Stable Isotope Food Web Analysis of a Large Subtropical Lake: Alternative Explanations for N 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, δC increased ~4o/oo and δN increased ~10o/oo from primary producers to fish. The δN of fish was ~9o/oo in the littoral zone, ~10 o/oo in the ecotone, and ~12o/oo in the pelagic zone. The cross-habitat enrichment in fish N corresponded with both an increase in the size of fish and an increase in the δN 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 3.4o/oo per trophic level, and normalizing to the δN of primary consumers.