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projects > application of stable isotope techniques to identifying foodweb structure, contaminant sources, and biogeochemical reactions in the everglades > 2001 Proposal

Application of Stable Isotope Techniques to Identifying Foodweb Structure, Contaminant Sources, and Biogeochemical Reactions in the Everglades

Project Proposal for 2001

Title: Application of Stable Isotope Techniques to Identifying Foodweb Structure, Contaminant Sources, and Biogeochemical Reactions in the Everglades

Project Chief: Carol Kendall, U. S. Geological Survey, 345 Middlefield Road, MS 434, Menlo Park, CA 94025; tel: 650-329-4576; fax: 650-329-5590; email:

Co-investigators: Cecily C. Chang, Steven R. Silva, Bryan E. Bemis, and Scott D. Wankel U. S. Geological Survey, 345 Middlefield Road, MS 434, Menlo Park CA 94025

This proposal is part of the larger USGS project funded through the Placed Based Studies Program: "Integrated Biogeochemical Studies in the Everglades, South Florida", Project Chiefs: W.H. Orem and D.P. Krabbenhoft

Statement of Problem

The south Florida ecosystem, including the Everglades, has been greatly impacted by many disturbances. Over 35 percent of the original natural ecosystem has been converted to agricultural or urban use, and most of the remaining portions are threatened by altered (unnatural) hydroperiods; over 2,250 km of canals dissect what was once continuum of wetlands, water pollution, fires, and a steady loss of wildlife habitat (McPherson et al., 1976; McPherson and Halley, 1996). Nearly 1 million acres of the Everglades are under a health advisory that discourages the human consumption of large mouth bass and several other species of fish because of high mercury contents. Other contaminants of concern include anthropogenically produced organic substances (pesticides, herbicides, polycyclic aromatic and aliphatic hydrocarbons).

Although present in very low concentrations in water, methyl mercury (MeHg) biomagnifies in the foodweb to toxic concentration levels. It is believed that the rates of MeHg production and biological uptake have increased (Winfrey and Rudd, 1990; Gilmour and Henry, 1991) as a result of the environmental change to more anoxic water conditions. One of the main research questions currently under investigation by a multi-agency taskforce in the Everglades is how MeHg and other organic and organometallic contaminants are bioaccumulated up the food chain. A clear understanding of the aquatic foodweb -- the relative trophic positions of different fish, birds, and other aquatic life -- is essential in determining the entry points and subsequent biomagnification rates of contaminants in the Everglades. The primary focus of this study will be to determine the trophic structure of aquatic biota in the Everglades ecosystem through the analysis of bulk samples for carbon, nitrogen, and sulfur isotopes, and to investigate the usefulness of carbon-isotope analyses of essential and non-essential fatty acids as indicators of trophic position and base of the food chain. These data will be used to explain MeHg concentrations in organisms and to determine MeHg bioaccumulation factors. We will also investigate the effects of various nutrients and contaminants sources, and biogeochemical reactions, on the spatial differences in foodwebs.

Background and Previous Work

Mercury contamination of the Everglades ecosystem is one of the most severe cases in the published literature. A clear understanding of transfer mechanisms of contaminants into the Everglades foodweb(s) is essential for understanding how management decisions will affect exposure and risk. Kendall et al (1999) showed that although gut contents analyses by several groups have showed that periphyton was a major component of gambusia diets, that the isotopic compositions of gambusia indicated that they could not be assimilating much periphyton. Alterations in biota diversity caused by anthropogenic changes in nutrients and sulfur can significantly affect the entry points of MeHg by changing ecosystems from foodwebs dominated by algal productivity to ones dominated by macrophytes and associated microbial activity. These types of findings are critical for risk assessments, which are fundamentally based on knowing the exposure levels of contaminants for organisms of concern. Knowing exposure levels, however, requires understanding the relative trophic relationships of Everglades’s wildlife. This study will provide that critical information.

