Unveiling Short-Scale Responses: How Pico- and Nanoeukaryotic Plankton Navigate Environmental Variability in a Coastal Upwelling System

For decades, identifying pico- and nanoeukaryotic plankton has been challenging due to their small size, leaving a significant gap in our knowledge of their composition and dynamics in comparison with their larger counterparts. The advent of molecular techniques unlocked new possibilities for exploring this hidden diversity. We applied metabarcoding targeting the V9 region of 18S rDNA to discern the principal taxonomic groups of pico- and nanoeukaryotes in the Ría de Vigo during the upwelling season. Nanoeukaryotes (NE) exhibit greater diversity compared to picoeukaryotes (PE). Specifically, NE were mainly comprised of nano-sized diatoms and dinoflagellates, many of them uncategorized novel species. Within PE, Syndiniales and Marine Stramenopiles (MAST) were the main components identified. We also captured short-term changes in the biomass and composition of PE an NE, with advection emerging as one of the most significant drivers. Most notably was the ocean inflow of unassigned picoeukaryotes into the Ría, likely driven by the negative circulation during downwelling. Moreover, local grazing within the Ría seems to be important enough to alter NE dynamics, but has a minimal effect over PE. Our findings improve the understanding of the small eukaryotic plankton community in coastal upwelling systems, highlighting a significant potential for novel diversity within these environments.