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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 Priority Ecosystems Science (GE PES)
Fiscal Year 2005 Study Work Plan
Study Title: Integrated Biogeochemical Studies in the Everglades: Task 2, Mercury Cycling and Bioaccumulation
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.
(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.
(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.
Title of Task 1: Integrated Biogeochemical Studies in the Everglades: Task 2 - Mercury Cycling and Bioaccumulation
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:
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.
U.S. Department of the Interior, U.S. Geological Survey
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Last updated: 04 September, 2013 @ 02:08 PM(KP)
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