Determining dominant phytoplankton assemblages and their controlling factors in the winter sea-ice-covered southern sea of okhotsk and the spring open water through a multiple analytical approach.

The southern Sea of Okhotsk (SSO) is a semi-enclosed marginal sea of the North Pacific. Winter sea ice strongly influences the physical, biogeochemical, and ecosystem processes in the SSO. However, the succession of phytoplankton assemblages from winter to spring and their relationships with controlling factors remain unclear. Here, we used microscopy, algal pigment signatures, and 18S rRNA gene (rDNA)-based metabarcoding to identify predominant phytoplankton assemblages in the SSO from winter to spring during 2022 and compared these data with environmental factors. As a result, diatoms were the consistently dominant group in the SSO, although about half of the winter phytoplankton were other phytoplankton groups as estimated by pigment signatures. In winter, diatom communities were associated with the presence of ice-covered regions. Co-occurrence network analysis using 18S rDNA sequences showed that the centric diatoms Thalassiosira and Chaetoceros had higher co-occurrence relationships with ice-melted coastal waters in winter, contributing to the spring bloom with higher chlorophyll a. The ice or ice-related diatoms represented by Porosira, Bacterosira, and Fragilariopsis in the under-ice seawater more likely influenced the algal assemblages in the coastal seawater. We found that not only sea ice diatoms but also winter phytoplankton in East Sakhalin Current (ECS)-affected coastal waters can contribute to the large spring bloom in the study area, and that could be more prevalent in less sea ice conditions due to warming. The findings of this study highlight the importance of an integrated approach to comprehensively understand predominant phytoplankton communities and their ecological traits in the changing environment.