The traditional method of foodweb investigation focused on the determination of gut contents (literally, "who ate what"), and is still a valuable tool used today. However, the gut contents may not provide an adequate representation of the long-term diets of the organisms because gut contents do not indicate what is actually assimilated, and the labor-intensive nature of this work limits the number of specimens and the number of sites that can be examined. In the last decade, stable carbon, nitrogen and sulfur isotope analyses have been used to identify trophic relationships in both lacustrine (eg. Fry, 1986) and marine environments ( eg. Malej et al., 1993). This method is based on the observation that selective metabolism of the lighter isotopes during food assimilation and waste excretion causes animals to become isotopically enriched in 13C and 15N relative to their diets (DeNiro and Epstein, 1978; Fry and Sherr, 1984). Thus an organism is typically between 0 to 1 ‰ (permil, or parts per thousand) enriched in 13C and 2 to 3 ‰ in 15N relative to its diet. Studies of the isotopic compositions of organisms at pristine and nutrient-impacted sites in the Everglades have shown that foodwebs at the impacted sites uniformly have delta13C values (which is defined as the 13C/12C ratio of an organism relative to the 13C/12C ratio in a standard) that decrease with increasing trophic level; this is backwards of the expected trend found at pristine sites (Kendall et al, 1999). For example, we can see that some places have isotope values consistent with periphyton being an important C source (i.e., U3) and others where detrital materials appear to be very significant (i.e., ENR Cell 3). Samples collected at REMAP I sites confirm that there are spatial differences in foodwebs across the Everglades (Paper in preparation).

Continuing studies of the C, N, and S isotopic compositions of organisms at USGS, FGFFC and EPA sites have shown that there is a difference between what a fish eats and what it digests (Kendall et al., 1997a, 1999). This is an important contribution because many groups are attempting to determine trophic relations by use of only gut contents and fish observational data. Several researchers (e.g., Garrison, Trexler, Loftus) have presented estimates of diets for gambusia based on hundreds of gut contents analyses, and have indicated that periphyton or other plants compose as much as 20 to 40% of the contents. Mass balance calculations using the measured isotopic compositions of the items found in the guts show that these gut-based estimates are in error by more than 1 trophic unit, and that the simplest interpretation of this contradiction is that the periphyton in the guts is not being assimilated (Paper in preparation).

The spatial and temporal distributions of the C, N, and S isotopic values of organisms suggest that the values largely reflect variability in reducing conditions in the marshes that favor methane production, sulfate reduction and denitrification (Kendall et al., 1997b, 1998). The isotopic compositions of aquatic plants appear to integrate the more variable water-column isotopic compositions produced by redox reactions (and other factors) in the ecosystem, and these same patterns are incorporated throughout the food chain. The spatial isotope patterns are likely to provide a valuable integration of long-term environmental conditions in the Everglades. Therefore, zones frequently dominated by particular redox reactions appear to be labeled by the C, S, and N isotopic compositions of local organisms. For example, largemouth bass at some sites have distinctive ranges in delta15N, delta13C, and delta34S. These compositions suggest that sites such as L-7, ENR cell 3, and WCA 3 sites have significantly different environmental conditions that isotopically "label" the organisms living there (Kendall et al., 1999). Hence, we can make conclusions about migration habits of game fish. Furthermore, isotopic compositions of biomass may prove to be more cost-effective and reliable indicators of prevailing environmental conditions that favor MeHg production (which is mainly a product of sulfate-reducing bacteria) than other parameters currently being considered because biomass isotopic compositions are much more difficult to perturb than the more transient concentrations of aqueous species (like sulfate or sulfide). The spatial and temporal changes in the isotopic compositions of primary producers caused by these local biogeochemical reactions need to be subtracted from the biota isotopic compositions before trophic relationships can be determined. Paper in preparation.

