Project Work Plan

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: Total Ecosystem
Funding Source: USGS Greater Everglades Priority Ecosystems Science (GE PES)
Other Complementary Funding Source(s): (1) US Army Corps of Engineers, 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. (2) Sulfur Toxicity to Macrophytes in the Florida Everglades (I. Mendelssohn/LSU cooperator with Orem; FDEP), (3) Aquatic Cycling of Mercury in the Everglades: Linking Everglades Restoration, Land and Air Management (C. Gilmour/SI cooperator with Krabbenhoft and Orem, FDEP SP-632)
Principal Investigator(s): David P. Krabbenhoft (dpkrabbe@usgs.gov, 608.821.3843) and William H. Orem (borem@usgs.gov, 703.648.6273)
Study Personnel: D. Krabbenhoft, M. Olson, J. DeWild, and S. Olund
Supporting Organizations: SFWMD, FLDEP, ENP, USFWS, USEPA, BCNP
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) Contaminants Synthesis (D. Krabbenhoft, W. Orem, and G. Aiken); (4) Ecological Risk Assessment of Toxic Substances in the South Florida Ecosystem: Wildlife Effects and Exposure Assessment (Tim Gross, tim_s_gross@usgs.gov), (5) 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, surface water chemistry is greatly affected by contributions from agricultural runoff, and the native hydroperiod alterations from the past compartmentalization and present decompartmentalization, are the two most relevant land-management factors that affect mercury toxicity. The scientific examination of the intersection of these external forces (runoff and hydrologic change) with the deposition of mercury is the emphasis of this project. 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 surrounding the mercury research field, and the Everglades Restoration program: (1) What ecological benefit to the Everglades would be realized if mercury emissions reductions would be enacted, 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. The scientific focus of the project is to examine the complex interactions of these contaminants (synergistic and antagonistic), ecosystem responses to variations in contaminant loading (time and space dimensions), and how imminent ecosystem restoration steps may affect existing contaminant pools. The Everglades restoration program is prescribing ecosystem-wide changes to some of the physical, hydrological and chemical components of this ecosystem. It remains uncertain, however, what overall effects will occur as these components react to the perturbations (especially the biological and chemical components) and toward what type of "new ecosystem" the Everglades will evolve. The approaches used will be extensions of previous efforts by the lead investigators, whereby we will enhance our abilities to address land management and ecosystem restoration questions.

The overall goal of the Mercury Cycling and Bioaccumulation in the Everglades project is to provide relevant science to DOI managers and other agencies involved in the Restoration program. This information takes the form of new discoveries of ecosystem functions and controlling factors, but also possible solutions. Our results will provide CERP (3005-1;3050-1,2,3,6,7,11;3060-1;3080 3,4,8,9,10), and GEER management with quantitative information for critical decisions regarding water quality and other competing issues (e.g. hydroperiod and implications for ASR). For this task in FY05, we plan to execute six interrelated but independent efforts (1) to conclude the sulfate, DOC and Hg dosing studies initiated previously by executing a final sampling of the mesocosms in the Fall of 2004; (2) to examine potential mitigating (methylation abatement) procedures that may be implemented in critical or particularly sensitive parts of the ecosystem (e.g., iron and selenium additions to the intact mesocosms) (3) to support the Sulfur Toxicity mesocosm study by conducting a mercury/methylmercury sampling in those mesocosms in the spring/summer of 2005; (4) to extend our examinations of the potential for exacerbated methylmercury production in Big Cypress National Preserve by conducting a synoptic sampling effort in the spring/summer 2005, and initiating mesocosm tests there; (5) to conduct a limited survey of other coastal marsh settings (about 2-4 sites on differing coastal settings in south Florida) to extend the observations initiated in FY04; and, (6) to conduct a limited synoptic survey of the Everglades in locations we suspect the methylmercury hotspot may have migrated. Although we do not intend to initiate mesocosm experiments in coastal marsh settings in FY05, we would like to initially propose the idea to build off the expertise and knowledge gained from the tests conducted in the freshwater marshes and the coastal marsh surveys.

