Insights Into Protistan Plankton Blooms in the Highly Dynamic Patagonian Shelf and Adjacent Ocean Basin in the Southwestern Atlantic

Abstract Blooms of phototrophic protists play a crucial role in marine biogeochemical cycles, significantly impacting carbon fluxes and overall ecosystem productivity. Eukaryotes of the nano‐ and microplankton are responsible for the large spring and summer blooms visible from space in the highly dynamic southwestern Atlantic Ocean. Here, we investigated the composition and abundance of protistan plankton during late spring 2021 across three contrasting oceanographic regions of the southwestern Atlantic Ocean: the Patagonian continental shelf, the core of the Malvinas Current (MC), and the adjacent energetic open ocean in the Argentine basin. Using a combination of in situ sampling and satellite‐derived chlorophyll‐ a , particulate inorganic carbon, sea surface temperature, and geostrophic currents, we identified marked differences in water masses and plankton communities. High chlorophyll‐ a concentration over the outer continental shelf was related to blooms of phototrophic and mixotrophic dinoflagellates and large diatoms. This plankton accumulation over the shelf was associated with the permanent thermohaline front that develops along the shelf‐break, with upwelling as the main driver of high productivity. However, in waters of the MC and the open ocean protists exhibited lower biomass and diversity, with prevalence of nanoflagellates and coccolithophores. The results suggest that the water column stability and the N:Si and N:P nutrient ratios shape the distinct bloom‐forming functional types. This study contributes with data on taxonomic diversity identified by microscopy which, along with remote sensing approaches, provide insights into how protists respond to environmental changes at different spatial and temporal scales.