projects > application of stable isotope techniques to identifying foodweb structure, contaminant sources, and biogeochemical reactions in the everglades > work plan
Project Work Plan
Greater Everglades Science Program: Place-Based Studies
Project Work Plan FY 2003
A. GENERAL INFORMATION:
Project Title: Application of Stable Isotope Techniques to Identifying Foodweb Structure, Contaminant Sources, and Biogeochemical Reactions in the Everglades
A first step of the Everglades restoration efforts is "getting the water right." However, the underlying goal is actually to re-establish, as much as possible, the "pre-development" spatial and temporal distribution of ecosystems throughout the Everglades. Stable isotope compositions of dissolved nutrients, biota, and sediments provide critical information about current and historic ecosystem conditions in the Everglades, including temporal and spatial variations in contaminant sources, biogeochemical reactions in the water column and shallow subsurface, and trophic relations. Hence, the scientific focus of this project is to use stable isotope techniques to examine ecosystem responses (especially variations in foodweb base and trophic structure) to temporal and spatial variations in hydroperiod and contaminant loading for the entire freshwater Everglades.
This is the largest isotope foodweb study ever attempted in a marsh ecosystem, and combines detailed, long-term, trophic and biogeochemical studies at selected well-monitored USGS/SFWMD/FGFFC sites with limited synoptic foodweb data from over 300 sites sampled during 1996 and 1999 by a collaboration with the EPA-REMAP program. The preliminary synthesis of the biota isotopes at USGS and 1996 REMAP sites provides a mechanism for extrapolating the detailed foodwebs developed at the intensive USGS sites to the entire marsh system sampled by REMAP. Furthermore, this unique study strongly suggests that biota isotopes provide a simple means for monitoring how future ecosystem changes affect the role of periphyton (vs. macrophyte-dominated detritus) in local foodchains, and for predictive models for foodweb structure and MeHg bioaccumulation under different proposed land-management changes.
Project Objectives and Strategy:
The major "long-term" objectives of this project have been 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 for the entire freshwater marsh part of the Everglades, and (6) work with modelers to correctly incorporate food web and MeHg bioaccumulation information into predictive models. We have generally completed the sample analysis parts of objectives #1-5, and are writing interpretative reports on topics #1-4. We will continue to pursue "publication efforts" related to objectives #5 and #6 in subsequent years (these are #5 and #6 below)
More recent and specific objectives include: (1) investigate isotopic compositions and trophic structures in ENR and STA's, (2) extend our foodweb studies beyond fish to bird and alligator populations, (3) link our data on seasonal differences in foodweb bases and trophic levels with SFWMD, FGFFC and USGS Hg datasets, (4) investigate the effects of seasonal/spatial changes in nutrients, water levels, and reactions on the isotopic compositions at the base of the foodweb, (5) continue our efforts to link our foodweb isotope data from samples collected at USGS-ACME and EPA-REMAP sites with the spatial environmental patterns observed by the REMAP program, and (6) continue our efforts to work with ecosystem modelers to incorporate our isotope data and conclusions into current models.
Potential Impacts and Major Products:
Applications of our project to the Everglades Restoration include: (1) Biota isotopes provide a map of the current spatial distributions of the extent of several biogeochemical reactions (especially sulfate reduction) affecting nutrient and Hg uptake. (2) By comparing the spatial patterns in the biota with those in the shallow sediments, recent anthropogenic changes in biogeochemical processes at the landscape scale can be demonstrated and dated. (3) Isotopes provide detailed information about temporal and spatial changes in trophic relations that complements traditional gut-contents analyses used by the FGFWFC (and others) for understanding foodwebs and the bioaccumulation of contaminants. (4) The preliminary synthesis of the biota isotopes at USGS and 1996 REMAP sites provides a mechanism for extrapolating the detailed foodwebs developed at the intensive USGS sites to the entire marsh system sampled by REMAP. (5) Biota isotopes provide a simple means for monitoring how future ecosystem changes affect the role of periphyton (vs. macrophyte-dominated detritus) in the gambusia foodchain, and for predictive models for MeHg bioaccumulation under different proposed land-management changes. (6) Since the REMAP spatial data are likely to be an important "benchmark" for assessing ecosystem changes, it is critical that these data be critically evaluated in the context of data generated by the USGS and local FL agencies; we are among the few scientists who are trying to "justify" the different perspectives and data generated by the USGS and EPA teams.
Recent products include many talks/posters, a few minor publications, 2 major synthesis papers in review, currently 3 more in various stages of preparation, and a database ready for the web. Future products will include several more peer-reviewed synthesis papers, Scott Wankel's dissertation, a fact sheet about isotopes and foodwebs, and an edited database of average isotopic compositions of organisms collected at USGS sites.
Clients: Governmental and private agencies involved in the Comprehensive Everglades Restoration Program (CERP).
B. WORK PLAN
Title of Task 1: Continued evaluation and publication of data generated in previous collaborations with ACME, SFWMD, EPA, FGFFC, and FIU.
