Phaeocystis Dominance of a Large‐Scale Productivity Anomaly in the Northeast Atlantic Ocean During Spring 2024

Changes to the North Atlantic spring bloom phytoplankton community structure are expected in response to regional warming trends. In 2024 an extensive body of low chlorophyll water, covering approximately 720,000 km2 in area, dominated the Northeast Atlantic during the peak of the spring bloom period. Picoplankton (<2 μm) and small nanoplankton (2–10 μm) size fractions together accounted for ∼90% of total chlorophyll-a implying a significant contribution by smaller phytoplankton cells to net primary production. Flow imaging microscopy of the phytoplankton community confirmed the dominance of smaller phytoplankton (<18 μm), whilst chemical taxonomy of phytoplankton pigments indicated the widespread presence of haptophyte species, most likely Phaeocystis sp. These results contrast markedly with recent descriptions of the North Atlantic spring bloom community structure which report larger nano- and micro-phytoplankton species dominance. However, these new results are consistent with model predictions showing a shift toward smaller sized phytoplankton species as water temperatures rise and with the appearance of increasingly favorable conditions for Phaeocystis. Our results further suggest that large scale regionally significant changes to the phytoplankton community can rapidly emerge in response to ocean warming which, in the case of a Phaeocystis dominated community, will likely have implications for carbon export fluxes due to changes in vertical flux attenuation efficiency.