Climate-driven changes in phytoplankton community structure in the Northern Chukchi and East Siberian Seas, Pacific Arctic Ocean

Abstract The Northern Chukchi and East Siberian Seas, located in the Pacific sector of the Arctic Ocean, are undergoing rapid environmental changes; however, phytoplankton community dynamics in this region remain poorly characterized. Using pigment-based taxonomic analyses from summer surveys conducted between 2015 and 2020, we identified significant interannual and spatial variability in phytoplankton composition. Diatoms generally dominated in the cold saline waters of the western region, while small flagellates were more prevalent in the stratified eastern area. In 2017–2018, surface phytoplankton biomass in the western area increased in association with elevated nutrient concentrations and enhanced diatom biomass, likely driven by an expansion of Atlantic-origin water. In contrast, diatom dominance in this area declined in both 2019 and 2020, while flagellate groups, including prymnesiophytes on the Chukchi borderlands, became more abundant. These changes coincided with anomalously warm conditions and rapid sea ice retreat in 2019, followed by continued sea ice loss in 2020, and were potentially linked to increased inflow of warm Pacific water. Correlation analyses indicated that environmental factors such as temperature, salinity, sea ice cover, and nutrient concentrations exerted taxon-specific influences on community structure. These results suggest that phytoplankton communities in the Pacific Arctic are responsive to hydrographic changes associated with both atlantification and Pacific warming, with possible implications for carbon export and regional trophic dynamics.