© 1997 Walt Dineen Society

Walt Dineen Society Annual Conference '97

Session VI: Bird & Animal Studies Abstract #: 97604

Patricia L. Mumford and Brian Fry
Biology Department, Florida International University, University Park, Miami, FL 33199


Food web structures in lakes are often complex and poorly understood. We used stable isotopes to describe the food web linkages and functional similarity of fish communities at 5 sites across Lake Okeechobee. Stable isotopes provide a natural tag that allowed us to find different feeding patterns of fish subgroups within the lake. When averages for the entire fish communities were taken, the five sites sampled showed distinctive patterns of DELTA^13^C and DELTA^15^N isotopic compositions. The marsh site (MH24,000) had distinctly lower DELTA^13^C and DELTA^15^N values than the other sites, the offshore site (LZ-40) had high DELTA^13^C and DELTA^15^N values, and the other 3 sites that were close to the marsh-open water interface had intermediate isotope values. The DELTA^13^C isotope data were most useful for distinguishing which fish species feed offshore, which species use the marsh, and which species migrate between the two habitats. All sizes of black crappie were found to have similar DELTA^13^C values, indicating offshore feeding throughout their lives. In contrast, largemouth bass showed strong changes in DELTA^13^C with size, consistent with a migration from marsh to offshore areas asfish grow. The DELTA^15^N values are a trophic level indicator, with N isotope values increasing with increasing trophic level. The progression of DELTA^15^N across sites generally followed the size of the fish collected, with smaller fish from the MH site having lower values and larger fish from the other sites with higher DELTA^15^N values. Underlying these broad patterns, there was evidence for a finer-grained distribution of isotope labels, beyond the simple marsh vs. offshore contrast. The non-uniform distribution of isotope labels at the marsh site, indicated that small fish differ strongly in their foraging habits.


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