Task Summary and Objectives: Our isotope study in the Everglades is the largest marsh foodweb isotope study ever attempted -- in number of samples analyzed, size of the field area, and number of sites sampled. Our challenge was to link the detailed foodweb and process information available at the USGS-ACME sites (where we had data at some 15 sites, many sampled 2-3 times per year for several years) with the wealth of synoptic data but limited foodweb samples (mainly just periphyton and gambusia) collected by the EPA-REMAP program in their 1996 and 1999 field campaigns. It took 6 years and >7000 samples for us to develop an integrated and coherent model for major controls on spatial and temporal variations in foodwebs and foodweb bases, and we have started the time-consuming task of dividing up the many complicated stories into publishable units. With the large amount of data we have available -- both our isotope data and the ancillary geochemical, hydrological, or gut contents data provided by our collaborators -- data interpretation and paper writing go very slowly.
Work to be undertaken during the proposal year and a description of the methods and procedures:
We expect to finish all the needed analyses for this task in FY02 (unless we ask the SFWMD for selected STA samples to fill in some gaps in our dataset). We currently have 2 Everglades papers in journal review, 2 in colleague review, and several more at various preliminary stages. We will spend part of FY03 getting the papers in review to publication. But most of the year will be spent on continuing the statistical analyses, plots, and interpretation required for the three major synthesis papers that are "in progress" - one on spatial variations in foodwebs across the entire Everglades (linking the ACME and 1999 REMAP datasets), one on spatial patterns in biogeochemical reactions, and the last on temporal variations in biogeochemical reactions and foodweb relations.
The goal of these papers is to present a coherent explanation of how biota isotopes provide a simple means for (1) monitoring how future ecosystem changes affect the role of periphyton (vs. macrophyte-dominated detritus) in the foodchain leading to gambusia and its predators, and (2) for developing predictive models for MeHg bioaccumulation under different proposed land-management changes. With our recent success in linking the patterns observed in our ACME foodweb and REMAP 1996 synoptic samples, we can extrapolate the detailed foodwebs developed at ~15 well-studied ACME sites to the ~100 synoptic sites sampled by the REMAP in September 1996. This linkage allows us to determine what environmental variables (thus far, mainly water levels and a complicated reflection of the nature of the local reducing conditions - but a lot more statistics are needed to deal with the spatial patterns) correlate with spatial and temporal changes in the dominant base of the foodweb. We will have a completely independent test of the foodweb models we are developing, when we finish analyzing the samples from the spring/fall REMAP 1999 synoptics (we have been careful to not quite finish analyzing the most critical 1999 samples, and to not look at the data, until our synthesis paper is submitted, so that the test is truly rigorous). This independent evaluation might be part of Wankel's dissertation (and overlap with Task #2 below).
Other possible "report" activities for FY03-FY05 include: (1) collaborating on a comparison of gut-contents and isotope data for REMAP gambusia samples (with Trexler) and/or largemouth bass (with Lange); (2) working with Thorton on incorporating our REMAP isotope data into his spatial models (are the spatial patterns we both see due to spatial changes in foodweb complexity (Stober's interpretation) or foodweb base (our favored isotope interpretation); (3) writing a third, more comprehensive, paper using our isotope data to explain spatial and temporal variations in Hg levels (with Rawlik, Krabbenhoft, and/or Lange); (4) collaborate (with Rawlik and Fink) on a paper on foodweb differences at cells in the ENR and various STA's where we have analyzed samples collected by the SFWMD; or (5) we will finally report the results of our attempts at using compound-specific C isotopes as a tracer of trophic relations (these data generated by Dias, a previous NRC postdoc, have been sitting in limbo for 2 years, since he left).
Planned Outreach: Results will be presented to our colleagues and collaborating agencies through talks and posters at meetings. Data and information will be made available on the SOFIA web site. Several papers are "in journal review" and others are making their way towards submission, detailing the methodology, data, and interpretations. A fact-sheet about the use of isotopes in ecological studies in the Everglades will be prepared, based in part on the most frequent foodweb-type questions submitted to the SOFIA help desk (I understand that one of our isotope foodweb diagrams, available on a poster on SOFIA, is the most requested item from the help desk).
Title of Task 2: Investigation of the effects of temporal and spatial changes in nutrients, water levels, and reactions on the isotopic compositions at the base of the foodweb
Task Summary and Objectives: The main objective of this study (which will constitute much of Scott Wankel's dissertation at Stanford) is to test several hypotheses generated during our earlier studies (and papers) about how temporal and spatial changes in nutrients, water levels, and biogeochemical reactions affect the isotopic compositions of the algae, macrophytes, and bacteria at the base of Everglades foodwebs. This validation is a critical "missing link" between our biota isotope data and the chemical and hydrologic data (and models!) generated by the ACME, EPA, SFWMD, and FGFFC teams. Without these data, our "mechanistic" interpretation of spatial and temporal changes of foodwebs and foodweb bases will rest largely on statistical correlations made possible by our huge dataset at hundreds of sites and dates, and will not be easily related to actual variations in nutrient sources and environmental sources at specific locations. However, we want to have more than just statistical data and arguments to support our process-based interpretation of how the temporal and spatial biota isotope patterns are established. In other words, we want to better establish exactly how spatial and temporal differences in the isotopic compositions of periphyton and macrophytes are established from the temporally and spatially varying isotopic compositions of dissolved carbon, nitrogen, and sulfur in the environment (which are directly linked to nutrient sources and biogeochemical processes).
