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Project Summary Sheet

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

Fiscal Year 2004 Study Summary Report

Study Title: Application of Stable Isotope Techniques to Identifying Food Web Structure, Contaminant Sources, and Biogeochemical Reactions in the Everglades
Study Start Date: 1996 (as part of the ACME project); separate project since ~2000. Study End Date: 9/05
Web Sites:;
Location (Subregions, Counties, Park or Refuge): Entire freshwater system
Funding Source: USGS's Greater Everglades Priority Ecosystems Science (PES) Initiative, NRP
Principal Investigator(s): Carol Kendall,, 650.329.4576.
Study Personnel: Bryan E. Bemis,, 650.329.5603; Scott D. Wankel,, 650.329.4303; Steven R. Silva,, 650.329.4558.
Supporting Organizations: South FL Water Management District, FL Game and Fresh-water Fish Commission, US EPA
Associated / Linked Studies: Integrated Biogeochemical Studies in the Everglades, South Florida (Orem,, 703.648.6273; Krabbenhoft,, 608.821.8181); Spatiotemporal Patterns and Ecological Effects of Canal-Water Intrusion into the A.R.M. Loxahatchee National Wildlife Refuge (McCormick,, 304.724.4478)

Overview & Objective(s): The primary scientific focus of this study is to examine ecosystem responses (esp. variations in food web base, trophic structure, and marsh biogeochemistry) to variations in hydroperiod and contaminant loading, and how restoration steps may affect spatial/temporal changes in food webs and MeHg bioaccumulation. Specifically, the major goals have been to: (1) determine the stable C, N, and S isotopic compositions of Everglades biota; (2) determine the relative trophic positions of major organisms; (3) examine spatial and temporal changes in food web structures across the ecosystem, especially with respect to the effect of anthropogenically derived nutrients and contaminants from agricultural land uses on food webs; (4) evaluate the effectiveness of isotopic techniques versus gut contents for determining trophic relations in the Everglades; (5) evaluate the role of algae versus detrital/microbial food webs for the entire freshwater marsh part of the Everglades; and (6) work with modelers to correctly incorporate food web and MeHg bioaccumulation data into predictive models.

Status: Our main goals for FY05 are to finish several papers for publication in peer-reviewed journals and to link our isotope data with the REMAP model. Several papers are in various stages of completion. Accidental data management errors of a colleague have delayed at least one paper from recent submission; revision of papers will begin as the data are corrected. In addition to writing papers, we have begun 4 focused pilot studies designed to answer some questions raised by the earlier studies and to further our ability to use isotopes to assess ecosystem changes: (1) a study with Paul McCormick on tracing the intrusion of canal water into interior refuge areas in WCA1; (2) a related study with Paul McCormick on the effect of nutrients on algal mats and other plants in WCA1; (3) a study with Scot Hagerthey (SFWMD) on the effect of N recycling on P levels and biota isotopes along the nutrient gradient in WCA2A; and (4) a study with Larry Fink (SFWMD) to evaluate the effects of different treatment strategies in the STAs on food web structure and MeHg bioaccumulation. Data from studies 2&3 will be reported in the in-progress Wankel et al. paper. We are actively involved in several ecosystem restoration projects in the San Joaquin River and Delta (funded by CALFED) where we have concentrated on tracing sources of organic matter and nitrate; we would like to develop a cross-comparison of the usefulness of isotope techniques for wetlands/river restoration programs (we will present a poster on our CA studies at the NCER conference).

Recent Products: Recent products include: Development of a web site presenting summary isotope data and food web plots representing (; 6 GEER presentations; several posters on SOFIA; McLaughlin et al., 2004 (phosphate-delta18O methods paper); Havens et al., 2003 (fish in Lake Okeechobee); Rumbold et al., 2002 (alligators and Hg).

