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The importance of small phytoplankton in warming and stratified water conditions has been emphasized, but little information on the small phytoplankton contribution is currently available in the Southern Ocean. In this study, we analyzed temporal variations in the carbon uptake rates of large and small phytoplankton and the small phytoplankton contributions to the total primary production. The data were collected in the Amundsen Sea polynya region using three different field observations (early January, January-February, and February-March) from 2012 to 2016. The total primary production in the polynya region decreased from 0.84 ± 0.18 g C·m −2 ·day −1 in early January to 0.19 ± 0.04 g C·m −2 ·day −1 in February-March, which is typical of the seasonal variation in the primary production of phytoplankton in the Amundsen Sea. In contrast, the small phytoplankton contributions to the total carbon uptake rates increased considerably from early January (14.9 ± 8.4%) to February-March (50.8 ± 18.4%) in the polynya region. This substantial increase of small phytoplankton contribution was not caused by increasing biomass and the specific uptake rates of small phytoplankton, but rather by seasonally declining biomass and the specific uptake rates of large phytoplankton during the study period. The overall contribution (25.5 ± 15.7%) of small phytoplankton to the primary production in the polynya region based on the three different time observations is significantly higher than the previous result (14.9 ± 8.4%) based on one time observation. Therefore, a seasonal variation in the small phytoplankton contribution in the Amundsen Sea should be considered in further studies.

Abstract. Carbon and nitrogen uptake rates by small phytoplankton (0.7–5 µm) in the Kara, Laptev, and East Siberian seas in the Arctic Ocean were quantified using in situ isotope labeling experiments; this research, which was novel and part of the NABOS (Nansen and Amundsen Basins Observational System) program, took place from 21 August to 22 September 2013. The depth-integrated carbon (C), nitrate (NO3-), and ammonium (NH4+) uptake rates by small phytoplankton ranged from 0.54 to 15.96 mg C m−2 h−1, 0.05 to 1.02 mg C m−2 h−1, and 0.11 to 3.73 mg N m−2 h−1, respectively. The contributions of small phytoplankton towards the total C, NO3-, and NH4+ varied from 25 % to 89 %, 31 % to 89 %, and 28 % to 91 %, respectively. The turnover times for NO3- and NH4+ by small phytoplankton found in the present study indicate the longer residence times (years) of the nutrients in the deeper waters, particularly for NO3-. Additionally, the relatively higher C and N uptake rates by small phytoplankton obtained in the present study from locations with less sea ice concentration indicate the possibility that small phytoplankton thrive under the retreat of sea ice as a result of warming conditions. The high contributions of small phytoplankton to the total C and N uptake rates suggest the capability of small autotrophs to withstand the adverse hydrographic conditions introduced by climate change.