Marine Heatwaves Disrupt Phytoplankton Communities Through Trait-dependent Selection

Hyojeong Kim , Stephanie Dutkiewicz, Junwoo Lee, Ibrahim Hoteit, Yixin Wang, Soon-Il An, Oliver Jahn, Hajoon Song

Phytoplankton communities, key regulators of marine ecosystems and the global carbon cycle, are susceptible to sea surface temperature variability, such as marine heatwaves (MHWs). However, their trait-dependent responses and the resulting compositional shifts during MHWs remain poorly understood. Here, using an advanced global ocean–biogeochemical model that resolves 310 phytoplankton trait types, we show that MHWs act as trait-selective disturbances. Decreases in biomass during MHWs, particularly in the equatorial Pacific and Indian Ocean, are generally accompanied by significant diversity loss and an enhanced dominance of small, warm-preferring types. However, short and strong heatwaves in the Pacific can promote elevated diversity. These contrasting patterns are shaped by the interplay between heatwave characteristics and background environmental conditions. Such restructuring in phytoplankton communities may have cascading effects on the food web and the biological carbon pump, highlighting the need to account for diverse traits when assessing climatic impacts on phytoplankton communities.