Specific Relevance to Major Unanswered Questions and Information Needs Identified: (Page numbers below refer to 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 (siting 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.

Status:

(1) Mercury Cycling in the Everglades - The use of Mesocosms to Unravel Ecosystem-Level Complexities. 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.

(2) Mercury fluxes in tidal marshes - In FY04, we initiated a field sampling exercise to determine whether coastal zones may be important landscape positions leading to the formation of methylmercury. Presently, a major scientific gap exists between our understanding of mercury cycling in the environment, which is almost entirely based on freshwater environments, and the fact that the vast majority of mercury exposure to humans is through the consumption of marine fishes. In FY03, we assisted the SFWMD in assessing the potential for methylmercury formation in Florida Bay, where the entire ecosystem is under a mercury advisory for commercially harvestable fish. In that brief assessment study, it was observed that Florida Bay sediments had similar potential to methylate as Everglades peat soils, which was a surprising result based on the literature. In addition, in FY04, we conducted a synoptic sampling effort over a complete tidal cycle to test whether more methylmercury was entering or leaving tidal marshes along the southern coast (Florida Bay) of the Everglades. From that effort we learned that there is a substantially greater amount (about 5X) of methylmercury leaving the marsh as entering from the tidal pulse, which suggests estuarine zone could be receiving methylmercury from this natural “pump”. Further sampling efforts to determine seasonal and spatial variability in these tidal-fluxes of methylmercury are sorely needed.

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. (2) Wiener, J. G., D. P. Krabbenhoft, and G. H. Heinz, Ecotoxicology of Mercury, Chapter 16 in Handbook of Ecotoxicology, 2003; (3) (5) 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. (6) 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:

See Mercury cycling and bioaccumulation in the Everglades synthesis FY05 work plan.

WORK PLAN

Title of Task 1: Integrated Biogeochemical Studies in the Everglades: Task 2 - Mercury Cycling and Bioaccumulation
Task Funding: USGS Greater Everglades Priority Ecosystems Science (GE PES)
Task Leaders: David Krabbenhoft
Phone: 608-821-3843
FAX: 608-821-3817
Task Status (proposed or active): active
Task priority: High
Task Personnel: D. Krabbenhoft, M. Olson, J. DeWild, and S. Olund

Task Summary and Objectives:

This task addresses one of the major contaminant issues in the greater Everglade: mercury contamination and its relations to ecosystem management and the restoration. The basic understanding provided by the Aquatic Cycling of Mercury in the Everglades (ACME) project was an essential first step in understanding this complex problem. As additional scientific understanding was developed, and linkages to land-management and restoration ties were established, the importance of the mercury problem became more evident. Now, our research is aimed at developing mitigation or resource management strategies to minimize the impacts of mercury on the Everglades, or other environments where our research can be applied. Emphasis is placed on ecosystem responses to variations in contaminant loading (changes in external and internal loading over time and space dimensions), and how imminent ecosystem restoration may affect existing contaminant pools and their impacts on natural resources in the ecosystem. The major objectives of this task include: (1) to extend our understanding of the man-related activities that affect water chemistry and water movement in the greater Everglades ecosystem, but that also affect mercury cycling (most importantly methylation) and bioaccumulation; (2) develop a predictive capability to assess the level of impact, and locations, where changes to water chemistry and flow due to the restoration efforts will occur, and (3) extend our understanding of mercury cycling and bioaccumulation processes to Big Cypress National Preserve where rapid changes in water chemistry are already occurring, and to coastal tidal zones, where a current understanding gap (where to marine fish get their methylmercury) exists. The approach used includes a combination of field surveys, contaminant monitoring at key sites, experimental studies in the ecosystem using experimental chambers (mesocosms), and laboratory experiments using microcosms. This approach will not only provide information to enable us to predict when and where we might expect changes to occur relative to the levels of methylmercury present in local food webs, but also what corrective measure may be attainable. Results from our study will continue to be made available to risk assessors/managers, and other ecosystem managers, such TMDL modelers. In the past, this project has collaborated with state and federal scientists and land managers to provide the best possible science to support decision makers.

