Organic Matter Flow or Organism Movement- Evaluating Mechanisms of Food Web Connectivity in Puget Sound Estuaries

Cross-ecosystem transfer of materials, energy and organisms often controls population, community and food web structure, highlighting the importance of connectivity at the landscape level.  However, ecosystem-scale changes that affect lateral transfer, such as disruptions to boundary permeability or habitat fragmentation, can alter or destabilize food web structure and function.  Thus, understanding the nature of cross-ecotone boundary transfer is critical to maintaining ecosystem integrity.  In this paper, we focus on two ways in which energy flows across ecotone boundaries in Puget Sound estuaries: the passive transport of water-advected organic matter (OM) and the active movement of organisms across space.  We investigate whether differences in life history traits correspond to differences in the diets and isotopic signatures of two fish species, one that displays site fidelity to eelgrass beds (Sygnathus griseolineatus), and one that moves throughout the estuarine delta during its juvenile growth stage (Parophrys vetulus).  We use Bayesian mixing models coupled with multivariate statistics to identify differences in the supporting OM sources for each fish species, asking whether transitory fish display greater food web connectivity by assimilating OM originating from more ecosystems within the estuarine environment.   We also examine the degree of food web connectivity between two estuaries, one displaying limited freshwater inputs (Padilla Bay), and the other containing a major river system that exhibits frequent flooding and pulsed, seasonal outflow (Skagit Bay).  Our results indicate that while the energy flow mechanisms of OM and organism movement are integrated, the conditions under which one mechanism controls the OM subsidy depends on freshwater discharge and an organism’s life history strategy.