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Abstract 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. 

Abstract Doliolids are common gelatinous grazers in marine ecosystems around the world and likely influence carbon cycling due to their large population sizes with high growth and excretion rates. Aggregations or blooms of these organisms occur frequently, but they are difficult to measure or predict because doliolids are fragile, under sampled with conventional plankton nets, and can aggregate on fine spatial scales (1–10 m). Moreover, ecological studies typically target a single region or site that does not encompass the range of possible habitats favoring doliolid proliferation. To address these limitations, we combined in situ imaging data from six coastal ecosystems, including the Oregon shelf, northern California, southern California Bight, northern Gulf of Mexico, Straits of Florida, and Mediterranean Sea, to resolve and compare doliolid habitat associations during warm months when environmental gradients are strong and doliolid blooms are frequently documented. Higher ocean temperature was the strongest predictor of elevated doliolid abundances across ecosystems, with additional variance explained by chlorophyllafluorescence and dissolved oxygen. For marginal seas with a wide range of productivity regimes, the nurse stage tended to comprise a higher proportion of the doliolids when total abundance was low. However, this pattern did not hold in ecosystems with persistent coastal upwelling. The doliolids tended to be most aggregated in oligotrophic systems (Mediterranea and southern California), suggesting that microhabitats within the water column favor proliferation on fine spatial scales. Similar comparative approaches can resolve the realized niche of fast‐reproducing marine animals, thus improving predictions for population‐level responses to changing oceanographic conditions.