Work to be undertaken during the proposal year and a description of the methods and procedures:

For this task in FY05, we plan to execute six interrelated but independent efforts (1) to conclude the sulfate, DOC and Hg dosing studies initiated previously by executing a final sampling of the mesocosms in the Fall of 2004; (2) to examine potential mitigating (methylation abatement) procedures that may be implemented in critical or particularly sensitive parts of the ecosystem (e.g., iron and selenium additions to the intact mesocosms) (3) to support the Sulfur Toxicity mesocosm study by conducting a mercury/methylmercury sampling in those mesocosms in the spring/summer of 2005; (4) to extend our examinations of the potential for exacerbated methylmercury production in Big Cypress National Preserve by conducting a synoptic sampling effort in the spring/summer 2005, and initiating mesocosm tests there; (5) to conduct a limited survey of other coastal marsh settings (about 2-4 sites on differing coastal settings in south Florida) to extend the observations initiated in FY04; and, (6) to conduct a limited synoptic survey of the Everglades in locations we suspect the methylmercury hotspot may have migrated. Brief details of each follow:

1. Starting in 2001, we initiated the use of mesocosms to provide constituent-specific quantitative estimates of the relative effects of mercury, sulfate and DOC loading on the methylation process in the Everglades. These experiments have lead to many novel and paradigm forming observations that had never been even proposed by the scientific community, including the old vs. new mercury discovery. In the more recent experiments, we discovered that the old paradigm of DOC inhibiting mercury availability to methylation was not only wrong, the opposite was actually true! What we observed was that not only did DOC additions make new mercury more available for methylating microbes, but that old mercury (previously buried in sediments) is also activated by DOC loading. These findings are especially important given the parallel discovery that much of the DOC present down-stream of the EAA canal discharges is due to mobilization of humic acids from the drained soils there.
2. Although our mesocosm tests to date have focused on the factors known to exacerbate (or speed) methylation in the environment, these test chambers can be equally well adapted for testing whether there may be solutions to the problem. Mercury methylation is almost entirely regulated by microbial processes, and as such competing processes, or those that limit mercury uptake, are certainly possible. Recent literature on test-tube scale incubations under laboratory conditions suggest that selenium and iron [Fe(III)] additions may prove useful in some situations for limiting methylmercury production. Such a strategy may prove useful in a setting like the STAs, where downstream transport of de novo produced methylmercury is a concern ecologically, and for permit renewal. It is interesting to note that one common feature of the STAs that yield little or no excess methylmercury is that excess iron that is also present. For this part of Task 2 we will conduct a limited number of trial Fe(III) and selenium (added as sodium selenite). For selenium, hypothesize that an important mechanism diminishing the availability of mercury to aquatic food webs occurs through complex interactions between insoluble mercury selenides that may result in long-term retirement of mercury from actively cycling (and bioaccumulating) pools in the environment. We will investigate this hypothesis through study of the effects of graduated additions of small amounts of selenium to mercury-contaminated aquatic ecosystems and determine the impact of this supplemental selenium on accumulation of mercury at different trophic levels of indigenous food webs
3. Sulfur Toxicity Experiment Mercury Sampling - This experiment will continue in FY05, with major sampling efforts in November 2004, and March and August 2005 (see details in Orem Integrated Biogeochemistry Task 1). The experiment tests the hypothesis that excess sulfate entering the Everglades from agricultural runoff has a toxicological impact on native macrophytes in the ecosystem by resulting in high levels of sulfide in porewaters of affected areas. Sulfide toxicity has been shown in several environments to negatively impact freshwater aquatic plants. This experiment will test the hypothesis that excess sulfate entering the Everglades from agricultural runoff has a significant effect on macrophytes in the ecosystem. ACME researchers have concluded that excessively high sulfide levels also limit inorganic mercury bioavailability, and thus reduced levels of methylmercury are coincident with sulfidic zones. Sampling conducted annually (spring summer 05) in these mesocosms will help to confirm or refute this hypothesis.
