Shifts in microbial community composition during the 2019 Pacific marine heatwave in the northern Gulf of Alaska

Abstract

Climate change has led to a gradual increase of sea surface temperatures in the Northern Gulf of Alaska (NGA) interspersed with marine heatwaves (MHW) that impose a rapid but temporary perturbation of sea surface temperature. MHWs have the potential to alter marine microbial community structure, which may impact the production and transfer of carbon to higher trophic levels. The year 2019 was characterized as an MHW in the North Pacific, with sea surface temperatures in the NGA reaching ~2.5 º C above average during 2019 and ~1 º C above average during 2020, while 2021 had near-average sea surface temperatures. To characterize shifts in the NGA's microbial community, samples for DNA and flow cytometry were collected on NGA Long Term Ecological Research cruises in summers 2018-2021. Flow cytometry sample analysis revealed higher abundances of picoeukaryotes, Synechococcus, and nanoeukaryotes in the summer of 2019 relative to 2020 and 2021 on the continental shelf. The diversity of eukaryotic microbes was lower during 2018-19 than 2020-2021, with similar patterns observed within the diversity of individual eukaryotic taxa. Conversely, the diversity of prokaryotic microbes was higher during 2019 than 2020. Different environmental conditions were correlated with small cell abundance and microbial diversity. Elevated picoeukaryote abundance was associated with higher temperature and inversely correlated with chlorophyll a concentration, while Synechococcus abundance was anti-correlated with the concentration of nitrate and phosphate. Shannon diversity of 18S reads correlated with lower salinity measurements while Shannon diversity of 16S reads was not significantly correlated with any tested biological or environmental variables. These correlations indicate that increases in sea surface temperature, along with associated changes in nutrient concentrations and salinity, act as environmental drivers with the potential to shifts the NGA's microbial community structure. Such a community shift towards pico-nanophytoplankton may reduce trophic transfer efficiency and decrease the production of fisheries and other higher trophic levels in a warmer NGA.