Field observations were made in the southern Beaufort Sea and the Amundsen Gulf as part of CASES 2003 expedition. Samples were collected during the first two legs of the expedition from 13 September to 14 October 2003 (open water) and from 15 October to 25 November 2003 (newly formed sea ice) onboard the Canadian icebreaker CCGS Amundsen. Temperature and salinity profiles were obtained using a SeaBird 911 Conductivity¿Temperature¿Depth (CTD) probe. Nutrient concentrations (nitrate plus nitrite, phosphate, and silicate) were determined on fresh samples (within 1 h of sampling) using standard colorimetric methodologies adapted for use on an Auto-Analyzer 3 (Bran þ Luebbe). A RAMSES ASC scalar hyperspectral irradiance sensor (TriOS) was mounted on the flight bridge to measure downwelling irradiance with 1 nm intervals during the day. A total of 50 and 102 discrete water samples were collected over the euphotic zone at 13 and 20 stations for legs 1 and 2, respectively for fluorometrically chlorophyll a measurements and absorption analysis of phytoplankton, non-algal particules (NAP) and colored dissolved organic matter (CDOM). Water samples (preserved in acidic Lugol solution) were enumerated and identified to the lowest possible taxonomic rank using an inverted microscope. Concentrations of chlorophyll a and chlorophyll b were obtained by HPLC measurements and these two pigments were used for the calculation of the ratios of chl b to chl a.