​An experiment was performed in order to analyze the effects of the increase in watertemperature and CO2partial pressure expected for the end of this century in a presentphytoplankton community inhabiting the Arctic Ocean. We analyzed both factors actingindependently and together, to test possible interactions between them. The arcticplanktonic community was incubated under six different treatments combining threeexperimental temperatures (1, 6, and 10◦C) with two different CO2levels of 380 or1000 ppm, at the UNIS installations in Longyearbyen (Svalbard), in summer 2010. Underwarmer temperatures, a decrease in chlorophyll a concentration, biovolume and primaryproduction was found, together with a shift in community structure toward a dominanceof smaller cells (nano-sized). Effects of increased pCO2were more modest, and althoughinteractions were weak, our results suggest antagonistic interactive effects amongstincreased temperature and CO2levels, as elevated CO2compensated partially thedecrease in phytoplankton biomass induced by temperature in some groups. Interactionsbetween the two stressors were generally weak, but elevated CO2was observed tolead to a steeper decline in primary production with warming. Our results also suggestthat future increases in water temperature and pCO2would lead to a decrease in thecommunity chl a concentration and biomass in the Arctic phytoplankton communitiesexamined, leading to communities dominated by smaller nano-phytoplankton groups, withimportant consequences for the flow of carbon and food web dynamics.