The extent of sulfur contamination in the Everglades was first documented by Bates et al. (1998) and Orem et al. (1999). High levels of sulfate entering the Everglades have stimulated sulfate reduction and greatly increased concentrations of toxic and reactive hydrogen sulfide, and play a key role in controlling mercury toxicity (see below). Based on biota isotope measurements at over a hundred sites throughout the Everglades, we conclude that the entire biomass at many sites (including all plants, animals, and recent sediments) shows the effect of long-term, in-situ, sulfate reduction in the marsh sediments that has altered local biogeochemical conditions enough to isotopically "label" local biota by producing distinctive changes in delta34S values (Kendall et al., 1997b, 1998). The differences in the delta34S patterns in modern biota and older sediments (mostly < 50 years) is strong evidence for a significant increase in sulfate reduction in the marshes in the last few decades, probably in response to increased S loading from the EAA. The biomass isotope data confirms the patterns seen in dissolved sulfate samples by the Orem et al. (1999), and shows that these sulfate-reducing conditions have persisted for several decades.

The environmental overprint on the C, N, and S isotopic compositions of aquatic organisms has made it difficult to determine the trophic relationships many locations. Bulk N isotopes are proving very useful for establishing relative trophic positions in our research to date, despite the large environmental effects caused by denitrification and ammonium uptake in eutrophic waters. However, the large range in delta13C values of plants and higher organisms caused by various environmental effects (such as methane production and oxidation, plant respiration, water depth, etc.) has made bulk C isotopes of little use in establishing trophic positions. Although bulk isotope measurements are an accepted method for determining foodweb relationships, it has been suggested that sometimes they might only crudely discern relative trophic level because they measure the isotopic composition of all the C, N, and S in the sample (eg., whole fish or muscle tissue), irrespective of chemical speciation. Since organisms are composed of a variety of compound classes such as proteins, lipids, carbohydrates, etc., each of which can have a unique carbon-isotope signature (eg. Parker, 1964; Degens et al., 1968), the isotopic composition for the whole organism is therefore the weighted sum of the isotope values for all the compounds found in that organism. Although bulk stable-isotope methods are useful as a screening tool, it might be possible to better refine relative trophic position by the analysis of specific compounds in organisms for delta13C.

The trophic structure in an aquatic foodweb could be resolved through the carbon-isotope analyses of essential and non-essential organic compounds. A non-essential compound is one which an organism biosynthesizes from dietary carbon. The carbon-isotope composition of this non-essential compound would therefore exhibit an increase in delta13C of between 0 to 1 ‰ relative to the dietary carbon. An essential compound is one which an organism cannot biosynthesize and must therefore acquire directly from the diet. This compound would be incorporated by the organism without chemical alteration or isotopic fractionation. And preferably, the essential organic compound would originate at the very bottom of the food chain via photosynthetic biosynthesis. The difference between the delta13C values of the non-essential and essential organic compounds (deltadelta13C) would then define the trophic position of the organism relative to photosynthesis. Increasingly-larger deltadelta13C values would indicate increasing trophic level separation from primary production. Direct comparisons of trophic structure between different aquatic environments would be possible because the carbon-isotope data would be normalized to local primary productivity.

The essential and non-essential compounds of choice for this study are fatty acids. The non-essential fatty acids of interest are the C16 and C18 homologues which contain 16 and 18 carbons, respectively, and are produced ubiquitously from dietary carbon via lipid biosynthesis. Their delta13C should reflect a one trophic level increase of about 1 ‰ over that of the dietary carbon. The essential compounds are the C18:2, C20:5 and C22:6 fatty acids. They contain 18, 20 and 22 carbons with 2, 5 and 6 double bonds, respectively, and are believed to be exclusively produced by phytoplankton. Although present in fish, shrimp, etc., they are not biosynthesized by the consumer and therefore must be incorporated directly from the diet (Enright et al., 1986). The delta13C values of essential fatty acids found in higher aquatic animals should be similar to those found in the algae. By determining the deltadelta13C values for organisms in an ecosystem, one should be able to discern the trophic position occupied by a given aquatic organism relative to the primary producers with greater certainty than with the use of more conventional bulk-isotope methods.


