[A-OS13] Marine ecosystems and biogeochemical cycles: theory, observation and modeling

[AOS13-09] Differences in phytoplankton assemblages and their controlling factors in the southern Sea of Okhotsk between winter and spring

*YANXU SUN¹, Jun Nishioka¹, Takenobu Toyota¹, Koji Suzuki¹ (1.Hokkaido University)

Keywords:Marine phytopalnkton, Sea ice, Spring bloom, Sea of Okhotsk

The Sea of Okhotsk, a semi-enclosed marginal sea of the North Pacific, is strongly influenced by sea ice, which is transported to the southern Sea of Okhotsk (SSO) by the East Sakhalin Current (ESC) and northerly winds in winter. The melting of the sea ice leads to a phytoplankton bloom in early spring. Still, the succession of phytoplankton assemblages from winter to spring and their relationships with controlling factors remain unclear. In this study, we used scanning electron microscopy (SEM), algal pigment signatures with ultra-high performance liquid chromatography (UHPLC), and DNA metabarcoding to investigate the spatial and temporal differences in phytoplankton assemblages. Then, we constructed a co-occurrence network based on 18S rRNA gene sequences to elucidate the influence of environmental factors and the interactions within dominant algal groups. As a result, diatoms were the dominant group in the SSO from winter to spring. However, about half of the winter phytoplankton comprised other phytoplankton groups, such as cryptophytes, dinoflagellates, and chlorophytes, which could be mixotrophic organisms capable of surviving in light-limited environments. In winter, diatom assemblages were mainly associated with ice-covered regions. Co-occurrence network analysis showed that the centric diatoms Thalassiosira and Chaetoceros had a high co-occurrence relationship in ice-melted coastal waters in winter, contributing to the initial bloom with higher chlorophyll a concentrations in spring. The sea ice algae or ice-related phytoplankton represented by Porosira, Bacterosira, and Fragilariopsis in the ice-covered seawater were more likely to influence the algal assemblages in the coastal seawater. The initial spring bloom by large centric diatoms ended due to lower inorganic nitrogen availability and then entered the second half of the bloom dominated by the smaller Chaetceros and the pennate diatom Fragilariopsis. At the same time, the proportion of other phytoplankton represented by dinoflagellates and cryptophytes increased. Co-occurrence network and amplicon sequence variant (ASV)-based ecological trait analyses showed that major diatom groups, including the nutrient-dependent Thalassiosira and temperature-sensitive Chaetoceros, were considered r-strategy-selected genera. In contrast, the ice-related Fragilariopsis appeared to be a K-strategy-selected genus in the SSO from winter to spring. Our study suggests that not only sea ice diatoms but also winter phytoplankton in ESC-affected coastal waters significantly contributed to the beginning of spring bloom in the SSO.