LAUSR

We determined the stable nitrogen isotope composition (δ15N values) and body size of taxonomic groups in a zooplankton community in the Cantabrian Sea (southern Bay of Biscay) to explore seasonal and depth (0–2000 m) variations in the size‐based trophic structure and their coupling to the production cycle. The positive linear relationship between δ15N values and log‐transformed body size reflects the dominance of new vs. regenerated production. The slope of the relationship (b) is high during productive periods and low when herbivory declines and the food web is more dependent on recycled production. This variation can be attributed to high δ15N values of the smallest plankton after repetitive cycles of microbial degradation. Downward transport of organic matter after the spring phytoplankton bloom was captured by a steady variation from low values of b at the surface to high values at the bathypelagic zone, where the imprint of the spring production pulse could be detected. Variation in b reveals that the mesopelagic and bathypelagic zooplankton communities are as dynamic as their epipelagic counterparts. This shows the efficiency of δ15N vs. body size relationships to capture fast, transient ecosystem processes without need for lengthy incubations or complex rate measurements.