Future warming stimulates growth and photosynthesis in an Arctic microalga more strongly than changes in light intensity or carbon dioxide partial pressure

Abstract We assessed responses of Arctic Isochrysis sp. grown under a matrix of temperature (2°C vs. 6°C), light intensity (55 vs. 160 μ mol photons m −2 s −1 ) and pCO 2 (400 vs. 1000 μ atm CO 2 ). Next to acclimation parameters (growth rates, particulate and dissolved organic C and N, chlorophyll a content), we measured physiological processes in vivo (electron transport rates and net photosynthesis) using fast‐repetition rate fluorometry and membrane‐inlet mass spectrometry. Within the applied driver ranges, elevated temperature had the most pronounced impacts, significantly increasing growth (~ 40%) and particulate organic carbon production (up to ~ 140%). Light stimulations manifested prominently under high temperature (~ 30–35%), underlining its role as a “master‐variable.” pCO 2 was the least effective driver, exerting mostly insignificant effects. The obtained data were used for a simplistic upscaling simulation to investigate potential changes in Isochrysis ' bloom dynamics in the Fram Strait with increasing temperatures over the 21 st century. Results suggest that global warming will accelerate bloom dynamics, with earlier onsets of blooms and higher peak biomasses. Despite remaining uncertainties about the magnitude of these effects, data strongly suggest that increasing temperatures over the coming century will affect the phenology of Isochrysis and other Arctic phytoplankton with likely important implications for higher trophic levels.