Increased atmospheric CO2 concentrations lead to ocean warming and acidification, with fastest rates of change occurring in the Arctic. Concurrent changes in sea-ice cover and optical properties as well as in surface stratification also cause strong changes in the light regimes phytoplankton encounter in the water column. Each of these environmental changes alone has the potential to strongly alter primary production and phytoplankton community structure, but their simultaneous changes probably lead to strong and unexpected interactive effects. To shed light on this understudied topic, a natural phytoplankton assemblage from the Kongsfjorden (Svalbard) was exposed to a matrix of two pCO2, two temperatures and two light levels. Performance and development of the phytoplankton assemblages under controlled conditions were monitored for 3 weeks. Preliminary results for the observed changes in species composition, stoichiometry, primary production and photophysiology will be presented. The likely consequences in terms of productivity changes or biodiversity shifts and their implications for higher trophic levels and biogeochemical cycles will be discussed.