Bidirectional energy subsidies for fish in river mouths of a large lake as revealed by stable isotopes and fatty acids

Abstract

Freshwater river mouths may facilitate cross-habitat resource use and bidirectional subsidization through (a) active cross-habitat movement by consumers like fish and (b) water and material movement both downstream, from tributary to nearshore lake, and in the opposite direction due to backflow processes. Despite potential importance of freshwater river mouths, relative contributions of lentic and tributary energy sources to fishes along these ecotones remain understudied, in part because measuring cross-habitat subsidization is not straightforward. We assessed cross-habitat subsidization of three fish consumers round goby (Neogobius malanostomus), yellow perch (Perca flavescens), and alewife (Alosa psuedoharengus) in three river mouths of Lake Michigan, USA. Specifically, we (a) measured stable isotope ratios (δ13C, δ15N, δ2H, δ18O) in ambient water, bivalve shells, potential invertebrate prey, and fish soft tissue, and we assessed fish nutritional status based on essential fatty acid composition, and (b) estimated Bayesian ellipses and mixing models including either two (δ13C-δ15N) or three (δ13C-δ15N-δ2H) isotope ratios. Results revealed evidence of bidirectional habitat subsidies for all fishes, but energy subsidies varied by species and across river mouths; likely related to differences in species-specific behavior and hydrologic differences among river mouths. Relative to nearshore oligotrophic Lake Michigan, fish in productive tributary environments contained relatively low essential fatty acid content, suggesting a possible trade-off between prey quantity and quality across habitats. This study advances our understanding of resource connectivity in lake ecosystems and the combined use of multiple stable isotopes (including mixing models) and fatty acids to document cross-habitat subsidies along freshwater ecotones.