Unprecedented differences in phytoplankton community structures in the Amundsen Sea Polynyas, West Antarctica

In the Antarctic coast, ice shelves are rapidly thinning and retreating due to global warming. Basal melt water influences marine life, particularly the phytoplankton, which are directly affected by changes in physicochemical environments. However, there is limited in situ data over large areas in the Amundsen Sea, which is currently a hotspot for rapidly thinning ice shelves in West Antarctica. During the austral summer cruise of 2020, phytoplankton species abundance was investigated along the Amundsen Sea coast using an automated continuous observation instrument, the Imaging FlowCytobot. The phytoplankton community was dominated by Phaeocystis antarctica in most coastal waters of the Amundsen Sea, as previously reported; however, unexpected blooms of diatom Dactyliosolen tenuijunctus were observed throughout the Pine Island Bay region at a high dominance rate (similar to 90%) and abundance (>10(7) cells l(-1)). D. tenuijunctus is a weakly silicified diatom and its massive bloom in the water column has been rarely reported from the Antarctic Ocean. The dramatic difference in phytoplankton compositions between these adjacent polynyas probably indicates an unstable response of phytoplankton to ice melting conditions. They could play a different role in the marine food web and carbon flux compared to other diatoms and P. antarctica. Therefore, further research is warranted to predict the biological and biogeochemical impacts of future melting conditions.

Automated summary

The thinning and retreating of ice shelves in the Antarctic coast have been accelerated by global warming. Basal melt water influences marine life, particularly phytoplankton, which is directly affected by changes in physicochemical environments. Limited in situ data over large areas in the Amundsen Sea, a hotspot for rapidly thinning ice shelves in West Antarctica, hinder our understanding. In a 2020 summer cruise, phytoplankton species abundance was investigated along the Amundsen Sea coast, revealing unexpected blooms of diatom Dactyliosolen tenuijunctus in the Pine Island Bay region.