Contribution of small phytoplankton to total primary production in the Chukchi Sea

• Contribution to total primary production by small phytoplankton in the Chukchi Sea is determined.
• Total carbon uptake rate of small phytoplankton is about 34%.
• Total ammonium and nitrate uptake rates of small phytoplankton are 59% and 38%, respectively.
• Small phytoplankton have higher POC per chlorophyll-a unit than large one.

Given a projection of thriving small phytoplankton in the Arctic Ocean under climate-induced environmental changes, it is important to estimate the contribution of small phytoplankton (0.7–5 μm) to the total primary production in the Chukchi Sea, which is an important conduit of organic matter from the North Pacific to the Arctic Ocean. Based on a 13C–15N dual isotope tracer technique, small phytoplankton productivity measurements were taken during two consecutive cruises in the Chukchi Sea in 2004. The total phytoplankton carbon uptake rates ranged from 0 to 25.38 mg C m−3 h−1, whereas the uptake rates of small phytoplankton ranged from 0 to 2.87 mg C m−3 h−1. In comparison with the carbon uptake rates, total phytoplankton nitrate uptake rates ranged from 0 to 4.40 mg N m−3 h−1 while small phytoplankton nitrate uptake rates ranged from 0 to 0.39 mg N m−3 h−1. Ammonium uptake rates ranged from 0 to 8.34 mg N m−3 h−1 and from 0.01 to 2.18 mg N m−3 h−1, for total and small phytoplankton, respectively. Small phytoplankton contributed 24.80% (S.D.=±23.0%) to the total chlorophyll-a concentration, and 59.41% (S.D.=±52.12%) to the total carbon biomass due to its higher particulate organic carbon per chlorophyll-a unit during the two cruises in 2004. In the Chukchi Sea, the average contributions of small phytoplankton to carbon and total nitrogen (nitrate+ammonium) uptake rates were 31.72% (S.D.=±23.59%) and 37.31% (S.D.=±26.06%), respectively.