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The Northern California Current (NCC) system is a productive coastal ecosystem with a mosaic of temporal and spatial features. The phytoplankton community plays a crucial role in supporting the rich ecosystem and economically important fisheries of the NCC. Our study integrates data across two years (2022-2023) and multiple transects to investigate the community composition of two major phytoplankton groups in the NCC: picocyanobacteria and photosynthetic picoeukaryotes (PPE). The abundances and cell sizes of the phytoplankton were measured using flow cytometry. We found PPE present at similar concentrations in both summer and winter, while picocyanobacteria were much more abundant in the summer than the winter. The relationship between the picocyanobacteria and PPE varied across on- to off-shore transects with different coastal bathymetry. Abundances of both picophytoplankton increased with distance from shore. Cell size also varied along these gradients. Sampling during a marine heatwave in summer 2023 revealed a shift towards smaller picophytoplankton. Overall, these data reveal a dynamic microbial community underlying a productive coastal system, which could inform management decisions and future ecosystem models in the context of climate change and marine heat waves. 

Abstract Doliolids have a unique ability to impact the marine microbial community through bloom events and filter feeding. Their predation on large eukaryotic microorganisms is established and evidence of predation on smaller prokaryotic microorganisms is beginning to emerge. We studied the association between microorganisms and wild‐caught doliolids in the Northern California Current system. Doliolids were collected during bloom events identified at three different shelf locations with variable upwelling intensity. We discovered doliolids were associated with a range of prokaryotic microbial functional groups, which included free‐living pelagic Archaea, SAR11, and picocyanobacteria. The results suggest the possibility that doliolids could feed on the smallest members of the microbial community, expanding our understanding of doliolid feeding and microbial mortality. Given the ability of doliolids to clear large portions of seawater by filtration and their high abundance in this system, we suggest that doliolids could be an important player in shaping the microbial community structure of the Northern California Current system.