Planned Products: Planned products for FY05 include: Bemis and Kendall USGS Fact Sheet on isotopic determination of food web variations across the system (11/04); 3 NCER presentations (12/04); Kendall et al. paper (mini-synthesis, USGS report - waiting for editor for > 2yr); Bemis et al. paper (Hg and isotopes in largemouth bass, being revised to correct Hg data management errors recently discovered with colleagues); Bemis et al. paper (Hg and sulfur isotopes in mosquitofish, submit to coauthors 12/04); Pete Rawlik et al. paper (temporal variation in food webs and Hg, waiting for > 1 yr for author to revise and resubmit); Wankel et al. (changes in solute and biota isotopes along nutrient gradients in WCA1 and 2A, submit to coauthors 7/05); Kendall et al. paper (food web/biogeochemistry synthesis paper, submit to coauthors 9/05).

Specific Relevance to Information Needs Identified in DOI's Science Plan in Support of Ecosystem Restoration, Preservation, and Protection in South Florida (DOI's Everglades Science Plan): [See Plan on SOFIA's Web site:]
This study supports several of the projects listed in the DOI science plan, specifically:

The new study with McCormick supports the Arthur R. Marshall Loxahatchee NWR Internal Canal Structures Project (p. 39) as it: (1) helps to understand the ecological effects of hydrology and water quality on refuge resources (p. 40) by studying isotope tracers of canal water intrusion into the refuge interior; and (2) monitors and assesses indicative geochemical responses of periphyton and macrophytes to changes in water quality (e.g., conductivity, nutrient loads) (p. 40).

This study and the new study with McCormick support the Water Preserve Areas Project (p. 43) as it: (1) provides baselines surveys of water quality and isotopic response of biota for long-term monitoring comparison (p. 44); and (2) provides critical information for environmental risk assessments of water quality contaminants (p. 44) by investigating patterns of isotopes and mercury concentration in biota throughout the Water Preserve Areas, with the goal of identifying mercury sources and pathways.

This study and the new study with McCormick support the Water Conservation Area 3 Decompartmentalization and Sheetflow Enhancement Project (p. 66) as it: (1) provides data to understand and reduce the effects of hydrologic barriers on ecological connectivity (p. 68) by studying interactions of flow with nutrient and carbon cycling and transport (as determined by isotopic and standard chemical tracers); (2) provides research to understand and reduce the effects of canals on the spread of exotic species (p. 68) by geochemically tracing the timing and extent of canal water intrusion into surrounding marsh habitats; and (3) provides additional research to understand the effects of different hydrologic regimes and ecological processes on restoring and maintaining ecosystem function (p. 69), via isotopic determination of changes in food web structure across hydrologic gradients and over time.

This study supports the Landscape Scale Modeling Project (p. 81) as it: (1) provides data that facilitate the simulation of nutrient transport and biogeochemical cycling in the soil and water column (p. 81); (2) facilitates Everglades Landscape Model (ELM) development (p. 82) by providing data on how periphyton respond to changes in water quality, which can be incorporated into the model to improve its accuracy in predicting landscape responses to different water management scenarios; and (3) facilitates Regional Simulation Model (RSM) ecological module development (p. 82) by providing data on how trophic interactions vary temporally at the landscape scale.

Key Findings:

  1. Our isotope and Hg data indicate that MeHg availability (not trophic shifts) appears to be the primary control on Hg concentrations in fish and seasonal changes in hydroperiod, which result in seasonal changes in the importance of algae versus detrital food sources, appear to explain much of the seasonal changes in Hg.
  2. Our various datasets suggest that the most likely explanation for spatial differences in food web structure across the Everglades is spatial changes in food web base (i.e., algae versus detrital), not spatial changes in food chain length.
  3. The preliminary synthesis of the biota isotopes at USGS-ACME and EPA-REMAP sites provides a mechanism for extrapolating the detailed food webs developed at the intensive USGS sites to the entire marsh system sampled by REMAP.
  4. Biota isotopes provide a simple means for monitoring how future ecosystem changes affect the role of periphyton (vs. macrophyte-dominated detritus) in the Gambusia food chain, and for predictive models for MeHg bioaccumulation under different proposed land-management changes.
  5. 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.

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