Major objectives of this project are to: (1) determine the stable C, N, and S isotopic compositions of Everglades biota, (2) use bulk and compound-specific isotopic ratios to determine relative trophic positions for major organisms, (3) examine the spatial and temporal changes in foodweb structures across the ecosystem, especially with respect to the effect of anthropogenically derived nutrients and contaminants from agricultural land uses on foodwebs, (4) evaluate the effectiveness of isotopic techniques vs. gut content analysis for determining trophic relations in the Everglades, (5) evaluate the role of algae vs. detritus/microbial materials in foodwebs across the nutrient gradients, (6) work with modelers to correctly incorporate foodweb and MeHg bioaccumulation information into predictive models, and (7) evaluate the effect of STA’s on foodweb relations and MeHg bioaccumulation.

Results to date show that human activities have dramatically altered the isotopic compositions of biota throughout the Everglades in the last many decades. Hence, interpretation of the isotopic compositions of organisms in terms of foodweb relations has been dependent on first developing a good understanding of the spatial and temporal changes in the isotopic compositions of the major primary producers. We can now build on this dataset to show that nutrient-impacted areas produce very distinctive delta13C patterns that suggest an expanded role of phytoplankton and/or macrophyte materials as a nutrient source at these sites. Preliminary evaluations of several estimates of gambusia diets based on gut contents by using isotopic data suggest that the algal and plant material found in the guts is not being assimilated by the organisms. Surprisingly, algal material in the periphyton mats does not seem to be a major food source in many areas. We are now applying more sophisticated compound-specific isotopic techniques to better defining the role of algae and microbial biomass as food sources and entry points for MeHg into foodwebs.

CONTINUING FOODWEB-RELATED FIELD STUDIES: We plan to continue our field studies. These studies involve the collection of surface water, vegetation, sediments, and animals (zooplankton to fish to bird feathers/eggs) for isotopic analysis. Sampling areas will be selected to answer specific biogeochemical questions, and to build on sampling programs and datasets of local collaborators. Thus, samples for examining sources of nutrients to the Everglades have focused on both marsh sites and canals draining eutrophied and pristine marsh sites. Laboratory and field-enclosure (mesocosm) studies are used to validate and extend field observations about actual isotopic trophic fractionations between organisms.

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. Finally, we intend to integrate all the information from this project into one consistent data base.

Understanding trophic level interactions within aquatic foodwebs is accomplished primarily by determining the bulk C, N and S stable isotopic compositions of organisms (zooplankton to fish to birds), and using a multi-variate approach to assigning relative trophic positions to organisms. Analysis of essential and non-essential fatty acids in selected organisms for delta13C will verify trophic positions determined using delta15N, and will provide more detailed information about primary producers and whether algae or bacteria constitute the base of the food chain at a particular site. Gut content analyses of selected fish provide "snapshots" of what fish have recently eaten but not necessarily digested; this information provides critical boundary conditions for the estimates of average diet of organisms determined using isotopic techniques. These diet estimates will then be compared with the MeHg concentrations in organisms and their diets to determine how MeHg is bioaccumulated in the foodweb. These observations will be verified using Hg isotopes within field-enclosure studies. The isotopic compositions of fish can be used to determine whether the fish migrate in and out of the marshes in response to changes in water levels or food availability. These findings should prove useful for determining where some populations of game fish are acquiring elevated levels of MeHg, at a much lower cost than conventional tag-and-release programs. Estimates of average diets for major organisms and an evaluation of the relative role of several primary producers are being made at about 10 sites currently (which include MeHg hotspots, high-nutrient sites in and near canals, and more pristine marsh sites) and for about 10 new sites (mostly STA's). Plants, sediments, and gambusia from several hundred marsh "survey" sites will also be analyzed for isotopic composition to provide critical background information about local biogeochemical processes that affect isotopic compositions. This data set will allow us to extend our foodweb estimates at well-characterized sites to the entire Everglades. Hence, we will be able to provide wetlands managers with the kind of small-scale, detailed foodweb information needed for predictive restoration models. Such an assessment should prove useful for the better regulation of hydrologic, geochemical, and biological conditions most favorable to the restoration of the Everglades.

