Size-Fractionated Primary Production and Chlorophyll in the Kara Sea during the First-Year Ice Retreat

The distribution of primary production (PP), chlorophyll a concentration (Chl a), and size structure of the phytoplankton community were studied in the Kara Sea during the first-year ice retreat in late June 2021. The maximum value of water column PP (IPP) reached 1352 mgC m(-2) day(-1). The ice-edge phytoplankton bloom was characterized by high averaged of IPP and Chl a integrated in the photosynthetic layer (Chl(phs)) values: 740 mgC m(-2) day(-1) and 81.40 mg m(-2), respectively. The highest IPP values were observed at sites where Chl a was concentrated in the upper mixed layer or where the subsurface chlorophyll maximum coincided with the pycnocline. Over the area of phytoplankton bloom, the contribution of microphytoplankton (>20 mu m) to the total IPP and Chl(phs) was 92 and 82%, respectively. Contribution of picophytoplankton (<3 mu m) to the total PP increased along the depth until reaching the lower margin of layer of photosynthesis, from 3 to 70%, on average. No similar pattern has been observed for vertical distribution of Chl a. This pattern was evidenced by an increase in the chlorophyll specific carbon fixation rate (assimilation number) of picophytoplankton with depth under low insolation conditions.