Seasonal variability in stable isotopes of estuarine consumers under different freshwater flow regimes

Abstract

Freshwater inflow from riverine sources is essential for estuarine productivity. How- ever, human alterations to freshwater flow have resulted in modifications to the seasonal complex- ities of estuarine communities. To quantify changes in energy transfer at the trophic guild and consumer levels that result from anthropogenic-altered flow, we evaluated seasonal trends over a 1 yr period in stable isotopes of carbon (δ 13 C), nitrogen (δ 15 N) and sulfur (δ 34 S) in nekton species sampled from 2 subtropical tidal rivers, one that is relatively natural and one that experiences reg- ulated flow discharges that result in autumnal high-flow events. Primary and secondary con- sumers in the flow-altered estuary exhibited a significant depletion in 13 C and enrichment in 15 N (~2‰ in both isotopes respectively) with the onset of the wet season, while the stable isotope val- ues of high trophic level species (i.e. ≥ tertiary consumers) remained relatively consistent, trends were not apparent in the natural estuary. These isotopic trends were evident in both benthic and pelagic consumers, suggesting a food web-wide influence of altered flow on nutrient dynamics. Although the relative trophic structure, based on δ 15 N, was consistent across seasons and among estuaries, the δ 34 S values were less seasonally variable in the flow-altered estuary and were indicative of more terrestrial/freshwater-influenced values compared to the natural estuary, suggesting a lengthened influence of high flow. Although limited in temporal scope, our results indicate a noteworthy difference in seasonal isotope dynamics of lower trophic level consumers in a flow-altered estuarine food web. The importance of this effect on ecosystem function warrants further attention.