MESOCOSMS: One of the difficulties of performing research in the Everglades is the inability to close mass balances due to the boundless nature of the ecosystem. Our larger USGS team proposes to include the use of mesocosms (about 2 meter diameter circular enclosures) at multiple locations throughout the Everglades. Within the mesocosms, we will perform chemical and hydrological manipulations to test the response of the ecosystem to various stresses. Potential manipulations include the addition of stable Hg isotopes to trace and better quantify the biogeochemical pathways mercury can take in the environment, including bioaccumulation. By the use of Hg isotopes, we can trace amendments made to the water column, sediments or periphyton and determine whether new mercury is causing this problem, or whether it is a "recycling" phenomenon. Isotopes of sulfur, phosphorus and nitrogen will also be used in the mesocosms to examine uptake, primary production, and recycling processes in the marshes. These sites might also be suitable for controlled-growth experiments to determine the actual isotopic fractionations for selected organisms.

Although periphyton mats are the most important source of primary production in many parts of the Everglades (McCormick et al., 1998) it does not appear that the higher organisms assimilate much of this food base (Kendall et al., 1997a, 1999). However, periphyton is generally quite high in MeHg, and while many of the nutrients in periphyton may not become incorporated, the MeHg may. We propose to use mesocosms to assess whether periphyton possibly promotes MeHg transfer to the foodweb either by direct consumption or by providing a "MeHg-rich" habitat for consumable organisms like zooplankton which are consumed once they enter the water column. One likely restoration scenario is the establishment of larger periphyton communities due to emplacement of a larger network of nutrient interception wetlands or STA’s. Another possible biological control that could be investigated would be to vary the amount of senescent plant material or detrital material on the sediment surface within mesocosms. Decaying plant material contains more MeHg that living plants in the Everglades, and plant material and detritus is commonly seen in the guts of gambusia. By combining light stable isotope measurements with the Hg isotope measurements, we should be able to settle some the long-standing discrepancies between the short-term observed diets of fish (as determined by gut contents), the actual long-term material assimilated (as determined by C, N, and S isotopes), and the actual entry point(s) for MeHg (i.e., is the Hg in periphyton assimilated and incorporated into biomass by the actually eating and digesting of periphyton or zooplankton, or by selective eating and digesting bacteria living on the mats).

COMPOUND-SPECIFIC ISOTOPE RATIO MASS SPECTROMETRY: This work will investigate the 13C content of specific molecular markers and their application to the understanding of aquatic foodwebs; in future years, we may investigate the 15N content of amino acids for the same purpose. The deltadelta13C between essential and non-essential fatty acids will be used to identify trophic relationships in the Everglades and may serve as a proxy for the bioaccumulation pathway of contaminants. This work will produce a relationship between deltadelta13C of fatty acids (a proxy for trophic level) and methylmercury bioconcentration which could be used in modeling efforts designed to predict changes in MeHg concentration caused by changes in water and land management practices. Samples will be analyzed from several pristine and eutrophic sites, including ones where algae is believed to be the base of the foodweb and ones where detrital materials or particulates are the dominant sources of nutrients at the base of the food chain.

Specific Plans:

- Continue collaborative efforts with Lange (FGFFC) on isotopic tracing of game fish migrations in and out of marshes. Preliminary work suggests that some sites in the Northern Everglades have distinctive C-N-S isotopic compositions that "label" the organisms that life there. Isotopic tracing may be more cost-effective than tag-and-release programs for determining how and where game fish acquire their high MeHg loads.

- Continue our investigations of foodwebs, biogeochemical reactions, and MeHg bioaccumulation in the ENR (in collaboration with Fink and Rawlik, SFWMD), and initiate similar studies in new STA’s. Will these sites be significant sources of MeHg to the foodwebs of migratory organisms?

- Continue our collaborative investigations with McCormick (SFWMD) on the impacts of nutrient concentrations along the F1-U3 transect on spatial variability of foodwebs. This study is also focusing on the question of whether movement of organisms in and out of microhabitats (e.g., sawgrass or cattail stands) in response to water levels, food availability, and predation might explain some of the spatial variability in delta13C (and perhaps foodwebs) observed.

