Distinct phytoplankton assemblages underlie hotspots of primary production in the eastern North Pacific Ocean

Marine eastern boundary current ecosystems, such as the California Current System (CCS), involve productive, mesotrophic transition zones. The CCS exhibits highly variable primary production (PP), yet factors driving the variability and underlying phytoplankton communities remain poorly understood. We integrated physicochemical and biological data from surface waters sampled during 10 CCS expeditions, spanning 13 yr, and resolved regimes with distinct phytoplankton communities. Additional to an oligotrophic regime (OR), mesotrophic waters beyond the coastal area partitioned into Meso‐High and Meso‐Low regimes, differing in nitrate concentrations and PP. The OR was dominated by Prochlorococcus High‐Light I (HLI), and eukaryotic phytoplankton were largely predatory mixotrophs. Eukaryotes dominated Meso‐Low and Meso‐High phytoplankton biomass. Within the Meso‐Low, Pelagomonas calceolata was important, and Prochlorococcus Low‐Light I (LLI) rose in prominence. In the Meso‐High, the picoprasinophyte Ostreococcus lucimarinus was abundant, and Synechococcus Clade IV was notable. The Meso‐High exhibited the highest PP (38 ± 16 mg C m−3 d−1; p < 0.01) and higher growth rates for photosynthetic eukaryotes (0.84 ± 0.02 d−1) than for Prochlorococcus (0.61 ± 0.01 d−1) and Synechococcus (0.31 ± 0.05 d−1). An experiment simulating seasonal oligotrophic seawater intrusion into the Meso‐High resulted in growth rates reaching 1.18 ± 0.10 d−1 (O. lucimarinus), 0.75 ± 0.21 d−1 (Prochlorococcus LLI), and 0.50 ± 0.04 d−1 (Synechococcus EPC2). Thus, variable PP is underpinned by distinct phytoplankton communities across CCS mesotrophic regimes, and their dynamic nature is influenced by the rapidity with which specific taxa respond to changing environmental conditions or possibly transient nutrient release from viral encounters. Future work should assess whether these dynamics are consistent across eastern boundary current ecosystems and over temporal variations.