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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
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U.S. Geological Survey Greater Everglades Science Initiative (Place-Based Studies)
Fiscal Year 2004 Project Work Plan
A. GENERAL INFORMATION:
Project Title: Integrated Biogeochemical Studies in the Everglades; Mercury Cycling and Bioaccumulation
Other Investigator(s): George R. Aiken, USGS, Boulder, CO; Cindy Gilmour, Academy of Natural Sciences, St. Leondard MD.
The overall programmatic goal of this project is to examine the complex interactions of mercury, sulfur, carbon, and nutrient contamination (synergistic and antagonistic) among the major sub-ecosystems present in south Florida to provide direct feedback to aid the Everglades Restoration Plan development and implementation. This information is needed at many levels, from those who are concerned with the restoration of natural flow (decompartmentalzied) to the Everglades, restoring "healthy" water quality, the potential for water treatment wetlands (STAs) to yield unsafe loads of methylmercury, to those responsible for possible implementation and enforcement of mercury air emission reduction laws, and to those who are concerned over the possible ecological effects of the ASR program.
Although ecological impacts from phosphorous contamination have become synonymous with water quality in south Florida, especially for Everglades restoration, there are several other contaminants presently entering the Everglades that may be of equal or greater impact, including: pesticides, herbicides, polycyclic aromatic hydrocarbons, and trace metals. This project focuses on mercury, a sparingly soluble trace metal that is principally derived from atmospheric sources and affects the entire south Florida ecosystem. Mercury interacts with another south Florida contaminant, sulfur that is derived from agricultural runoff, and results in a problem with potentially serious toxicological impacts for all the aquatic food webs (marine and freshwater) in the south Florida ecosystem. The scientific focus of this 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. However, it remains uncertain 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 by this study have been purposefully chosen to yield results that should be directly useable by land management and restoration decision makers. 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, in any, ecological benefit to the Everglades would be realized if mercury emissions reductions would be enacted, and over what time scales (years or tens of years) would improvements be realized? (2) What is the role of old mercury (previously deposited and residing in soils and sediment) versus new mercury (recent deposition) in fueling the mercury problem? (3) In the present condition, is controlling sulfur or mercury inputs more important for reducing the mercury problem in the Everglades? (4) Does sulfur loading have any additional ecological impacts that have not been realized previously (e.g., toxicity to plant and animals) that may be contributing to an overall decreased ecological health? (5) Commercial fisheries in the Florida Bay are contaminated with mercury, is this mercury derived from Everglades runoff or atmospheric runoff? (6) What is the precise role of carbon (the third member of the "methylmercury axis of evil", along with sulfur and mercury), and do we have to be concerned with high levels of natural carbon mobilization from agricultural runoff as well? (7) Hundreds of millions of dollars are being, or have been spent, on STA construction to reduce phosphorus loading to the Everglades, however, recently constructed STAs have yielded the highest known concentration of toxic methylmercury; can STA operations be altered to reduce methylmercury production and maintain a high level of phosphorus retention over extended periods of time? The centerpiece of our research continues to be the use of environmental chambers (enclosures or mesocosms), inside which we conduct dosing experiments using sulfate, dissolved organic carbon and mercury isotopic tracers. The goal of the mesocosm experiments is to quantify the in situ ecological response to our chemical dosing, and to also determine the ecosystem recovery time to the does. Ultimately, we intend to provide a more definitive means addressing specific management questions concerning mercury contamination in the Everglades, and also more generally.