- Investigate whether delta13C analysis of essential vs. non-essential fatty acids in biota will provide more quantitative determinations of relative trophic positions of organisms and of the role of algae in foodwebs. Preliminary work indicates that the differences in delta13C of selected essential and non-essential fatty acids increase with trophic levels above the algal source of the essential fatty acid. We are starting with U3 and 3A-15 (sites with "normal" foodwebs) and will later investigate trophic relationships at more nutrient-impacted sites (e.g., F1, ENR, canals).

- Examine the foodweb structures in the proposed mesocosm experiments, in collaboration with the Aquatic Cycling of Mercury in the Everglades (ACME) team.

- Examine the role of microbial biomass in marsh foodwebs. Preliminary work suggests that bacteria living within algal epiphyton might be a significant food source at some sites (e.g., ENR cell3). Furthermore, it has been suggested that MeHg in microbial biomass within periphyton mats might be a significant source of MeHg to local foodwebs at some impacted sites. We will investigate whether microbial biomarkers might be used to assess whether significant amounts of microbial biomass from algae (which can contain significant amounts of MeHg) might be assimilated when fish eat, but do not digest, algae or other plants.

- Investigate how to incorporate isotopic information about site-specific foodweb relations into predictive models being developed by Harris and Pullman (Tetratech). Can a multi-variate approach be used to estimate "average" trophic relations from a complete dataset of C, N, and S isotopic compositions of organisms?

- Continue our efforts to "map" spatial changes in foodwebs in marsh sites by evaluating the differences in the isotopic compositions of gambusia and algae (in collaboration with REMAP II), and comparing them with more detailed foodweb studies at USGS sites. This information is critical for separating geochemically derived isotopic variability from foodweb-related variability.

- Investigate the usefulness of delta34S for determining trophic relations and/or food sources. It has proved very useful as an indicator of sulfate reduction and, hence, sites dominated by such reactions. Thus far, foodweb efforts have focused almost entirely on delta13C and delta15N.

- Initiate controlled growth experiments (as part of the planned mesocosm experiments, or in the lab) to determine the actual C-N-S isotopic fractionations associated with the assimilation of selected important food sources (e.g., diatoms, other algae, chironomids) by specific organisms (e.g., zooplankton, shrimp, gambusia). It is hoped that some of these lab experiments can perhaps be "piggybacked" onto the existing collaborations with McCormick (SFWMD) and Anderson (MacArthur Res. Cen.) on the effects of nutrient conditions on growth rates.

- Assemble collaborators at the SFWMD (and elsewhere) to develop a workplan (for FY2001) for extending our isotope foodweb studies beyond game fish to higher trophic levels (e.g., birds) and other endangered species. We hope to obtain bird feather and/or egg samples, and possibly animal fur.

- Develop a workplan (for FY2001) to extend our foodweb-MeHg bioaccumulation studies into the freshwater marsh-mangrove zone (coastal waterway zone) in the ENP by interfacing with ongoing work by Loftus and McIvor in the ENP. Wading bird populations in this under-studied, extremely productive, environment have dropped drastically in recent decades. We are currently studying foodwebs and MeHg bioaccumulation in game fish from Rockery Branch and Lostmans Creek in the ENP, in collaboration with Lange (FGFFC). We also are analyzing gambusia, periphyton, floc, sediments, and macrophytes at over 100 ENP sites sampled by the EPA REMAP I and II programs.

- Investigate whether delta34S analysis of sediments provides new insights into marine vs. terrestrial sources of organic matter in Florida Bay, in collaboration with Orem’s team.

- Begin analysis of marsh biota (periphyton, gambusia, macrophytes) and sediments collected by REMAP II for delta34S (and delta13C and delta15N) to extend our understanding of the spatial and temporal extent of S contamination and anthropogenic changes in marsh redox conditions.

- Begin investigation of whether there are significant amounts of S-35 in biota and DOC in selected areas, as an indicator of significant contributions of rain-derived S.

- Begin analysis of selected already-dated cores (USGS or SFWMD) for bulk delta34S (and delta13C and delta15N), in areas where the modern biota show high values of delta34S (suggesting anthropogenically-caused sulfate reduction) compared to the underlying peat.

