In an era of dramatic anthropogenic perturbations of the global carbon cycle, the Arctic and Southern Oceans are experiencing rapid environmental changes, including ocean acidification, warming, sea-ice loss and changes in stratification. Given their important roles in ecosystems and biogeochemistry, the responses of polar phytoplankton to climate change variables (predominantly temperature and pCO2) have received increasing attention in recent years. However, the interactive effects of multiple stressors remain poorly understood, as is the interplay of species-specific physiological responses and ecological dynamics. Work in the PHYTOCHANGE research group at the AWI, had focused on understanding the physiological and ecological processes that determine the effects of multiple stressors on polar phytoplankton. In this talk, I will present some of our recent work on the interactions between ocean acidification and other environmental drivers, including ocean temperature as well as iron and light availability and their combined effects on phytoplankton assemblages and key species from the Southern and Arctic Oceans.