The study investigates the responses of Arctic phytoplankton communities to stable warming and repeated heatwaves, aiming to understand how these extreme events affect primary productivity and community composition. The results show that even extreme stable warming increases productivity, while the response to heatwaves is complex and depends on the specific scenario applied. The cool phase following a heatwave is found to be at least as important as the warm phase in determining overall phytoplankton productivity. The study highlights the need for a mechanistic understanding of heatwave responses, as the cool phase can act as an additional stressor and significantly influence the overall response. The findings suggest that the interplay between warm and cool phases in heatwaves is crucial for predicting the ecological impacts of these events.The study investigates the responses of Arctic phytoplankton communities to stable warming and repeated heatwaves, aiming to understand how these extreme events affect primary productivity and community composition. The results show that even extreme stable warming increases productivity, while the response to heatwaves is complex and depends on the specific scenario applied. The cool phase following a heatwave is found to be at least as important as the warm phase in determining overall phytoplankton productivity. The study highlights the need for a mechanistic understanding of heatwave responses, as the cool phase can act as an additional stressor and significantly influence the overall response. The findings suggest that the interplay between warm and cool phases in heatwaves is crucial for predicting the ecological impacts of these events.