Irreversible phytoplankton community shifts over Subpolar North Atlantic in response to CO ₂ forcing

Abstract. Marine phytoplankton play a crucial role in the ocean's food web, marine ecosystems, and the carbon cycle. Their responses to external forcing vary across phytoplankton species, and phytoplankton community shifts can have important implications for their roles in the Earth's system. Here, we find that phytoplankton communities in the Subpolar North Atlantic shift toward smaller species under greenhouse warming that are not easily recovered even under CO2 removal scenarios. Despite negative CO2 emissions, the persistent collapse of larger-celled diatom populations and the shift toward smaller phytoplankton communities is a consequence of lower surface nutrient availability following the slowdown of the Atlantic Meridional Overturning Circulation (AMOC). This weakening of AMOC and associated nutrient transport exhibits delayed recovery. Depleting nutrients disrupts trophic dynamics by altering primary limiting nutrient components, contributing to the continued decrease in diatoms and an increase in smaller phytoplankton. Consequently, the downsizing of the phytoplankton community indicates a large reduction in the ocean's biological carbon export capacity.