Winter microalgal communities of the southern Sea of Okhotsk: A comparison of sea ice, coastal, and basinal seawater

Abstract

The Sea of Okhotsk is a marginal sea of the western North Pacific Ocean and a key region for the seawater circulation of the North Pacific. In winter, a large amount of sea ice forms over the northern shelf and off the eastern Sakhalin coast. The sea ice is transported to the southern part of the sea by the fresh and cold East Sakhalin Current (ESC). Microalgae are the main players in the primary production of the sea ice ecosystems. We investigated the winter microalgal communities of the sea ice ecosystem in the southern Sea of Okhotsk, including the communities living in sea ice and seawater using microscopy and DNA barcoding. The results showed that diatoms were the major component in the microalgal communities. Sea ice algal communities were dominated by Thalassiosira spp., Porosira glacialis, and Fragilariopsis cylindrus, while other typical sea ice pennate diatoms were rarely found. The seawater stations were separated into two groups by hydrological conditions: coastal stations strongly affected by ESC; basinal stations influenced by the deep mixing and the basinal gyre. Small-sized diatoms and naked nanoflagellates were more abundant at the ESC-influenced stations that had higher concentrations of dissolved Fe (DFe) and ammonia (NH4). In particular, spores of Chaetoceros dominated the ESC-influenced seawater. Representative algal taxa specific to the basinal stations, including the diatom Shionodiscus spp., the haptophyte Coccolithus pelagicus, and the dinoflagellate Prorocentrum minimum, were either typical for the basinal gyres or the remnants of the autumn algal blooms. The results suggested that the winter phytoplanktonic communities of the southern Sea of Okhotsk were dependent almost entirely on whether they were within the influence of the ESC. Ice algae were far more abundant than phytoplankton, indicating their major role in the seeding of the spring bloom.