4. Methylation Controls in BCNP - Results from a preliminary water quality survey in BCNP conducted in FY03 and FY04 indicate that some areas of BCNP have higher than anticipated methylmercury (MeHg) concentrations. While most of the Preserve has very low levels of sulfate, much higher concentrations are found in canals outside the Preserve, especially the L28. Restoration plans call for diverting water from the L28 into BCNP to increase water levels. However, the resulting increased sulfate load entering the Preserve in this canal water may have the unwanted effect of stimulating MeHg production and bioaccumulation here. To test this hypothesis we propose to conduct limited mesocsom studies of MeHg production in response to increased sulfate loads in BCNP, similar to studies we have already conducted in the central Everglades. Results from these tests will provide managers with information on the effects of diverting water of high sulfate concentrations into BCNP, so that costs and benefits of this planned diversion can be assessed, and also provide possible confirmatory results for our other mesocosm studies.
5. MeHg Production in the Coastal Zone - As mentioned above, there is a major disconnect between past work on mercury cycling (freshwater emphasis) and human exposure to methylmercury (primarily from consumption of marine fish). Work in FY05 will begin studies of the mechanism by which MeHg is produced in the coastal zone of the greater Everglades. Task 2 will specifically focus on the occurrence and distribution of mercury and methylmercury in coastal marshes (water, sediment and lower trophic levels), as well as a few key process measurements (methylation and demethylation). FY05 efforts will will focus on field surveys, similar to our approach in the freshwater Everglades, which will be followed by experimental work in subsequent years. Investigations aimed at understanding MeHg production and bioaccumulation in the coastal marine environment has been identified as a principal objective of future Hg research at a recent USGS mercury Workshop for DOI scientists and land managers.
6. Methylmercury hotspot survey - Overall levels of methylmercury in the central Everglades have been decreasing for several years, but until recently it was difficult to ascertain the controlling reason(s). Our mesocosm tests confirmed that mercury, sulfate, and DOC declines could be responsible, but all were suspected to have changed over the past 4-5 years due to Decompartmentalization and Sheetflow Enhancement. Recently, the State of Florida's DEP conducted a TMDL study and concluded that regulations emplaced in 1989 to reduce mercury emissions where the driving force. However, upon careful examination of ACME (Task 1) data, we found striking evidence to suggest that dramatic (>95%) declines in sulfate levels in the central Everglades were the more likely cause. If this is the case, this observation has profound implications for the overall benefit from reduced sulfate loads, and scientifically challenges the long held belief that the Everglades have always had a mercury problem. In order to test our conclusion, we propose to conduct a systematic survey of sites across the Everglades (some traditional ACME sampling sites and some new) in order to determine whether the sulfate plume has moved, and the corresponding methylmercury hotspot.

Specific Task Product(s): (see details in Contaminant Synthesis Work plan)

Mercury Synthesis Report in USGS Publication Form, with mercury-specific chapters addressing mercury cycling processes, geochemistry, and bioaccumulation, sulfur geochemistry effects on mercury cycling and toxicity, and the role of dissolved organic carbon. Final report expected late FY05 or early FY06). Several other science synthesis reports will also be produced during this effort. David Krabbenhoft will be synthesizing information for two additional reports intended for journal publications, including: (1) a report on photochemical processes regulating mercury speciation and cycling; and (2) a report on bioaccumulation of mercury in lower trophic levels of the Everglades (through Gambusia). These reports are expected to be Direct approved by late FY05 or early FY06. Bill Orem will author a second synopsis report that will bring together nutrient and major ion data collected in the greater Everglades from 1995 to the present. This report is expected in FY06. The report will include information on concentrations of nutrients and major ions in surface water, porewater, soils, and sediments. George Aiken will assemble a manuscript on the overall importance of DOC in regulating the speciation, cycling and bioaccumulation of mercury in the Everglades. This report is expected to be Direct approved by late FY05 or early FY06.