In situ imaging across ecosystems to resolve the fine-scale oceanographic drivers of a globally significant planktonic grazer

Doliolids are common gelatinous grazers in marine ecosystems around the world and likely influence carbon cycling due to their large population sizes and 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 particular regions that do 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 chlorophyll- a fluorescence and oxygen. The relative abundance of the nurse stage tended to increase when total doliolid abundance was low, but this pattern did not hold in upwelling ecosystems, indicating that nurses occupy less favorable habitats in established populations with wider shelf habitats. The doliolids tended to be most aggregated in oligotrophic systems (Mediterranean 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 of population changes in response to oceanographic conditions. ### Competing Interest Statement The authors have declared no competing interest.