Anthropogenic alteration of freshwater flows to estuarine systems is of particular importance, as estuaries serve as nursery areas for many top predators. In this study, we analyzed natural abundances of stable-carbon, nitrogen and sulfur isotopes to assess the response of a top predator, the bull shark (Carcharhinus leucas) to anthropogenically induced freshwater flow in the Caloosahatchee River of southwestern Florida, and compared that response to individuals inhabiting an adjacent unmodified river, the Myakka. Spatial differences in baseline isotopic signatures were corrected using a two end-member mixing model.
Results/Conclusions Isotope analysis indicated that juvenile bull sharks occupy a higher trophic level in the low-flow relative to the high-flow season; food chain length was reduced with high-flow disturbance with a shift in top predator from bull shark to gafftopsail catfish (Bagre marinus). However, a reduction of food chain length was supported by a community-wide decrease in trophic level. Analysis based on convex hull area in isotopic niche space suggests trophic niche width of the bull shark is broader under high-flow conditions. These shifts were not identified in the Myakka River, which had similar isotope values to the low-flow season of the Caloosahatchee River, suggesting that high freshwater flow induces a measurable alteration to the food web and that juvenile bull sharks broaden their trophic niche width and feed at a lower trophic levels in response to increased freshwater flow.
