Trophic Analysis Armed with a Trident: A Three-pronged Approach to Characterizing the Role of the Ctenophore Pleurobrachia bachei in Pelagic Food Webs

Jessica Masterman, United States, Aaron Winson Elias Galloway, University of Oregon, Oregon Institute of Marine Biology, Charleston, OR, United States, Robert Cowen, Oregon State University, Hatfield Marine Science Center, Newport, OR, United States, Su Sponaugle, Oregon State University, Department of Integrative Biology, Corvallis, OR, United States and Kelly Sutherland, University of Oregon, Oregon Institute of Marine Biology, Eugene, United States
Abstract:
Gelatinous zooplankton make up a substantial portion of the overall plankton, yet the trophic relationships between these organisms and the surrounding plankton community--including larval fishes-- remain poorly understood. Various methods may be used to understand pelagic food webs, yet each brings its own limitations. Gut content analyses provide a snapshot of recently ingested prey but don’t account for soft-bodied prey. Biomarkers such as stable isotopes and fatty acids provide a longer term view, yet fine scale determinations may be confounded by prey items exhibiting similar stable isotope or fatty acid signatures. Working in the productive northern California Current (NCC), we are employing a three-pronged approach of 1) gut content, 2) stable isotope, and 3) fatty acid analyses to provide a robust characterization of trophic relationships between gelatinous zooplankton and the surrounding plankton community. Here we present results from the ctenophore Pleurobrachia bachei, as this gelatinous predator is abundant within the NCC and has been shown to significantly influence food webs in upwelling zones. Because the NCC exhibits high variability both spatially and seasonally, samples of larval fishes, P. bachei, and their potential prey items were collected from cross-shelf transects from two latitudes, and during two seasons over the years 2018-2019. Preliminary results from fatty acid and gut content analyses indicate that calanoid copepods make up a substantial portion of P. bachei diet (0-80%), suggesting that P. bachei may affect larval fishes in the NCC through competition for shared prey items. The results of this study will not only help characterize the trophic relationships among P. bachei and the surrounding plankton community, but should also further demonstrate the value of a multi-method approach in the trophic study of gelatinous zooplankton.
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