Within the world ocean, the western subarctic Pacific is known as the region with the largest seasonal drawdown in the partial pressure of CO2 due to biological activity, i.e., high spring primary production and particulate organic carbon flux. These distinctive features are mainly caused by intense spring diatom blooms in coastal Oyashio (COY) and Oyashio (OY) waters. Although phytoplankton assemblages in OY waters are rather well studied, little is known about COY waters. In this study, photophysiological properties and phytoplankton community composition in COY waters were investigated during the pre-bloom and bloom periods from March to April 2015. Next-generation sequencing targeting the 18S rRNA gene revealed that the diatom Thalassiosira generally dominated the phytoplankton community and showed distinct differences in the diatom communities in shelf and offshore waters of the COY. Additionally, the  PSII [nm 2 quanta-1 ]

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Sea ice algae contribute up to 25% of the primary productivity of polar seas and seed large‐scale ice‐edge blooms. Fluctuations in temperature, salinity, and light associated with the freeze/thaw cycle can significantly impact the photophysiology of ice‐associated taxa. The effects of multiple co‐stressors (i.e., freezing temperature and high brine salinity or sudden high light exposure) on the photophysiology of ice algae were investigated in a series of ice tank experiments with the polar diatom Fragilariopsis cylindrus under different light intensities. When algal cells were frozen into the ice, the maximum quantum yield of photosystem II photochemistry (PSII; Fv/Fm) decreased possibly due to the damage of PSII reaction centers and/or high brine salinity stress suppressing the reduction capacity downstream of PSII. Expression of the rbcL gene was highly up‐regulated, suggesting that cells initiated strategies to enhance survival upon freezing in. Algae contained within the ice‐matrix displayed similar levels of Fv/Fm regardless of the light treatments. Upon melting out, cells were exposed to high light (800 μmol photons · m−2 · s−1), resulting in a rapid decline in Fv/Fm and significant up‐regulation of non‐photochemical quenching (NPQ). These results suggest that ice algae employed safety valves (i.e., NPQ) to maintain their photosynthetic capability during the sudden environmental changes. Our results infer that sea ice algae are highly adaptable when exposed to multiple co‐stressors and that their success can, in part, be explained by the ability to rapidly modify their photosynthetic competence – a key factor contributing to algal bloom formation in the polar seas.