Project Objectives and Strategy:
This proposal identifies work elements that are logical extensions, and which build off, our previous work. Our overall scientific objective is to provide a complete understanding of the external factors (such as atmospheric mercury and sulfate runoff loads) and internal factors (such as hydroperiod maintenance and water chemistry) that result in the formation and bioaccumulation of MeHg in south Florida ecosystems, and to conduct this research is such a way that it will be directly useable by land and water resource managers. Although this work will have direct implications for decision-making in south Florida, past experience has shown that our results have practical applications for other areas of the United States and the world. Presently, four other know research teams across the US and internationally are planning mesocosm research involving mercury, and will be applying the same methods we have developed in south Florida. Overall, we will continue to apply technologies, sampling strategies and experimental designs that extend from the centimeter scale (incubation experiments), to the meso scale (in-field mesocosms), to the ecosystem scale. More specifically, we will seek to achieve the following sub-objectives (1) Extend our mesocosms studies to provide a more comprehensive examination of the newly discovered "new versus old" mercury effect by conducting studies under differing hydrologic conditions and sub-ecosystem settings so that our experimental results will be more generally applicable to the greater south Florida ecosystem including the STAs that have been recently constructed and are yielding very high levels of methylmercury but the cause is currently unknown; (2) Seek to further identify the mechanisms that result in extremely high levels of MeHg after natural drying and rewetting cycles in the Everglades and which have major implications for the Restoration Plan; (3) Further our studies on the production of methylmercury in south Florida estuaries and tidal marshes by conducting mass-balance studies of tidal marshes; (4) Begin to partner with wildlife toxicologists funded by the State of Florida to unravel the complexities surrounding methylmercury exposure and effects to higher order wildlife in south Florida; and , (5) Continue to participate with mercury ecosystem modelers who are funded by the State of Florida and the USEPA to evaluate the overall ecological effects of reducing mercury emissions and the risks associated with methylmercury exposure (Reed Harris, Everglades and TMDL mercury modeling)
Potential Impacts and Major Products:
This project is purposefully designed, and has as one of its major goals, to provide integrated science for state and federal natural resource managers and decision makers who need relevant scientific information to aid in three critical areas of resource management: Restoration Plan Design, Environmental Regulations and Action Plans, and Risk Assessments. Several major questions are looming related to Everglades Restoration, and several of which are directly related to our research efforts, including: Will the altered hydrology of the "New Everglades" promote, have no effect, or reduce the level of mercury methylation and bioaccumulation? The original test STA showed very depressed levels of methylmercury production, however, more recently constructed treatment wetlands have yielded some of the highest methylmercury levels ever observed in any ecosystem, but what is the underlying cause and long-term consequences of this effect? Sulfate loading from agricultural runoff is directly linked to mercury methylation it the Everglades, but what is would be the long-term benefit of reduced sulfate loads if the existing pools of sulfur that now exist in the peat are capable of maintaining elevated levels of methylmercury production? Florida Bay is currently under a mercury advisory of contaminated fisheries, but is the source of the methylmercury from marsh runoff or in situ production in the Bay? Will increases in freshwater discharge to Florida Bay, one of the probably out comes of the Restoration effort, have an effect on methylmercury production in Florida Bay? Currently, plans for the Aquifer Storage and Recovery (ASR) project are to discharge recovered water to the marshes that has sulfate levels about the same as current Lake Okeechobee conditions (about 30-60 ppm). Will methylation occur in the aquifer or will the recovered stimulate mercury methylation upon discharge to the marshes? The ACME project has shown that "new mercury" inputs are more available to methylating microbes than "old mercury" already existing in the peat and sediments, precisely how quickly and to what new baseline conditions would the ecosystem would adjust to if absolute mercury emissions reductions were emplaced? Lastly, although the ACME project has provided a great deal to our overall understanding of mercury methylation and bioaccumulation, a logical extension of our research is to interface with new plans by the State of Florida to fund researchers at the University of Florida (Drs. Peter Frederick and Marilyn Spalding) to perform toxicological studies on wading birds exposed to methylmercury. Our work will be very useful for these studies to define the actual exposure levels that are present in the environment, and in addition, we may be able to provide analytical tools not available elsewhere to employ isotopically labeled mercury in their experiments. We will continue to be closely aligned with the Everglades Mercury Model development (EPA, SFWMD, and FDEP funded) to assure our field and laboratory studies are in concert with the model construction, coding, and the predictive questions being asked of the model.
The scientific understanding by the ACME project for the general topic of mercury contamination of aquatic ecosystems has had many far-reaching impacts as well. Many "paradigm shifting discoveries" have been made by the ACME effort that have altered the way we view mercury cycling in the environment generally, or the overall understanding of the biogeochemistry of the Everglades. For example, our studies have demonstrated for the first time that dissolved organic carbon plays a direct role in the mercury methylation process, whereas before it was only perceived to be a transport, or stabilization ligand for mercury and methylmercury in solution. In the Everglades, we have discovered that dramatic changes in mercury and methylmercury concentrations occur on a diel basis and other investigators have followed our lead and shown this to be the case in other systems. In addition, before our work, the existence of the sulfate contamination plume originating from the Everglades Agricultural Area, and its impact on methylmercury formation was a direct outcome of ACME. Finally, the notoriety of the ACME project has resulted in David Krabbenhoft being frequently called upon to give technical briefings of our results to DOI, USEPA, the US Congress, and the House Science Committee.