Work to be undertaken during the proposal year and a description of the methods and procedures:
This project has two major elements. The first element is a collaboration with the SFWMD where they will collect us water and plant (and some animal) samples from selected sites (among the sites along the nutrient gradient that they are currently studying), for us to analyze for stable isotopes. Our main focus will be to analyze the water for d15N/d18O of nitrate, d15N of ammonium, d13C of dissolved inorganic and organic carbon (DIC and DOC), d34S of sulfate, and d18O of phosphate (selected samples); biota will be analyzed for d15N/d13C/d34S. These data will allow us to (1) trace the sources of the nutrients in the water column (eg, original, recycled, or mixed-sources of N, S, C, and P), (2) provide information about the nature of the recycling reactions in the water column and floc layer (eg, denitrification vs nitrification of organic N, methane oxidation vs respiration vs exchange with the atmosphere, sulfate reduction), and (3) provide needed empirical data on specifically how changes in water level affect the complex interplay of biogeochemical reactions that result in the isotopic compositions of aquatic plants at the base of local foodwebs.
There are virtually no data on the d15N of nitrate or ammonium, or d18O of phosphate, in the Everglades because such samples pose extreme analytical challenges. After several years of work, we finally have usable methods (some we developed as collaborations, and some new published methods that we are adopting), and we are currently analyzing our first few test samples. In particular, this study is now possible because some recently published methods will allow the analysis of d15N/d18O of nitrate on ml-sized samples instead of the L-sized samples required with our earlier methods (making it much more feasible for our collaborators to collect and transport us bimonthly samples from a number of sites); a newly purchased automated device that makes it easy to analyze d13C of DIC; and a successful collaboration with colleagues at Stanford that has resulted in the development of an improved method for d18O of phosphate (co-authored methods paper to be written this fall). If resources permit, we would like to use phosphate d18O to see if we can distinguish between sources of phosphate along the nutrient gradient and into FL Bay.
The second element is a collaboration with Bill Orem, Chuck Holmes, and others who have collected and age-dated sediment cores from the freshwater marshes; we are particularly interested in changes in the last 100 years. We want to analyze bulk organic matter for d13C, d15N, and d34S (and perhaps organic phosphate-d18O) to test our models for how the isotopic compositions in plants change over time in response to changes in water levels, nutrient conditions, and biogeochemical reactions (especially ones like sulfate reduction and denitrification, that have been significantly affected by recent anthropogenic activities).
These organic matter isotope data will also be extremely useful for refining our understanding of the role of S in the environment. For example, we have preliminary d34S data from a few age-dated cores where we can date the spatial distribution of the timing of the on-set of the massive ecosystem response to overuse of agricultural S - namely, a proliferation of sulfate reducing bacteria that have caused d34S of the remaining S in the entire environment to shift by up to 15 permil. Since these same sulfate reducers methylate mercury if available, these maps reflect the spatial and temporal patterns of one of the main parameters controlling the "potential" for MeHg production. Arguably, since Hg is a transient chemical in the water column, and measured Hg methylation rates are highly dependent on temporal changes in water chemistry, the d34S of biota may prove to be more cost-effective and reliable indicators of prevailing environmental conditions that favor MeHg production 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). Hence, the spatial isotope patterns are likely to provide a valuable integration of long-term environmental conditions in the Everglades.
We will compare these core isotope data with our sediment isotope data for the 300+ REMAP 1996/1999 sites where we have samples, to show how one aspect of the methylation potential (as indirectly measured by sulfate reduction) has changed over time and space. These S results should be useful for the development of a sulfur module for Everglades predictive Hg models, and for modeling or land-management attempts to predict the potential for methylation based on easily measured environmental parameters (like those measured by REMAP). If resources permit, we would be interested in looking at temporal changes in d13C/d15N/d34S at some of the USGS cores recently collected to assess the evolution of tree islands.
We hope to complete much of the sample collection and analyses of the water samples for element 1 in FY03. However, this will extend into FY04 if we have problems getting samples from the SFWMD, encounter significant method and/or analytical difficulties (not unreasonable for such challenging analytes), or if the preliminary data cause us to shift our hypotheses a bit. We plan to complete all of the d13C/d15N and most of the d34S analysis of the existing core samples (element 2) in FY03, assuming we get prompt access to the desired cores.
Planned Outreach: Results will be presented to our colleagues and collaborating agencies through talks and posters at meetings. Data and information will be made available on the SOFIA web site. Peer-reviewed papers will follow, detailing the methodology, data, and interpretations.