Heat Stress-Driven Shifts in Marine Phytoplankton Trait Composition in a Global Ocean-Biogeochemical Model

Marine heatwaves (MHWs) are becoming more frequent, intense, and prolonged, posing increasing threats to marine ecosystems, including phytoplankton communities. Yet, understanding the impacts of MHWs on phytoplankton community structure remains challenging, given the limited number of observational and process-resolving modeling studies. Here, we develop a modeling framework using an advanced coupled ocean–biogeochemical model (MITgcm–Darwin), in which biogeochemical processes for 310 types of phytoplankton are explicitly resolved. In this model, 310 types are defined by different combinations of key traits: 14 size classes, 10 temperature preferences, and 8 ecological functions. We find an overall shift in phytoplankton composition toward small and warm-preferring types during MHWs. However, detailed features differ substantially across regions and traits. For example, in the tropical Pacific Ocean, the magnitude of shifts tends to increase with heatwave intensity, for both size and temperature traits. A moderate influence of the duration on the temperature trait is also found. In the Indian Ocean, on the other hand, heatwave intensity is the primary factor that affects size composition, while no significant shifts in temperature preference are detected. For both regions, these composition shifts are accompanied by significant losses in biodiversity, reflected in decreased richness and evenness. These results indicate that even short-term climatic extremes can substantially disrupt phytoplankton communities, with potential increasing consequences for marine food webs and ecosystem functioning that depend on phytoplankton as such perturbations intensify.