Major products from the study include USGS Open-File Reports, articles in peer-reviewed international scientific journals, USGS Fact Sheets, abstracts and presentations at national and international scientific meetings and at client agencies, contributions to USGS and interagency synopsis reports, databases, and the electronic posting of reports and databases on the Web. Input of geochemical data into ecosystem models and risk assessment studies will also be a principal product of this project.
Collaborators: William Orem, USGS, Reston, VA; George Aiken, USGS, Boulder, CO; Cindy Gilmour, Academy of Natural Sciences, Estuarine Research Center, St. Leonard, MD; Darren Rumbold, SFWMD, Ft. Meyers, FL; Paul McCormick, USGS, Leetown, WVA; Eduardo Patino, USGS, Miami, FL; David Evans, NOAA, Raleigh, NC; Reed Harris, Tetra Tech, Inc., Toronto, Canada;Clients: Florida Department of Environmental Protection (T. Atkeson, D. Axelrad), South Florida Water Management District (L. Fink); U.S. Fish and Wildlife Service, Arthur R. Marshall Loxahatchee National Wildlife Refuge (L. Brandt and B. Arrington); National Park Service, Everglades National Park (T. Armentano, Nick Aumen); Big Cypress National Preserve; USEPA, Office of Air and Radiation, Washington DC (Ellen Brown, Tamara Saltman
B. WORK PLAN
Title of Task 2: Integrated Biogeochemical Studies in the Everglades: Mercury Cycling, Methylation and Bioaccumulation
D. Krabbenhoft (Research Geochemist), M. Olson (Lab Manager, Analytical Chemist), J. DeWild (Analytical Chemist), S. Olund (Analytical Chemist), M. Tate (Analyst and Technical Support), M. OKeefe (Analyst and Technical Support), M. Schmidt (Data Base & Web Page design)
Task Summary and Objectives:
Task 1 of this project focuses on biogeochemical processes and the sources and cycling of nutrients, sulfur, and organics in the ecosystem. It coordinates with other tasks to examine the complex involvement of nutrients, organics, and especially sulfur and ties intimately to Task 2, Mercury Cycling, Methylation and Bioaccumulation, through the complex intertwining of the sulfur and mercury cycles. Our principal objective is to determine the ecosystem-level factors governing methylmercury production and exposure to wildlife, and to ascertain the ecosystem responses to changes that result from the Restoration Program and have a direct influence on methylmercury production. Specifically, we are examining the methylmercury production responses to changes in related chemical constituents (e.g., inorganic mercury, sulfate, DOC), non-chemical ecosystem changes or natural perturbations (e.g., fire, water level maintenance), and how imminent and ongoing ecosystem restoration (e.g., decompartmentalization, ASR, STA construction and operations) may affect future levels of methylmercury contamination of this fragile ecosystem. Our Phase I studies reveal the ecosystem-wide trends of methylmercury contamination in the Everglades, and what were the controlling factors of these trends. Phase II studies will continue to build off our major findings and seek to provide a more meaning full and direct connection to CERP, and to expand our investigations to other parts of the ecosystem where time and staff limitations prohibited examinations in the past but that are now feasible. The sub tasks described here have a wide range of operating scales, from small scale lab tests to whole-ecosystem mass flux estimates, that will allow for a more expanded use of our data, as well as working in sub ecosystems not examined previously, including Florida Bay and the interfacing tidal wetlands, Big Cypress National Preserve, and Shark River Slough and the southwest coast.
