Stable isotopes delineate regional pelagic food web structure in British Columbia's coastal ocean

The pelagic food webs of British Columbia's (BC) coastal oceans have never had a comprehensive review of their trophic structure. In this study, we analyzed carbon (813C) and nitrogen (815N) stable isotope ratios of pelagic food web components collected from four regions in southern BC: Juan de Fuca Strait, the Strait of Georgia, Queen Charlotte Strait, and Queen Charlotte Sound during an August 2019 survey. In addition, conductivity, temperature, and depth (CTD), chlorophyll, and nutrient data were collected to assess the oceanographic basis for regionalization. Between regions, we observed differences in the isotopic baseline driven by regional oceanography. Likewise, we also observed oceanography-driven differences in regional food chain length, carbon range, and isotopic overlap. Species-specific trophic level and isotopic niche were determined. For common pelagic species, we described how trophic level varied regionally, was not always in line with previously published dietary data, and instead largely tracked regional changes in food chain length. We conclude that variable food web properties and trophic ecology can manifest across the small spatial scales of the BC coast's discrete regions.