How marine heatwaves are reshaping phytoplankton in the Northeast Pacific

In 2015 and 2019, the Northeast Pacific has undergone significant heatwave events that strongly disrupted marine ecosystems functioning and services. Here we use the data collected by the Continuous Plankton Recorder (CPR) survey to investigate the changes in summer phytoplankton abundance and morphology and show an increase in prolate (i.e., elongated) taxa during heatwave events, in response to warmer temperatures and declining nitrate. We also investigate the impacts of the morphological shifts on the carbon cycle and show that warmer periods were dominated by prolate cells with high carbon content, whereas cooler periods were dominated by oblate (i.e., flattened) cells with low carbon content. By altering community composition and the overall cells sinking velocity, these morphological shifts likely impacted the carbon cycle, as they were correlated with changes in surface chlorophyll a and particulate organic carbon concentrations.