Assessing Spatial and Seasonal Variation in Trophic Structure in Nearshore Pelagic Fish and Jellyfish Using Community-Level Stable Isotope Metrics

Pelagic estuarine systems worldwide are exhibiting alarming changes in community structure: the causes of which are often unclear. A holistic, ecosystem-level approach is important to understand these communities and why they may be changing. For example, traditional community attributes such as composition and abundance can be complemented by analysis of trophic structure, which reflects energy flow among consumers, to understand the processes underlying community shifts. We used stable isotopes of carbon and nitrogen to examine trophic structure of pelagic fish and jellyfish in Puget Sound, a large fjord-estuary complex in Washington State, USA. We collected stable isotope compositions for 1,800 fish and jellyfish species across six distinct oceanographic subbasins from April to October 2011. Using a novel community-level analysis, we derived metrics of population and community trophic structure. Specifically, we assessed spatial and seasonal variation in community niche width, trophic length and redundancy as well as potential trophic overlap between fish and jellyfish. Preliminary results indicate strong spatial patterns in community metrics mediated by season. Trophic overlap between fish and jellyfish was also spatially variable but consistently declined seasonally. We discuss the implications of these results, future analyses and the utility of stable isotope community metrics in food web studies.