Heatwave responses of Arctic phytoplankton communities are driven by combined impacts of warming and cooling

Marine heatwaves are increasing in frequency and intensity as climate
change progresses, especially in the highly productive Arctic regions.
Although their effects on primary producers will largely determine the
impacts on ecosystem services, mechanistic understanding on phytoplankton
responses to such extreme events is still very limited. We experimentally
exposed Arctic phytoplankton assemblages to stable warming, as well as to
repeated heatwaves, and measured temporally resolved productivity,
physiology and composition. Our results show that even extreme stable
warming increases productivity, while the response to heatwaves depends on
the specific scenario applied, and are not predictable from stable warming
responses. This appears to be largely due to the underestimated impact of
the cool phase following a heatwave, which can be at least as important as
the warm phase for the overall response. We show that physiological and
compositional adjustments to both, warm and cool phases drive overall
phytoplankton productivity, and need to be considered mechanistically to
predict overall ecosystem impacts.