• Dinoflagellates dominate the winter protist community • Fast-growing diatoms dominate the spring bloom with high nutrients. • Diverse mixo- and heterotrophic protist community dominates during stratified summer. • Ciliates peak in warm, nutrient-poor Atlantic Waters with low sea ice and high light. • Phytoplankton and copepod grazers mismatch occurs during spring in Atlantic Waters. Unicellular marine eukaryotes (protists) are important components of the biological carbon pump and form the base of marine food webs, ultimately supporting commercially important fish stocks. However, seasonally resolved data on protist composition, abundance and biomass are rare, especially in high latitude ice-covered oceans. This study presents unique seasonal data on protist community composition and abundance from the northwestern Barents Sea, spanning a latitudinal gradient from open Atlantic waters south of the Polar Front to the seasonally ice-covered Barents Sea shelf and across the shelf break into the Arctic Nansen Basin. Observations were conducted during seven cruises (2018 to 2022) covering periods from early winter to late summer. Protist standing stocks and community composition exhibited pronounced seasonality. Dinoflagellates, particularly Gymnodinium species, dominated the low protist stocks in winter whereas summer communities were more diverse and characterized by dinoflagellates, ciliates and flagellates. Mixo- and heterotrophic feeding modes enable these taxa to persist through the polar night and thrive during the stratified and nutrient-poor conditions in summer. Seasonal differences between dinoflagellates and ciliates likely reflect contrasting feeding and survival strategies. The onset of the phytoplankton spring bloom, dominated by rapidly growing centric diatoms of the genera Thalassiosira and Chaetoceros , occurred in a deeply mixed water column in both open and ice-covered waters. This may have been facilitated by diatoms’ effective defense against protozoan grazers and temporal mismatch between the phytoplankton spring bloom and copepod grazers in Atlantic influenced waters. Our seasonally resolved observations highlight the strong seasonality in both the quantity and quality of protist plankton in the Barents Sea, dynamics which will likely shift with the ongoing decline in sea-ice cover.
Goraguer et al. (Sun,) studied this question.