Hysteresis of phytoplankton communities over Sub-polar North Atlantic to CO2 forcing

Abstract. Marine phytoplankton play a crucial role in the ocean’s food web, marine ecosystems, and the carbon cycles. 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 Sub-Polar North Atlantic shift towards smaller species under greenhouse warming that is not easily recovered even under CO₂ removal scenarios. Despite negative CO₂ emissions, the persistent collapse of larger-celled diatom populations and shift toward smaller phytoplankton communities is a consequence of lower surface nutrient availability followed by the slowdown of the Atlantic Meridional Overturning Circulation (AMOC). This weakening of AMOC and associated nutrient transport exhibit delayed recovery. Depleting nutrients disrupt trophic dynamics, by altering primary limiting nutrient components, contributing to the continued decrease in diatoms and increase in smaller phytoplankton. Consequently, the downsizing of the phytoplankton community indicates a large reduction in the ocean’s biological carbon export capacity.