Work to be undertaken during the proposal year and a description of the methods and procedures:
(1) Mesocosm Studies
Background/Objectives-We are currently using environmental enclosures placed in the ecosystem (mesocosms) to examine the effects of changing water quality variables on methylmercury production and bioaccumulation. These experiments are designed to examine the effects of individual variables and multivariable interaction effects on methylmercury production. Variables of interest with regard to methylmercury production include: inorganic mercury (added in isotopic form), sulfur, nutrients (especially phosphorus), DOC, and iron. All Tasks on the project cooperate on the mesocosm work: Task 1 focuses on the effects of sulfur and nutrients on methylmercury production, Task 2 focuses on mercury, and Task 3 on DOC
Methodology- The proposed 2003 mesocosm experiments will be conducted at the 3A-15 site in the central Everglades, that provided the highest MeHg response in the 2002 experiments, and also provides an ideal location for the DOC and sulfate addition experiments, and for the sulfur toxicity experiments described later. In total, we will have over 60 mesocosm at site 3A15 involved in our research, and in essence each mesocosm is the equivalent of a sampling site. MeHg mesocosm experiments will be conducted between mid June and the end of October 2003. Conducting the experiment during this wet season/summer period should produce the maximum MeHg production signal, due to overall higher microbial activity in summer months. Mesocosms used in the experiment will either be newly purchased or previously used mesocosms that have never had mercury isotopes added (such as controls, sulfate only, or DOC only additions). All previously used mesocosms will be relocated at the 3A-15 site for this experiment. Installation of the new mesocosms and relocation of existing mesocosms (some moved from other sites in the Everglades) took place during mid-April 2003 to allow time for reequilibration of the sediment and water prior to initiating the experiment in June. After installation, mesocosms are left open (six two-inch breather holes drilled on the perimeter of each mesocosm) to its surroundings, which allows for free exchange of water. At the start of experiments, the holes are plugged with silicone stoppers to isolate the interior environment of the mesocosm from the surroundings and maintain the presence inside the mesocosm of the chemical.
Ten mesocosms will be used for sulfate plus mercury additions. The ten mesocosms will be grouped in sets of two for addition of sulfate at five different dosing (i.e. concentration) levels and a single mercury level of 1X ambient atmospheric (22 ug/m2; or 14.3 ug Hg). Experiments performed in 2000-2002 have adequately defined the mercury-only addition response over a range of 0 to 2X ambient dosing level. The sulfate dosing levels are 4, 8, 12, 16, and 20 mg/l, based on the results of our 2002 mesocoms. The sulfate is added to the mesocosms as sodium sulfate dissolved in site water. The appropriate amount to be added to each mesocosm to reach the target concentration is calculated based on the volume of water in each mesocosm. Each dosing level has a duplicate mesocosm for quantifying natural variability in the response, which is epically high for sediment-based measurements (e.g., net methylation rates). A group of 6 mesocosms (three sets of duplicates for each dosing level) will be used to examine the effects of DOC and mercury isotope dosing at 3 different dosing levels. DOC isolated from eutrophied sites near canal discharge in Water Conservation Area 2A will be used for the experimental dosing. Target addition levels for DOC will be about 30, 40 and 50 mg/l. The DOC is added to the mesocosms as a concentrated solution and mixed by gentle stirring of the surface water. Finally, a group of two mesocosms will have DOC, sulfate, and mercury isotope added. These mesocosms are intended to evaluate the synergistic effects of sulfate, DOC, and mercury on MeHg production. The dosing level to be used in this mesocosm pair will likely be about 14.3 ug Hg, 12 mg/l sulfate and 40 mg/l DOC. As with out previous mesocosm experiments, we will employ control mesocosms to monitor the natural variability in the system and to evaluate whether there are any unnatural "mesocosm" effects. To establish natural variability and to control for mesocosm effects, two mesocosms will be set aside as controls, and in addition two sites will be established in the marsh near the control mesocosms as ambient controls. The mesocosm controls will be plugged with silicone stoppers and treated in a fashion similar to the experimental mesocosms, but no dosing of any kind will be added.
