COS 25-5 - Terrestrial-aquatic linkages support food web productivity in an estuarine ecosystem mosaic

Abstract: In estuarine ecosystems, terrestrial primary producers such as riparian and marsh vegetation are believed to promote resilient food webs through direct (detrital) and indirect (consumptive) pathways, but the inherent interconnectivity of estuarine systems often obscures the origins and importance of these terrestrial inputs. The goal of this study was to determine the relative contributions of terrestrial and aquatic primary producers to the estuarine food web and the pathways by which these primary sources influenced food web structure. To accomplish this, we analyzed carbon, nitrogen, and sulfur stable isotopes from multiple trophic levels, environmental strata (i.e., terrestrial, pelagic, benthic), and estuarine habitat types—from tidally influenced forests to outer delta mudflats. We used a Bayesian stable isotope mixing model (SIMM) to elucidate relationships among primary producers, invertebrates, and a pelagic and demersal fish species (juvenile Chinook salmon [Oncorhynchus tshawytscha] and sculpin [Leptocottus spp.]) in the Nisqually River Delta (NRD), Washington, USA. The NRD is a macrotidal estuary with a complex habitat mosaic, making it an ideal system in which to examine terrestrial-aquatic linkages in estuaries. The SIMM indicated that terrestrial salt marsh plants and marine algae and eelgrass formed the base of the NRD food web. Demersal sculpin diets were largely supported by aquatic primary producers via the consumption of benthic and pelagic crustaceans. Conversely, juvenile salmon, which migrate from river to sea, derived their energy from a mix of terrestrial and aquatic primary producers, where the contribution of terrestrial sources to salmon diets ranged between 26 and 43%. Terrestrial inputs were generally limited to indirect consumptive pathways whereby salmon ate dipterans and insects that fell into the water column from the surrounding marsh and riparian zones (as opposed to aquatic crustaceans responsible for processing terrestrial detritus). These findings demonstrate how connectivity among estuarine habitat types and environmental strata promotes robust food webs, especially for ecologically important, migratory species such as salmon.