Stratification intensity structures zooplankton functional trait composition in a continental shelf system

Zooplankton composition and distribution influence prey quality and availability for higher trophic levels, yet ecological forces structuring communities are not often resolved on spatial scales relevant to predator–prey encounters (1–10 m). Because continental shelf water columns are often vertically stratified, fine-scale interactions may influence overall biological productivity. Using a towed imaging system, we measured meso- and macrozooplankton abundances (>2.2 mm equivalent spherical diameter) in the South Atlantic Bight between the 25 and 45 m isobaths in August 2021. Zooplankton were parsed into four key traits (size, carbon content, trophic strategy, and swimming speed), and buoyancy frequency was used to identify discrete vertical oceanographic zones. Trait diversity was less variable in mixed waters due to the dominance of low carbon content zooplankton or passive swimmers. Upwelling intrusions generated high chlorophyll-a and sharp stratification, which favoured high-carbon, fast swimming zooplankton. Trait group abundances were often higher in these deeper, sharply stratified waters, suggesting that intrusions generally favour secondary production, with gelatinous organisms gradually becoming more dominant as the pycnocline weakens. The distribution of size classes, however, did not change among water masses. Stratification and mixing generate distinct environments and consistent trait assemblages, potentially improving predictions of community responses to oceanographic structure.