The experiment will commence on June 23, 2003. Samples of surface water, porewater, Gambusia and sediments will be collected at the mesocosm and outside controls to define the initial conditions of the site. After sampling, all mesocosms will be plugged, and appropriate chemical doses will be added. Follow-up sampling of the experimental mesocosms, mesocosm controls, and outside controls for surface water, porewater, Gambusia, and sediments will continue on days 1, 61, and 119. In addition, we will conduct a diel sampling study of a least 4 mesocosms (a control, Hg+sulfate, Hg+DOC, and Hg+sulfate+DOC) in which scientists remain on site for approximately 30-36 hours and sampling each mesocosm approximately every three hours. After the initial doses, subsequent sulfate dosing is scheduled for days 14, 28, 42, 63, 78, 91, and 105. Mercury-clean procedures are followed for all sampling, which provides minimal contamination acceptable for all analytes. Sediments are collected using a small push core to minimize disturbance of the mesocosm interior. Analytes measured in surface water, suspended particulates and porewater include: total mercury and MeHg (ambient pools and isotope spikes), anions, cations, sulfur species (sulfate, thiosulfate, sulfite, sulfide), nutrients (nitrate, ammonium, and phosphate), DOC, iron and manganese, redox, dissolved oxygen, and pH. Sediment geochemical analyses include: total mercury and methylmercury (ambient pools and isotope spikes), total sulfur, sulfur species (AVS, sulfate, disulfides, and organic sulfur), total and organic carbon, total nitrogen, total phosphorus, and metals. Sediments are also measured for various microbial parameters, including mercury methylation rate, and sulfate reduction rates. Time-sensitive parameters are measured in motel-room laboratories within hours of sample collection. Samples for later analyses are stored in an appropriate fashion (frozen, cool, etc.) and shipped back to laboratory facilities at the various PIs labs (Middleton, WI; St. Leonard, MD; Reston, VA; Boulder, CO). At the termination of the experiment (currently scheduled for October 14, 2003), the silicone stopper plugs are removed from the mesocosms, and all equipment is removed from the site, with the exception of mesocosms, which are left in place for potential future studies.
(2) Developing a predictive model for methylation and sulfur cycling
A new feature in FY03 will be the collaborative efforts to develop a biogeochemical sulfur model with researchers funded by the State of Florida. This model will allow us to have a better understanding of the changes in sulfur geochemistry, its effects on methylmercury production, and time lag that may result from sulfur recycling should external loads be reduced. The Everglades Mercury Cycling Model (EMCM) has a major information gap in that it does not directly simulate sulfur cycling and linkages to methylation, primarily due to a lack of previous research on sulfur cycling rates, fate and speciation. The development of a sulfur model to interface with the EMCM model would be a substantial advancement for mercury research everywhere. In FY03 we propose to increase our level of investigation of the STAs. These constructed wetlands can behave as zones of low methymercury production (such as ENR), but also can produce very high levels of methylmercury (STA-2). Our hope is to identify the cause of the high methylation rates in new STAs, and to determine if there are operational procedures that could be used to minimize the amount of methylmercury discharged to down-gradient marshes.
Methodology- Our studies suggest that there are three main, manageable, controls on MeHg production in the STAs: antecedent soil chemistry, inflowing water chemistry, and interior water level maintenance. Since MeHg production is substantially dependant on the amount and type of sulfur present in soils, and on the mercury content of soils. Agricultural and non-agricultural soils may have very different sulfur and mercury levels because of land-use history, although to our knowledge little information is available on soil sulfur and mercury chemistry in the STAs except for research sites in ENR and STA 2 Cell 1. Inflowing water contains three critical constituents that strongly relate to methylmercury formation, transport and bioaccumulation: sulfate, organic carbon and Hg. In addition, these constiutents may change the character of soils in the long run. Last, the timing and duration of flow-through of water of the STAs (i.e., hydroperiod) can dramatically affect MeHg production through the initiation of drying/rewetting cycles that have been shown to dramatically increase MeHg in Everglades soil.
Here we propose a set of studies conducted over two years that are designed to produce a predictive model for MeHg production in the STAs. The study will be carried out via agreements with USGS researchers and with the Academy of Natural Sciences Environmental Research Center (ANSERC) in St. Leonard, Maryland. The objective of this study is to develop a predictive capability based on soil geochemistry, quality of inflowing water, and hydrologic conditions. Although this research arises from the need to manage existing STAS, it will also be useful in site selection for any future treatment areas and for planning and operation of future water reservoirs. We seek to apply our knowledge of MeHg production gained in the Everglades to the STAs, through collection of comparative soil data for the STAs, and by additional study of the influence of drying and wetting cycles across a wider range of soil types. This new work will provide information toward management of MeHg production in existing and planned STAs of different soil types, through site selection, control of hydrology, and water quality. The proposed study has several linked components: (1) Survey of soil geochemistry, Hg and MeHg in STA soils; (2) Follow up examination of soil geochemistry, Hg and MeHg at ACME Everglades sites; (3) Examination of the influence of drying and wetting cycles across a wider range of soil types.
