MEPS 487:55-69 (2013)  -  DOI: https://doi.org/10.3354/meps10402

ABSTRACT: Freshwater inflow from riverine sources is essential for estuarine productivity. However, human alterations to freshwater flow have resulted in modifications to the seasonal complexities 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 (δ¹³C), nitrogen (δ¹⁵N) and sulfur (δ³⁴S) in nekton species sampled from 2 subtropical tidal rivers, one that is relatively natural and one that experiences regulated flow discharges that result in autumnal high-flow events. Primary and secondary consumers in the flow-altered estuary exhibited a significant depletion in ¹³C and enrichment in ¹⁵N (~2‰ in both isotopes respectively) with the onset of the wet season, while the stable isotope values 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 δ¹⁵N, was consistent across seasons and among estuaries, the δ³⁴S 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.

KEY WORDS: Disturbance · Benthic · Pelagic · Resource use · Stable isotopes · Trophic guilds