Reversible Regime Change: Climate‐Driven Phytoplankton Community Shifts in the Cariaco Basin, Venezuela

Abstract Phytoplankton communities are integral to oceanic biogeochemical cycles and are sensitive indicators of climate‐driven environmental variability. Long‐term time series capture this variability, allowing us to unravel the effects of environmental change on local communities. This study investigates changes in the phytoplankton community of the Cariaco Basin, Venezuela, from 1996 to 2016, using monthly water column observations integrated with climate indices and meteorological data. To investigate impacts of larger‐scale climate variability, we considered the Atlantic Multidecadal Oscillation (AMO) index and Multivariate El Niño Southern Oscillation index (MEI v.2), both linked to local weather and upwelling. Cluster analysis identified two distinct community states separated by a shift from high to low upwelling in 2004. In 2014, conditions shifted toward a stronger upwelling regime, and community composition began transitioning toward the pre‐2004 state. No significant differences in genus richness or Shannon diversity were observed between the two periods. The low upwelling period, however, was characterized by significantly higher community evenness and lower species turnover. To quantify effects of climate and bottom‐up drivers on phytoplankton community changes, we employed gradient forest analysis and found the AMO index was the strongest predictor of changes in the community, closely followed by in situ variables related to upwelling (sea surface temperature (SST) and nitrate concentration) and the MEI v.2. Our study emphasizes the impact of long‐term climatic oscillations, such as the AMO, on the phytoplankton community of a tropical coastal ecosystem. The transition toward pre‐2004 conditions from 2014 suggests a cyclic behavior of the ecosystem under decadal‐scale climate forcing.