Bryan E. Bemis Scott D. Wankel
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 #1-5
More recent and specific objectives include: (1) link our data on seasonal and temporal differences in foodweb bases and trophic levels with SFWMD, FGFFC, and USGS Hg datasets (first for large fish and, more recently, for lower trophic levels), (2) investigate the effects of seasonal/spatial changes in nutrients, water levels, and reactions on the isotopic compositions at the base of the foodweb (that affect our interpretation of relative trophic positions of organisms), and (3) 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.
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Bemis, Bryan Wankel, Scott Silva, Steve Chang, Cecily Campbell, Linda
Silva, S. R. Stober, Q. J. Meyer, P.
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 effort has two major elements. The first element is a collaboration with the SFWMD where they will collect 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 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. 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.
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.
1. Continued evaluation and publication of data generated in previous collaborations with ACME, EPA, FGFFC, SFWMD, and FIU to include a paper on environmental controls on mercury in large mouth bass, spatial/temporal changes in foodwebs, a first isotope synthesis paper, and possibly a paper on isotope-gut contents.
2. Investigation of temporal and spatial changes in nutruients, water levels, and reactions on the isotopoic compositions at the base of the foodweb
We tried several times in 1998 to connect the isotopic compositions in the water column to those of plants, but failed to get reliable d13C of DIC and d15N of nitrate and ammonium because of the high DOC concentrations. Scot Hagerthey (SFWMD) is very interested in collaboration since he feels that our isotope data will provide valuable data on the extent of N recycling along the nutrient gradient in WCA2 (and at other SFWMD sites) that will help them understand processes that affect P concentrations in water. Furthermore, after a lot of hard work in FY03 by Scott Wankel on developing and automating a new nitrate d15N-d18O method, and by Dan Doctor (an NRC postdoc working on C cycling at the Sleepers River Watershed) who got our new automated DIC/DOC- d13C device working, we now have the methods we need to see how temporal and spatial variability in the water column solutes are established, and how they affect the d13C and d15N in plants (and microinvertebrates) at the base of the foodweb. We will also analyze a small number of samples for phosphate-d18O, organic matter d34S, and water d18O/ dD. Hagerthey has a number of sites in WCA2 and elsewhere across the Everglades that he samples monthly, and he will collect us water samples, macrophytes, floc, periphyton, and (we hope) microinvertebrates from some 10-15 sites, several times during the year. We are in the process of developing a similar but smaller collaboration with Paul McCormick for sampling at WCA1. Our nitrate and DIC isotope preparation units are extremely well automated, so these samples will not require much manpower to analyze.
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U.S. Department of the Interior, U.S. Geological Survey, Center for
Coastal Geology
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