Phytoplankton and ice-algal communities in the seasonal ice zone during January (Southern Ocean, Indian sector)

In the seasonal ice zone (SIZ), sea-ice algae have been hypothesized to influence phytoplankton species composition in seawater after melting from sea ice. However, information on ice-algal species composition during summer (December–February) was small particularly in the Indian sector of the Southern Ocean. We investigated sea-ice macronutrient concentrations and relationships between phytoplankton and ice-algal species composition in this region during January of 2018–2020. The species composition of ice algae each year (after weighted-averaged by sample volume for each sea ice) was dominated by the diatom Fragilariopsis cylindrus (78–89% of total cells). This result differed from ice-algal compositions in spring but consistent with those in summer, indicating seasonal changes in species composition of ice algae at East Antarctic SIZ. Using all years’ sea-ice data, we found nitrate + nitrite: phosphate ratio declined significantly with increased algal concentrations. Diatoms’ uptake ratio of macronutrients and remineralization of phosphorus from their products (e.g., dissolved organic matter) may result in this relationship, which indicates algal accumulation especially of diatoms in sea ice. At the ice-sampling stations, phytoplankton was dominated by F. cylindrus, while at the northern stations, higher proportions of F. kerguelensis, Pseudo-nitzschia prolongatoides/subcurvata, and Gymnodiniales were observed. The similarity index of sea-ice and phytoplankton composition was negatively correlated with days elapsed since sea-ice retreat and surface seawater temperature at each sampled station. This suggests that phytoplankton species composition might be influenced by release of ice algae where the influence of sea ice is strong during summer.