Zooplankton in the temperate ocean display strong ties to pico‐plankton and seasonally variable trophic roles

Abstract Trophic connections between the food web base and zooplankton affect the structure of marine food webs and fluxes to higher predators. The food that zooplankton consume shifts in response to prey resources, but specific zooplankton trophic sources remain unresolved over the dynamic seasonal cycle for many species. We investigated the food web pathways from basal production to 10 meso‐ and macrozooplankton species at six times over an annual cycle at a temperate coastal site, using fatty acid and stable isotope trophic markers. In addition, we concurrently characterized the zooplankton prey field through three size fractions of particulate organic matter (POM) to relate zooplankton trophic sources to specific components of primary production. Distinct stable isotope and fatty acid signatures of POM size classes enabled the identification of different pathways that support zooplankton. Overall, we found strong ties to pico‐POM based production, challenging the expectation that productive coastal regions are sustained by micro‐phytoplankton like diatoms. Zooplankton displayed widespread trophic flexibility and large seasonal differences in trophic markers, including varying by up to > 1 trophic position within species, with several taxa exhibiting seasonally shifting trophic associations. Despite this, inter‐specific differences were the greatest source of variability for zooplankton stable isotopes and fatty acids, which generally corresponded to established trophic roles and feeding types. Defining the seasonal trophic sources to zooplankton is the first step to mechanistically link the food web base to higher trophic levels and to understand how shifts in phytoplankton size structure under climate change will influence higher trophic levels.