The southern Sea of Okhotsk (SSO), a semi-enclosed marginal sea of the North Pacific, is characterized by winter sea ice and massive spring diatom blooms. However, the succession of phytoplankton assemblages, including diatoms, from winter to spring and their relationships with controlling environmental factors remain unclear. Additionally, differences in phytoplankton photosynthetic activity between the two seasons are poorly understood. Here, we used microscopy, algal pigment signatures, and 18S rRNA gene (rDNA)-based metabarcoding to identify predominant phytoplankton assemblages in surface waters of the SSO from winter to spring in 2022, and compared these data with environmental factors. As a result, diatoms were the consistently dominant group in the SSO from winter to spring. Co-occurrence network analysis using 18S rDNA sequences showed that the centric diatoms Thalassiosira and Chaetoceros had stronger associations with ice-melted coastal waters in winter, contributing to the spring bloom with higher chlorophyll a levels. The ice- or ice-related diatoms represented by Porosira, Bacterosira, and Fragilariopsis in under-ice seawater were more likely to influence phytoplankton assemblages in coastal seawater. We compared the expression levels of the rbcL gene, which encodes the large subunit of the photosynthetic carbon-fixation enzyme RuBisCO, across dominant diatom families in the SSO. Centric diatoms exhibited significantly higher rbcL transcription efficiency than pennate diatoms throughout the study period. The transcription efficiency of three dominant diatom families—Chaetocerotaceae, Thalassiosiraceae, and Bacillariaceae—showed strong positive correlations with both the maximum quantum yield of photochemistry (F_v/F_m) in algal photosystem II and chlorophyll a concentration. We identified photosynthetically active diatom groups that contribute to high primary productivity in spring blooms and are sensitive to environmental changes in seasonally ice-covered regions.

Sea of Okhotsk,sea ice,spring bloom,diatoms,phytoplankton