These data are the first observations of Arctic winter microbial activity under sea ice in a west Greenland fjord (Lillefjord, 70°30'N 50°40'W), and are based on changes in dissolved oxygen content in light and dark in-situ incubations. The results can be used to calculate net community productivity, respiration and photosynthesis rates under the ice. Data were collected at two fully ice-covered sites during February 2013, shortly after the end of the polar night. This is the results summary. Methods are described in the 'Method.txt' file. Results for individual bottles in each experiment are provided in doi:10.1594/PANGAEA.912677.