Stable isotopes reveal that bottom-up omnivory drives food chain length and trophic position in eutrophic coastal ecosystems

Abstract In coastal marine food webs, food chain length and the distribution of trophic levels mediate the impacts of emerging threats such as overfishing, pollution, and climate change, through their effect on essential properties such as productivity, connectivity, and energy transfer efficiency. These two components of food web structure are therefore an essential element of ecosystem-based management; however, what drives them remains poorly understood. It has been hypothesized that high primary production drives lower trophic levels and shorter food chain length in coastal regions. Here, we evaluate this hypothesis on the British Columbia (BC) coast as this coastal marine ecosystem is comprised of regions with contrasting levels of primary production. We measured nitrogen stable isotopes from zooplankton, micronekton, and nekton collected from four of the main water bodies of southern BC during a survey completed in August of 2019. We used Sentinel-3 satellite data to determine overall production in each region across a 5-year climatology and during the 2019 season. Results showed that primary production varied significantly between the four regions and that increases in phytoplankton biomass were driven by increases in microphytoplankton. Nitrogen isotope data demonstrated a significant inverse relationship between a region’s primary production and both total food chain length and trophic level in species sampled in multiple regions, but no change in the trophic structure of the zooplankton community across regions. Changes in species’ trophic levels were therefore driven by shifts in the level of zooplanktivory. These results support a model of bottom-up omnivory driving coastal food web structure, where levels of primary production, specifically biomass of microphytoplankton, determine food chain length and the distribution of trophic levels. High microphytoplankton biomass supports a large biomass of large grazing zooplankton, which drive increases in omnivorous feeding behaviour among the micronekton and nekton.