Expected Results

The deltadelta13C of the essential and non-essential fatty acids should provide a direct measure of trophic position within the aquatic foodweb. Organisms at the bottom of the foodweb that feed primarily on algae should display deltadelta13C values close to those of the primary producers (0-1 ‰), while deltadelta13C values for organisms higher up the food chain should be larger (3-5 ‰ relative to the algae). Significant variation from the expected trend for a given organism may indicate that its diet lies outside that of the primary foodweb.

Other recent foodweb studies (e.g., Cabana and Rasmussen, 1994) have found a linear relation between the bulk delta15N of organisms and the respective dry weight of a bioaccumulated contaminant such as PCBs or MeHg. Preliminary work suggests that there will be a linear relation between deltadelta13C of fatty acids of organisms within the foodweb and the amount of bioaccumulated contaminant. In the Everglades, the contaminant of primary interest is methylmercury. If MeHg concentration data are co-linear with the deltadelta13C data for the foodweb, then the deltadelta13C of fatty acids could be a proxy for the bioaccumulation pathway of MeHg in the Everglades. It may be possible to relate a given change in deltadelta13C to a specific increase in MeHg concentration which can then be used in efforts to model the behavior of MeHg within the foodweb. This determination will be possible as ancillary data on the Everglades available through the USGS includes MeHg concentration data on many of the samples to be used in this study (L. Cleckner, pers. comm.).

- Maps of the temporal and spatial changes in C, N, and S isotopic compositions of so-called indicator species across the Everglades – showing the effect of local biogeochemical reactions on the environment.

- Sufficient site-specific isotopic data on temporal and spatial variability in so-called indicator species that we can successfully normalize our foodweb isotopic data at well-monitored sites (ie remove the baseline isotopic effects) for environmental effects – hence leading to site-specific foodweb diagrams showing trophic structures and energy flow.

- MeHg bioaccumulation factors based on trophic levels established by bulk delta15N and fatty acid delta13C values.

- Open-file reports (with a Web page) with all our isotope data for the Everglades.

- Table of relative trophic positions and foodweb structures in a form useful for modelers.

Planned Foodweb Papers (FY2001):

- "The delta13C of Essential and Non-Essential Fatty Acids and their Application for Trophic Level Determinations in Aquatic Foodwebs" (Dias and Kendall; in draft form now--planned submission winter 2001).

- "Development of a Method for Collection and Analysis of Phosphate from the Everglades for d18O" (Chang et al.; USGS Open-File Report; ready for review).

- "Spatial Changes in Aquatic Foodwebs in the Everglades" (Kendall et al., Limnology and Oceanography?; in draft form now--planned submission winter 2001).

- "Spatial and Temporal Changes in Marsh Redox Conditions using Isotopic Techniques" (Kendall et al.; Water Resources Research?; in draft form now--planned submission spring 2001).

- "Bioaccumulation of Mercury through Aquatic Foodwebs in the Everglades" (Kendall et al., Limnology and Oceanography?; in data-interpretation stage now—planned submission summer 2001).


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Orem, W.H., Bates, A.L., Lerch, H.E., Corum, M., and Boylan, A. (1999) Sulfur contamination in the Everglades and its relation to mercury methylation. USGS Open-File Report 99-181, U.S. Geological Survey Program on the South Florida Ecosystem, Proceedings of South Florida Restoration Science Forum, May 17-19, 1999, Boca Raton, FL, pp. 78-79.

Parker P.L. (1964) The biogeochemistry of the stable isotopes of carbon in a marine bay. Geochim. Cosmochim. Acta., 28, 1155-1164.

Winfrey M.R. and Rudd J.M.W. (1990) Environmental factors affecting the formation of methylmercury in low pH lakes. Environ. Tox. and Chem., 9, 853-869.


Dave Krabbenhoft/Bill Orem and their team (WRD/GD), Bill Loftus (BRD), Ted Lange (FGFFC), Paul McCormick, Larry Fink, Peter Rawlick, Karl Havens (SFWMD), Joel Trexler (FIU), Jerry Stober (EPA)

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