| Summary: | 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. Funding provided by: Deutsche Forschungsgemeinschaft Crossref Funder Registry ID: https://ror.org/018mejw64 Award Number: WO2452/1-1 Funding provided by: Alfred-Wegener-Institut, Helmholtz-Zentrum für Polar- und Meeresforschung Crossref Funder Registry ID: http://dx.doi.org/10.13039/501100023690 Award Number: Funding provided by: Svalbard Science Forum Project Nr Award Number: 310723 We exposed natural spring communities from coastal Svalbard (Norway) for 2-3 weeks to stable temperature treatments (2°C, 6°C, 9°C), where 2°C acted as a control treatment, as well as to repeated 5-day heatwaves of differing intensity (6°C and 9°C, Figure 1). By excluding grazers and ensuring nutrient replete and stable light conditions, we focused on the effect of temperature only. To understand the dynamics and mechanisms during changing temperature regimes, we explicitly investigated the different phases of a heatwave towards ...
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