Community shift to picophytoplankton during the 2019 North Pacific marine heatwave

Marine heatwaves (MHWs) can rapidly alter marine ecosystems and are increasing in frequency and duration. Several MHWs have been documented in the North Pacific over the past decade, including in 2019, with sea surface temperature anomalies of ~2.5°C punctuated by periods of upwelling in the Northern Gulf of Alaska (NGA). Here, we present a time series of microbial abundance and community structure collected from the top 50 m of the water column in the NGA during the summers of 2018-2021. Flow cytometry revealed significantly higher biomass of picophytoplankton and Synechococcus cells during the 2019 MHW, particularly over the continental shelf. The diversity of prokaryotic microbes was significantly higher during the 2019 MHW compared with 2018, 2020, and 2021, with a high relative read abundance of SAR11 bacteria and Marine Group II archaea. Eukaryotic microbial diversity was significantly lower in 2018 and 2019 than in 2020 and 2021, with an increase in the relative read abundance of chlorophytes, a decrease in dinoflagellates, and an increase in the ratio of pennate to centric diatoms during the 2019 MHW. Both prokaryotic and eukaryotic microbial communities differed between years from 2018 to 2021. Different environmental drivers correlated with key phytoplankton functional groups: picoeukaryote biomass was correlated with temperature, while Synechococcus biomass was inversely correlated with nitrate and phosphate concentrations. These patterns indicate how environmental changes, including warming, stratification, and short-term regional upwelling, may shape the distribution of key microbial taxa in the NGA, and provide a window into the possible future microbial community structure of the subarctic North Pacific.