Small phytoplankton contribution to the standing stocks and the total primary production in the Amundsen Sea

Abstract. Small phytoplankton are anticipated to be more important in a recently warming and freshening ocean condition. However, little information on the contribution of small phytoplankton to overall phytoplankton production is currently available in the Amundsen Sea. To determine the contributions of small phytoplankton to total biomass and primary production, carbon and nitrogen uptake rates of total and small phytoplankton were obtained from 12 productivity stations in the Amundsen Sea. The daily carbon uptake rates of total phytoplankton averaged in this study were 0.42 g C m⁻² d⁻¹ (SD  =  ± 0.30 g C m⁻² d⁻¹) and 0.84 g C m⁻² d⁻¹ (SD  =  ± 0.18 g C m⁻² d⁻¹) for non-polynya and polynya regions, respectively, whereas the daily total nitrogen (nitrate and ammonium) uptake rates were 0.12 g N m⁻² d⁻¹ (SD  =  ± 0.09 g N m⁻² d⁻¹) and 0.21 g N m⁻² d⁻¹ (SD  =  ± 0.11 g N m⁻² d⁻¹), respectively, for non-polynya and polynya regions, all of which were within the ranges reported previously. Small phytoplankton contributed 26.9 and 27.7 % to the total carbon and nitrogen uptake rates of phytoplankton in this study, respectively, which were relatively higher than the chlorophyll a contribution (19.4 %) of small phytoplankton. For a comparison of different regions, the contributions for chlorophyll a concentration and primary production of small phytoplankton averaged from all the non-polynya stations were 42.4 and 50.8 %, which were significantly higher than those (7.9 and 14.9 %, respectively) in the polynya region. A strong negative correlation (r² = 0. 790, p<0. 05) was found between the contributions of small phytoplankton and the total daily primary production of phytoplankton in this study. This finding implies that daily primary production decreases as small phytoplankton contribution increases, which is mainly due to the lower carbon uptake rate of small phytoplankton than large phytoplankton.