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projects > linking land, air and water management in the southern everglades and coastal zone to water quality and ecosystem restoration: task 1, mercury cycling, fate and bioaccumulation > work plan

Project Work Plan

U.S. Geological Survey, Greater Everglades Priority Ecosystems Science (GE PES)

Fiscal Year 2006 Study 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: 9/30/2006
Web Sites:;
Location: Total Ecosystem
Funding Source: USGS Greater Everglades Priority Ecosystems Science (GE PES)
Other Complementary Funding Source(s): (1) combined contribution from the Wisconsin Water Science Center, NAWQA and the Associate Director for Water to fund a National Research Council (NRC) Post Doctoral Candidate to conduct research methlylation mechanics of a variety of microbial species (e.g., iron reducers and sulfate reducers) (2) 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).
Funding History: Task 2 FY98; FY99; FY00; FY01; FY02; FY03; FY04; FY05; FY06;
Principal Investigator(s): David P. Krabbenhoft (, 608.821.3843) and William H Orem (, 703.648.6273)
Study Personnel: D. Krabbenhoft, M. Olson, J. DeWild, S. Olund, and T. Sabin
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,, (2) Interactions of Mercury with Dissolved Organic Carbon in the Everglades (George Aiken,, (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,, (5) The Effects of Water Flow on the Transport of Suspended Particles and Particle-Associated Nutrients in the Everglades (Judson Harvey,,

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), food-web characteristics (trophic position, food-chain length, and introduced exotics), and resource management activities that alter any of the above (such as the restoration), and can exacerbate the formation of methylmercury (MeHg) a potent neurotoxin. In south Florida, and the Everglades in particular, surface water chemistry is greatly affected by agricultural practices upstream of the Everglades that can alter mercury relevant parameters, such as sulfate, dissolved organic carbon, pH, and iron. In addition, past alterations to the native hydrologic regime due to man's activities (compartmentalization) and present decompartmentalization, are the two most relevant land-management factors that affect mercury toxicity. The scientific examination of the intersection of the combined effects of changes in water chemistry and hydrologic regime, with the importation and deposition of mercury on the Everglades 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: (l) 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; and, (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). Continued detailed measurements at specific sites in the Everglades has allowed the USGS to build a time series of data that is unparalleled in the mercury research realm, and has yielded major new discoveries on the effects of land management on mercury toxicity. 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. Overall, the scientific objective of our research program in the south Florida environment is to examine the complex interactions (synergistic and antagonistic) of the chemical and hydrologic factors regulate mercury cycling, fate, speciation, and toxicity. In addition, we are purposefully designing our research so that it will provide scientifically relevant results to the assist resource management decisions related to the Everglades restoration program. At the present time, we have documented large-scale changes to water chemistry (sulfate and dissolved organic carbon) that are the direct result of hydrologic alterations attributed to the Restoration program, and that have had demonstrable effects on spatial and temporal distributions of methylmercury. Given the large uncertainties in what the “new” Everglades hydrology and water chemistry will entail, however, the long-term mercury contamination levels, and concerns for mercury toxicity, remain unclear.

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 USGS science plan. The USGS 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 USGS 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 USGS 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 USGS 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: Specific areas of focused research for FY06 fall into two broad categories: (1) efforts that are continuations of previous work and that is specifically designed to seek possible management solutions to the methylmercury problem, and (2) efforts that are new, but are natural extensions of our previous research on mercury contamination in south Florida. Those two areas are discussed next.

Continued studies on Mercury Cycling in the Everglades - Process Based Studies: 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. Legislation under consideration in the US and elsewhere has language that affords for “co-benefits” of reducing sulfate emissions due in part to our research that has shown the close linkages between mercury methylation and sulfate loading. In addition, these observations led ACME researchers to hypothesize that sulfate contamination may have a much greater impact on the ecosystem than previously thought - direct toxicity of the sulfur itself. However, because several biogeochemical factors co-vary in space along the Everglades eutrophication gradient (e.g., sulfate, DOC, pH, DO, phosphate, iron, sediment redox), traditional field studies were unable to sort out the individual contributions of several factors and an experimental (mesocosm dosing) approach was developed. 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. Thus far, the ACME team has used Hg, sulfate, DOC, pH, phosphate, and iron in dosing trials. The results from the Hg, sulfate and DOC experiments are being written up as a series of journal papers, and the iron dosing experiments are just concluding. By employing the use of long-term data sets and in situ mesocosms, we have provided managers in south Florida with scientific results that are much more reliable in terms of what controls mercury methylation across the ecosystem, and what land management strategies might exacerbate or mitigate the problem. 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 (probably) the Everglades National Park. Data collected in the Everglades National Park (ENP) by the Florida Fish and Wildlife Conservation Commission in fact shows that during the time period when fish Hg levels were declining in the WCA's, levels in the ENP were correspondingly increasing. The mechanistic causes for the increase in bioaccumulation of MeHg in ENP fish warrants further evaluation.

