Identifying Local Scale Food Web Variation Using Stable δ¹³C and δ¹⁵N Isotopes in a Central Indiana Reservoir and Downstream River

Abstract

We used stable isotopes of δ¹³C and δ¹⁵N were measured from fish and macroinvertebrate consumers, terrestrial and aquatic primary producers, and detritus to describe food webs for a reservoir and downstream riverine habitat. Measurements of δ¹³C indicated that reservoir energy sources were primarily autochthonous while the riverine habitat was primarily a result of allochthonous carbon sources. Measurements of d 15N indicated that terrestrial primary producers were basal to organic matter, aquatic primary producers, macroinvertebrates, and fishes, respectively. We found significant differences between riverine and reservoir δ¹³C and δ¹⁵N levels among seven fish taxa representing Cyprinidae, Centrarchidae, Percidae, Ictaluridae, and Clupeidae families. Riverine and reservoir overall δ¹³C and δ¹⁵N levels differed by an average of 7.9% and 7.7% by taxa, respectively. Reservoir fish exhibited lower intra specific variation in δ¹³C and δ¹⁵N values compared to river fish indicating less variation in diet. Reservoir fish were also found to feed at more extreme trophic positions (high and low) compared to riverine fish. This evidence suggests the presence of habitat-induced variability in local scale food webs and has implications for understanding aquatic ecosystem diversity and organization.