Stable isotope food web analysis of a large subtropical lake: alternative explanations for 15N enrichment of pelagic vs. littoral fisheries.

The food webs of littoral, pelagic, and littoral-pelagic ecotone (interface) regions of a large subtropical lake were investigated using stable isotope ratio methods, expanding the focus of a previous fish-only study to include other food web components such as primary producers and invertebrates. In these food webs, delta13C increased approximately 4 per thousand and delta15N increased approximately 10 per thousand from primary producers to fish. The delta15N of fish was approximately 9 per thousand in the littoral zone, approximately 10 per thousand in the ecotone, and approximately 12 per thousand in the pelagic zone. The cross-habitat enrichment in fish 15N corresponded with both an increase in the size of fish and an increase in the d15N of primary consumers (mollusks). Despite larger body size in the pelagic zone, fish in all three habitats appear to occur at the same average trophic level (TL = 4), assuming an enrichment factor of 3.4 per thousand per trophic level, and normalizing to the delta15N of primary consumers.