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Hydrology and Primary Production Influence Food-Web Patterns in the Florida EvergladesEPA Grant Number: U916081
Title: Hydrology and Primary Production Influence Food-Web Patterns in the Florida Everglades
Investigators: Williams, Alissa J.
Institution: Florida International University
EPA Project Officer: Just, Theodore J.
Project Period: January 1, 2002 through January 1, 2004
Project Amount: $53,031
RFA: Minority Academic Institutions (MAI) Fellowships for Graduate Environmental Study (2002) RFA Text | Recipients Lists
Research Category: Academic Fellowships , Fellowship - Natural and Life Sciences , Biology/Life Sciences
The objective of this research project was to investigate energy exchange, nutrient flow, and food-chain length—emergent properties of community structure that exhibit complex relationships with abiotic factors, notably primary production and disturbance. One theory predicts that food-chain length will shorten along gradients of disturbance or lengthen along gradients of increasing productivity; however, in natural systems, these two environmental factors may vary simultaneously. In the Everglades, hydroperiod represents a gradient of disturbance because drying events yield high mortality of fishes and macroinvertebrates. Although oligotrophic, local nutrient availability and primary productivity also vary by a factor of two within the Everglades. In recent years, stable isotopes have become a widespread and reliable method in the examination of trophic feeding relationships over time and space.
We characterized food webs at 20 freshwater sites in three regions of the Florida Everglades. Sites were sampled in both the wet and dry seasons, and they encompass a wide range of productivity and hydroperiod conditions. Indicators of productivity were measured by analysis of soil, floc, and periphyton total phosphorus at each site, as well as by primary production of local periphyton samples using light/dark bottle incubations. A total of 702 samples of taxa representing basal, intermediate, and top-trophic levels were analyzed for 15N and 13C. Because of the highly variable nature of primary-producer isotopic analysis, primary consumers were used as the baseline values for food-web analysis. Amphipods and Seminole ramshorn snails were selected as the primary baseline detritivore and herbivore, respectively. Eastern mosquitofish and riverine grass shrimp represented the intermediate trophic levels, and Florida gar were the top carnivores at each study site. We measured food-chain length using 15N, and we estimated an index of detritivory (ID) with 13C to describe food webs.
More variation in food-chain length could be explained by interactions of environmental parameters than by single factors. Both productivity and hydroperiod disturbance were correlated with food-chain length, but these results were dependent on season and top consumer. After adjusting for spatial variation in 13C of baseline consumers, variation in our ID indicated a possible shift along environmental gradients of the relative role of detrital and algal carbon at the base of the food web. In the wet season, 9 of 20 sites were best explained by a detritus-based model and the remainder by a two-source, or detritus-algae model. Dry season data indicated that 10 of 19 study sites were consistent with a detritus-based model and the remainder with the detritus-algae model; only one site fit a purely algae-based model. The ID decreased with increasing productivity and lengthening hydroperiod in both seasons, although results were stronger in the dry season. In the dry season, the interaction of productivity and hydroperiod had a greater influence on basal carbon patterns than either of these two factors alone. Our results indicate that detritus is particularly important as an energy source in the Everglades, and suggest that the role of algae may be overestimated in this ecosystem. In the Everglades, hydroperiod disturbance may play a larger role in food-web structure over time and space than variation in productivity in areas not receiving anthropogenic nutrient enrichment, although productivity also influenced our results. Our results support multiple factors influencing ecosystem processes.