Phytoplankton productivity in newly opened waters of the Western Arctic Ocean

Phytoplankton carbon and nitrogen production in newly opened waters of the Western Arctic Ocean were measured using a 13C–15N dual isotope tracer technique in summer 2008, when Arctic sea ice was at its second lowest extent since 1979. Daily integrated carbon-uptake rates were highest (675.8±931.3 mg C m–2 d–1) in ice-free shelf regions of the Chukchi Sea. In deeper regions, the daily integrated carbon-uptake rates (73.5±45.0 mg C m–2 d–1) in newly opened deep waters with adjacent sea ice cover (10–90%) were significantly higher than those (32.0±22.8 mg C m–2 d–1) in ice-free deep waters. The average contributions of small cell-size phytoplankton (0.7–5 μm) to total carbon-uptake rates were 19.8%, 62.8%, and 55.5% in shelf regions, ice-free deep waters, and newly opened waters, respectively, although the average biomass contributions of the small phytoplankton was 54.9–96.5% of total algal biomass in the three study areas. Daily total nitrogen-uptake rates were similar between newly opened deep waters and ice-free deep waters, although nitrate assimilation rates were higher in newly opened waters compared to ammonium uptake rates in ice-free waters. Lower carbon- and nitrate-assimilation rates in ice-free waters were due to strong stratification in the euphotic layer caused by melted sea ice, which limited nutrient availability to phytoplankton, although phytoplankton had greater light availability in ice-free waters than in newly opened waters.