Using stable isotopes to discern mechanisms of connectivity in estuarine detritus-based food webs

In this paper, we focus on 2 mechanisms of cross-boundary food web connectivity in Puget Sound estuaries: passive transport of water-advected organic matter (OM) and active movement of organisms. Both mechanisms serve as potential vectors of food web connectivity, but little research has investigated whether landscape setting changes the dominance of one mechanism over another, or whether the influence of organism movement on food web connectivity can be detected in estuarine systems. We use fish diets, stable isotopes and Bayesian mixing models to identify differences in OM sources assimilated by estuarine fishes, testing whether increased organism mobility or increased fluvial influence results in greater food web connectivity. We compare food web connectivity in 2 different estuaries, one displaying limited freshwater inputs, and the other the terminus of a major river system. Within each estuary, we investigate whether differences in behavioral life history traits correspond to differences in the diets, isotopic signatures and OM assimilation of 2 fish species: bay pipefish Syngnathus leptorhynchus, which displays site fidelity to eelgrass beds, and the more transitory juvenile English sole Parophrys vetulus, which moves throughout estuarine deltas during the early demersal growth stage. Our results show water advection plays a dominant role in large-scale OM transport and delivery to adjoining ecosystems in the fluvial estuary, while organism movement provides the more important mechanism of food web connectivity in the estuary exhibiting minor fluvial discharge. However, the 2 mechanisms certainly interact to enhance food web connectivity across estuarine ecotones.