STA soil geochemistry - The Survey of soil geochemistry in STAs is an examination of STA soil geochemistry, especially sulfur, iron, and Hg/MeHg content. This component consists primarily of a field survey of soil geochemistry across the STAs. The objective of this survey is to test our Everglades-based understanding of MeHg production in the STAs. The primary drivers of MeHg within Everglades surface soils are sulfur, Hg, organic carbon and hydrologic conditions. A survey of soil conditions within the STAs will allow us to determine if the same drivers operate in STA soils, with their different land-use and hydrologic-maintenance histories. Currently operating STAs would be examined first in summer/fall 2003, then the survey would be expanded to STAs under construction and planned for construction in spring 2004. Site-selection criteria would include examination of a wide variety of soil and land-use types, and the management needs of the agencies responsible for Everglades restoration planning and operation. Specific objectives of this component are to provide baseline data for geochemistry and Hg/MeHg content of STA soils, and to evaluate the ACME conceptual model for control of MeHg production in Everglades soils for STA soils. Six sites within the STAs will be examined in fall 2003 and six more in 2004. Site access will be via helicopter. Mercury and MeHg will be measured in surface soils, interstitial waters, surface waters, periphyton and gambusia (should we do all these matrices?). Other standard ACME analytes to be measured include for bulk sediment: total sulfur, acid volatile sulfide, chromium reducible sulfur, organic sulfur, organic carbon, bulk density, and moisture content. Surface waters and pore waters will be analyzed for sulfate, partially reduced sulfate species, sulfide, total iron, total manganese, and dissolved organic carbon using the previously referenced methods.
Soil biogeochemistry at ACME Everglades sites - The ACME project examined eight discrete Everglades sites (ENR 103, F1, U3, 2BS, 3A15, 3A33, TS7, and TS9) in detail, 2-3 times per year from 1995 through 1998, and that covered most of the north-to-south extent of the ecosystem. These data have been used to generate a general conceptual model for control of MeHg production in the Everglades. There are a number of reasons to look at the sites again in 2003. First, decreases in MeHg in fish and wading birds have been observed in many areas of the central Everglades during that time period, but there is no information on any changes in MeHg in soils and water from the ACME sites. Second, additional data density, especially during a different hydrologic period, will provide a more robust data set for comparison with STA soils, and diagenetic modeling. Last, there are some additional parameters that are needed for the diagenetic model that were not collected during 1995-1998, particularly solid-phase Fe speciation, which is needed to model microbial Fe reduction. Periodic resampling of the ACME sites is relatively inexpensive, and will provide valuable long-term data on changes in Hg cycling in the Everglades ecosystem. Sampling conducted at site ENR103 will provide valuable insights into STA biogeochemistry after several years of operation, particularly how long-term sulfate loading has impacted geochemistry and MeHg production in this soil. ENR soils were agricultural prior to conversion, and the high S content of these soils has minimized MeHg production at this site since start-up.
Specific objectives of this component are: (1) Hgmeasure/MeHg concentrations in soils, soil interstitial waters, surface waters and gambusia at the eight main ACME sties; (2) examine potential changes in MeHg concentrations at ACME sites, in comparison with declines in MeHg in wading bird and largemouth bass in the central Everglades; (3) examine changes in soil geochemistry and MeHg in response to changing flow patterns and sulfate loading, particularly in 2BS where substantially increased sulfate loading has occurred since 2000; and, (4) collect information on iron cycling that is needed for construction of the diagenetic MeHg model.
Six to eight ACME sites in the Everglades will be revisited in June or July of 2003. Sites will include ENR103, F1, U3, 2BS, 3A33, 3A15, TS7 and TS9. Site access will be via helicopter. Mercury and MeHg will be measured in surface soils, interstitial waters, surface waters, periphyton and gambusia. Other standard ACME analytes to be measured include for bulk sediment: total sulfur, acid volatile sulfide, chromium reducible sulfur, organic sulfur, organic carbon, bulk density, and moisture content. Surface waters and pore waters will be analyzed for sulfate, partially reduced sulfate species, sulfide, total iron, total manganese, and dissolved organic carbon using the previously referenced methods.
