Due to the high CO₂ solubility and low carbonate saturation states of its cold surface waters, the Arctic Ocean is considered particularly vulnerable to ocean acidification. If anthropogenic CO₂ emissions continue at current rates, large parts of the Arctic Ocean will be corrosive for calcium carbonate before 2050. While several Arctic calcifying species have been shown to respond negatively to ocean acidification, the effects of ocean acidification on pelagic communities and biogeochemical cycling are presently unknown. To close this gap, a mesocosm CO₂ enrichment experiment was conducted in the Kongsfjord off Svalbard (78°56’N, 11°56’E) in June–July 2010 in the framework of the European Project on Ocean Acidification (EPOCA). This study, which involved 35 scientists from 12 institutes, yielded a comprehensive data set on community level responses to ocean acidification and their impacts on nutrient cycling and air–sea exchange of climate-relevant gases.