New efforts on mercury contamination in south Florida: Big Cypress National Preserve (BCNP) is historically not an area of major concern for mercury, primarily because it naturally has very low sulfate levels. However, over the past few years, surface flows from the L28 canal into BCNP have been increasing, and at the same time the L28 canal is receiving increasing amounts of sulfate-rich runoff from the EAA. Thus far, we have conducted two surveys in BCNP for Hg and MeHg concentrations, and observed a number locations where MeHg levels are substantially above concentrations we would have expected. At this time, we are not certain whether MeHg hot spots in BCNP are related to the discharge of sulfate-rich canal water, but if so, it could the unwanted effect of stimulating MeHg production throughout the Preserve.

Mercury fluxes in tidal marshes - In FY05, 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. Thus, any research and knowledge gained in this present understanding gap will potentially be valuable on a global basis. 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, and our understanding of methylation controls from the Everglades. However, the biogeochemistries of marine and estuarine systems are substantially different than freshwater systems, and it is not uncommon to have poor transferability of scientific results across such different ecological settings. Specifically, we would have anticipated based on Everglades results that the extremely high levels of sulfide in marine sediments would have prevented much, if any, methylation. In addition, in FY05, we conducted a synoptic sampling of coastal mangroves from the Shark River Slough region of the Everglades. The results from this sampling showed the highest levels of MeHg we have ever observed in anywhere in south Florida, and may provide a important leap forward in understanding why Florida Bay fish, as well as the Gulf of Mexico region more generally, commonly have high levels of mercury in game fish. If MeHg production is highly active along the coastal mangroves, the natural flow of water from the Shark River slough in combination with tidal flush may provide a very effective means for moving MeHg-rich waters from the coast toward the marine food webs. Further sampling efforts to determine seasonal and spatial variability in these high MeHg levels observed in coastal mangrove settings 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 Harris, 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.


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
Time Frame for Task 1: FY98; FY99; FY00; FY01; FY02; FY03; FY04; FY05; FY06;
Task Personnel: Krabbenhoft, M. Olson, J. DeWild, S. Olund, and T. Sabin

Task Summary and Objectives: This task addresses one of the major contaminant issues in the greater Everglades: 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 other areas of concern in south Florida (e.g., Big Cypress National Preserve where rapid changes in water chemistry are already occurring, and to coastal tidal zones, where we now know that very high levels of MeHg occur, and where a current understanding gap (where do 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 FY06, we plan to execute four interrelated but independent efforts (1) Continued monitoring of the Sufur Toxicity Mesocosms; (2) complete the initial examination of the iron and selenium dosing mesocosms; (3) participate in the Big Cypress National Preserve synoptic surveys; and (4) to repeat sampling along the coastal mangroves of the Shark River slough system. Brief details of each follow:

(1) Sulfur Toxicity Experiment Mercury Sampling - This experiment will continue in FY06, with major sampling efforts in November 2004, and March and August 2006 (see details in Orem Integrated Biogeochemistry Task 1). This experiment tests the hypothesis that excess sulfate entering the Everglades from agricultural runoff results in the generation of high sulfide levels porewaters of affected areas, which by itself can exhibit toxicity, but also many secondary deleterious effects (e.g., suffocation, limitation of nutrient uptake). 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 many native macrophytes in the ecosystem, but that cattail is largely robust to sulfide toxicity. The experimental design is explained in more detail in the Orem's Work Plan section for Task 1 - Nutrients, Sulfur, and Organics. Although the primary emphasis on this experiment is to evaluate the possible toxicity of sulfide on native Everglades vegetation, the opportunity to study the sulfur loading effects on Hg methylation and bioaccumulation is obvious. Sampling conducted in the mesocosms will help to confirm or refute this hypothesis.

(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) Methylation Controls in BCNP - Results from a preliminary water quality survey in BCNP conducted in FY04 and FY05 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.

(4) 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 FY06 will begin studies of the mechanisms by which MeHg is produced in the coastal zone of the greater Everglades. Our initial survey of 12 mangrove sampling sites at the Shark River slough-ocean interface reveal the highest levels of MeHg in water ever seen in our extensive work on Hg in south Florida. Work in FY06 will seek to determine whether this was a one time, or short term, occurrence, or a more general condition that may be extremely important for understanding why marine fisheries in the the Florida Bay and Gulf of Mexico region are commonly high in mercury.

Specific Task Product(s): (see details in Contaminant Synthesis Workplan) 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 in 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; (2) a report on the effects of hydroperiod, and hydroperiod alteration on MeHg generation and bioaccumulation, and (3) a report on the first round of mesocosm studies. 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.

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