(3) Sulfur Biogeochemistry and Toxicity Experiments
Background/Objectives- Sulfur Toxicity Mesocosm Experiments - Conventional wisdom holds that changes in macrophyte distributions in the Everglades (cattail replacing sawgrass) has resulted from excess phosphorus entering the ecosystem. However, areas of the ecosystem where these changes have occurred are also heavily contaminated with S. Sulfur enters these areas as sulfate from canal discharge (the sulfate has been shown to originate in the EAA from agricultural runoff and soil oxidation). The sulfate diffuses into the anoxic sediments, and microbial sulfate reduction reduces the sulfate to sulfide. Areas contaminated with high levels of sulfate also have very high levels of sulfide in porewater. Dissolved sulfide is highly reactive, and may also be toxic to both plants and animals. It may reduce the ability of oxygen to penetrate to macrophyte roots, can react with metals to make them unavailable for plant uptake, and can impact biochemical processes of plant metabolism, such as nutrient uptake. It is also worth noting that tree islands have largely disappeared from regions of the Everglades impacted by sulfur contamination. Our hypothesis is that high sulfide levels have played an important, yet previously unrecognized role in the proliferation of cattail in heavily S and P contaminated areas of the Everglades.
To test this hypothesis, we propose to employ mesocosms and sulfate dosing in sawgrass and cattail dominated sites of WCA3A. The sites will be in relatively close proximity to each other, probably near tree islands where cattails are often found. Mesocosms would be installed at these sites and allowed to equilibrate for a period of couple months. As with the other mesocoms (described above), holes in the sides of the mesocosms would allow exchange of water with the outside during this equilibration period. The experiment would involve addition of sulfate at three levels: 100 mg/l, 50 mg/l, and 20 mg/l; each level run in triplicate. A pair of control mesocosms would also be run at each site (no sulfate addition). A pair of external control sites (no mesocosm, monitoring of external environment) would also be employed at each location (cattail and sawgrass). Sulfate added to each mesocosm would be calculated based on the volume of water at the time of the experiment, and the amount needed to bring the mesocosms up to the desired concentrations. Sulfate additions would initially be conducted biweekly, and sulfate concentrations monitored to determine future addition needs. Surface water in each mesocosm and in controls (mesocosm and external controls) would be routinely collected (biweekly to monthly) and analyzed for anions, cations, and nutrients. More intensive sampling of surface water, and pore water, and biological sampling would be conducted at least 4 times per year during the initial period of the experiment. Surface and pore water will be analyzed for sulfur species, anions, cations, and nutrients. Biological studies to be conducted would include rates of respiration and photosynthesis in macrophytes, abundance and types of periphyton on submerged periphytometers, and numbers and types of macroinvertibrates. We anticipate that the toxicological effects to plants may require many months to be detectable, and therefore we are planning for this experiment to run for two years. At the end of the study, intensive surface water, pore water, sediment, and biological analyses will be conducted. Coring and removal of plants from the mesocosms for further study will be conducted at this time..
(4) Mass Balance Studies of Methylmercury in Tidal Marshes of Shark River Slough
Background/Objectives- One major looming question in the mercury research field is, "where do marine fish (especially commercial fisheries) get their mercury?" Generally, oceanic mercury and methylmercury levels are so low that extraordinary efforts have to be used to even quantify aqueous samples. One hypothesis is that marine fish acquire their mercury through migration patterns that bring them in proximity of coastal waters where plausible mercury methylation could occur (e.g., as ACME has shown for Florida Bay, see above); or, that these fish consume other fish that contact coastal settings. However, it has never been demonstrated whether tidal margins do in fact have active methylation, and tidal forces transport the methylmercury to deeper ocean waters where bioaccumulation commences. The goal of this work element is to collaborate with coastal zone researchers in the USGS (Eduardo Patino) to make use of the ongoing data gathering, sampling and instrumentation to determine if tides do serve to "pump" methylmercury from coastal environments to the oceans.
Methodology- To initiate this research, we will conduct a sampling effort at an instrumented (gauge height, flow rate and direction, pH, salinity, DO, and temperature) over an entire tidal cycle during the summer and winter periods to construct mass flux estimates for tidal creeks in Shark River slough. Samples will be taken at hourly intervals over the 8-12 hour tidal cycle. Samples will include surface water, suspended particulates, and porewater, which will be analyzed for total mercury, methylmercury, sulfate, sulfide, DOC, chloride, and major cations.
U.S. Department of the Interior, U.S. Geological Survey
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