The datasets present new data from microscopic counts and selected nutrient and physical data assembled from the following missions. Aboard the CCGS Louis St-Laurent: IPY Canada's Three Oceans 2007. Aboard the CCGS Amundsen: International Polar Year (IPY) Circumpolar Flaw Lead Study 2008; ArcticNet 2008. Temporal coverage is July-September. Cell density of the three sub-clades MAST-1A, MAST-1B and MAST-1C was measured by cell-counts using fluorescent in situ hybridization with taxa-specific probe. Biomass of phototrophic organisms, of different size classes, was obtained by counts of DAPI-stained cells under epifluorescence microscopy. Samples for total chlorophyll a (chl a) were filtered onto GF/F filters, extracted in either ethanol or acetone and analysed by spectrofluorometry. Chl a was also measured for the < 3 µm size fraction, and chl a in the < 3 µm size fraction was calculate by simple substraction. CCGS Amundsen data: Temperature, salinity, transmissivity, oxygen, and photosynthetically active radiation were provided by Dr. Y. Gratton (Institut National de la Recherche Scientifique, Québec). Nitrate and Phosphate concentrations were provided by Dr. J.-É. Tremblay (Université Laval, Québec). CCGS Louis St-Laurent data were supplied by Drs. E.C. Carmack and J. Nelson (Institute of Ocean Sciences, Sidney, B.C.). Distance to ice edge was obtained from the Canadian Ice Services ( http://ice-glaces.ec.gc.ca).