Stable Isotope Food Web Analysis of a Large Subtropical Lake: Alternative Explanations for ¹⁵N Enrichment of Pelagic vs. Littoral Fisheries

Abstract

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, δ¹³C increased ~4o/oo and δ¹⁵N increased ~10o/oo from primary producers to fish. The δ¹⁵N 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 ¹⁵N corresponded with both an increase in the size of fish and an increase in the δ¹⁵N 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 δ¹⁵N of primary consumers.