Land Use and Seasonality Influence Fish Trophic Position and Food Web Structure in Headwater Streams

We examined nine headwater streams with a gradient of riparian forest and watershed agriculture to determine the extent to which riparian buffers can mitigate the effects of agricultural activities on stream ecosystems. Sampling was conducted seasonally over two years to explore temporal patterns in fish assemblages related to land use, in-stream habitat, and water quality parameters. Stable isotopes analysis (δ13C and δ15N) was performed to determine how land use and seasonal changes affect energy flow and in-stream food web structure. Results suggest significant community-wide isotopic shifts occurred among streams and seasons. Agricultural streams with low percent forest buffer had compressed food webs based primarily on periphytic energy sources with shorter food chain length compared to forested streams. Streams with high percent riparian forest had higher variability in δ13C than low forest buffer streams, suggesting consumers utilized a greater variety of carbon sources and indicate a basal shift in reliance on detrital energy. Periphyton and consumers were more δ15N-enriched in streams without riparian buffers, but fish trophic position was significantly higher in streams with riparian forest after baseline corrections. The results highlight the importance of allochthonous energy inputs in headwater streams and have implications for restoring riparian areas in agricultural watersheds.