Based on data obtained during seven Kara Sea expeditions (2017–2023), seasonal variation in the contribution of phytoplankton size groups to the total values of primary production (PP) and chlorophyll a (chl a) are examined for the first time. Micro- and nanophytoplankton (MPh + NPh) (>3 µm) dominated in the community composition during the entire ice-free period (June–October). Its predominance was especially noticeable during the spring bloom immediately after first-year sea-ice retreat (up to 97

The aim of the work was to study the distribution of chlorophyll a (Chl-a) in the deepest part of the western Kara Sea — the Novaya Zemlya Trough and in St. Anna Trough. Materials : field studies of Chl-a, macronutrients, dissolved oxygen and physical parameters were held in September 2020 during the 81st cruise of R/V «Akademik Mstislav Keldysh». The results showed that Chl-a concentration within the upper 100 m layer ranged from 0.01 to 1.37 mg/m 3 (median value of 0.23 mg/m3). The maximum concentrations of Chl-a in the Novaya Zemlya Trough (0.39–1.18 mg/m 3 ) were observed at depths of 10–35 m, in the St. Anna Trough (0.25–1.37 mg/m3) — in the upper 15-meter layer. In the vertical distribution of Chl-a the subsurface chlorophyll maximum (SCM) was noted at depths of 10–42 m. In the southern part of the St. Anna Trough, the maximum concentrations of Chl-a were observed in the surface horizon. SCM existence depended on the runoff water distribution: well-pronounced SCM was observed at stations with high surface salinity which were characterized by a deeper euphotic depth, and a low concentration of Chl-a at the surface. In regions affected by river runoff, which had low salinity and high nutrient content in the upper layer as well as shallow depth of the photic layer, the maximum concentrations of Chl-a were observed at the surface. The share of pheophytin varied from 18 to 73% (median value of 41%) with minimum values in the layer of chlorophyll maximum. Integrated Chl-a concentration ranged from 5.75 to 24.54 mg/m 2 (median value of 9.51 mg/m 2 ), the highest values were observed in regions with deeper euphotic layer and pronounced SCM. Practical value : the data on the vertical distribution of Chl-a concentration allow to expand understanding of the conditions of organic matter formation and productivity of the Kara Sea.

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 (Chlphs) 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 µm) to the total IPP and Chlphs was 92 and 82%, respectively. Contribution of picophytoplankton (<3 µm) 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.

Spatial and vertical variability of primary production (PP) and Chl a were studied in the framework of the 76th cruise of R/V Akademik Mstislav Keldysh to the Kara Sea from July 7 to August 1, 2019. In the middle of the summer the subsurface chlorophyll maximum (SCM) was well pronounced in the regions outside of river run-off influence with surface salinity >25. The water column PP (IPP) values depended on the vertical PP and Chl a distribution and were meaningfully (4.5 times) higher in the regions with well pronounced SCM. The contribution of SCM to the IPP in the middle of the summer was 55% on average, and approximately four times higher than in the autumn according to previous studies. A statistically significant link between the contribution of SCM to IPP and the contribution of SCM to the photosynthetic layer integrated Chl a was established. The absolute values of PP formed in the SCM did not depend on Chl a concentration and were affected by assimilation activity of phytoplankton, which in turn depended on underwater irradiance at those depths. Our findings suggest that the development of regionally and seasonally adopted production models is relevant for the estimation of the annual Kara Sea IPP.