may have a much greater impact on the ecosystem than previously thought. However, several biogeochemical factors co-vary in space along the Everglades eutrophication gradient (sulfate, DOC, pH, DO, phosphate, sediment redox), and traditional field studies were unable to sort out the individual contributions of several factors. It should be noted, that the use of in situ mesocosms for the purposes of testing biogeochemical controls of mercury methylation are a novel contribution to the scientific community. Recently, the combined use of the mesocosms and field monitoring data have led ACME researchers to conclude that recent dramatic declines in methylmercury levels at our primary monitoring site in Water Conservation Area 3A is the result of almost quantitative loss of sulfate from the water column. We have hypothesized that the sulfate declines are a result of changes in water routing in the Everglades, and the methylmercury hotspot has moved elsewhere in the ecosystem, possibly the Everglades National Park.

Recent Products: (1) Krabbenhoft, et al., 2005, Water and sediment indicators for mercury monitoring in the environment; in Environmental Mercury Monitoring, (Saltman and Newman, eds) ACS publications (in press). (2) Krabbenhoft, Branfireun, and Heyes, Biogeochemical Cycles Affecting the Speciation, Fate, and Transport of Mercury in the Environment, in Mercury: Sources, Measurements, Cycles, and Effects (Parsons and Percival, Eds.), Mineralogical Association of Canada, vol. 34, pp. 139-156. (3) Branfireun, Krabbenhoft, Hintelmann, Hunt, Hurley, and Rudd, 2005, The Transport and Speciation of Atmospheric Mercury in Wetlands: A Stable Mercury Isotope Approach, 2005, Water Resources Research (2005). (4) Wiener, J. G., D. P. Krabbenhoft, and G. H. Heinz, Ecotoxicology of Mercury, Chapter 16 in Handbook of Ecotoxicology, 2003; (4) Krabbenhoft, Orem, Aiken, and Gilmour, 2004, Mercury Contamination and Land-Management: The Convergence of Two Issues in the Everglades to Control Methylmercury Contamination at the Ecosystem Scale, Invited Keynote presentation at the 7th International Conference on Mercury as a Global Pollutant, September 2004, Lujubjina, Slovenia, Program and Abstracts. (5) Gilmour, Krabbenhoft, Orem, and Aiken, 2004, The influence of drying and rewetting on Hg and S cycling on Everglades soils, Proceedings of The 7th International Conference on Mercury as a Global Pollutant, September 2004, Lujubjina, Slovenia, Program and Abstracts.

Planned Products: (1) Synthesis Report: Aquatic Cycling of Mercury in the Everglades: 1995-2004; (2) Mercury, sulfur and carbon: The complex web of biogeochemical interactions unraveled in the Everglades (journal paper); (4) Drying and Rewetting Effects on Mercury Methylation in the Everglades (Journal paper).

Specific Relevance to Information Needs Identified in DOI's Science Plan in Support of Ecosystem Restoration, Preservation, and Protection in South Florida (DOI's Everglades Science Plan) [Page numbers listed below are from the DOI's Everglades Science Plan. The Science Plan is posted on SOFIA's Web site: http://sofia.usgs.gov/publications/reports/doi-science-plan/]:

This study supports several of the projects and overall goals listed in the DOI science plan. The DOI science plan lists three overarching restoration questions (page 9) that this study has direct relevance and provides information toward answering, including: (1) What actions will improve the quantity, timing, and distribution of clean fresh water needed to restore the South Florida ecosystem? (2) What actions will restore, protect, and manage natural resources on DOI lands in South Florida? (3) What actions will recover South Florida's threatened and endangered species? Aquifer Storage and Recovery (ASR) has substantial potential to affect water quality everywhere recovered water is released to the south Florida ecosystem, and is an area of concern in the DOI Science Plan (page 27). This study has demonstrated links between water quality characteristics of waters to be injected (sulfate, DOC, DO, and pH), the water quality characteristics of water recovered, and the water quality characteristics of water within the receiving surface and ground waters. In addition, the Comprehensive Integrated Water Quality Feasibility Study (CIWQFS; page 84) identifies degraded water bodies, types and sources of waterborne pollution, establishing load reduction targets for pollutants, and the need to improve water quality. Findings from this study will assist the DOI in providing needed information to multiagency CIWQFS Project Delivery Team in identifying the linkages between water quality targets and ecosystem restoration. The need to understand the sources, cycling and fate of critical chemical constituents like mercury, and to quantify the types and sources of pollution is stated on page 85. Linked to cycling and fate, the Science Plan cites the need for water quality performance targets (page 85) that can be used to evaluate the progress of restoration, and to identify areas in need of adaptive management. This project has shown clear linkages between water quality, land management (sitting and operation of STAs; page 86), and restoration plans, which will be critical for evaluating the overall success of the Restoration effort. Finally, the Science Plan specifically identifies the need to predict the effects of hydroperiod alterations and soil and water chemistry on the bioavailability of mercury to methylation (Page 89). This project not only discovered these hydro-cycle mercury-methylation linkages, but continues to unravel its complexities. The intent of these studies is to help land managers to make decisions that reduce the effects of hydroperiod alterations on mercury methylation.

Key New Findings:

1. Although the “mercury problem” was discovered in freshwater ecosystems about 20 years ago and research has largely been focused on them, the fact remains that most human exposure to methylmercury is through consumption of marine fish. A recent effort conducted by this project has revealed that the mangroves and other brackish settings along the southern Florida coastline have substantially elevated methylmercury levels, bringing into question whether coastal zones possibly play a key role in contributing mercury to marine commercial fisheries. This may be especially important in south Florida, where the entire fishery in Florida Bay is under an advisory for high levels of mercury, yet little is known about how this problem has developed, or possible corrective actions.
2. It has long been held that all wetlands will methylate mercury, and the Everglades mercury problem has always existed. This project has recently shown the Everglades (and other wetlands) will not necessarily methylate if one of the “axis of evil” (in this case sulfate) is eliminated. This is a key finding for guiding restoration planners and for evaluating success. This is novel finding and has led mercury researcher world-wide to evaluate it in other settings.
3. Dry/rewet cycles in the Everglades play a major role in remobilizing sulfur from organic sediments and stimulating methylmercury production and bioaccumulation. Effective management of dry/rewet cycles is important for minimizing methylmercury levels in fish and wildlife, and will be a key link to the restoration program.