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.

В ходе научной экспедиции «Европейская Арктика – 2020» на НИС «Академик Мстислав Келдыш» (рейс №80) были проведены измерения концентрации хлорофилла а, спектральных показателей поглощения света пигментами фитопланктона, неживым взвешенным веществом и окрашенным растворенным органическим веществом в верхнем освещенном слое Норвежского и Баренцевого морей, а также в Северном Ледовитом океане в период с 28 июля по 23 августа 2020 г. Установлена высокая вариабельность биооптических характеристик, неоднородность исследованной акватории по относительному вкладу окрашенного растворенного органического вещества в общее поглощение света. Получена связь между общим поглощением света на 438 и 490 нм в поверхностном слое вод и глубиной зоны фотосинтеза.

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.

The vertical distribution of phytoplankton primary production (PP) and chlorophyll a (Chl) was studied based on the data carried out in August–September 2015, 2017, and 2018. The PP maximum was located at the surface or within the 0–5 m subsurface layer. The subsurface chlorophyll maximum (SCM) was recorded at 39% stations on the outer shelf and in the vicinity of the continental slope. The SCM was not detected along the northward transect (130° E) from the Lena River delta. As in other areas of the World Ocean, the SCM was located below the upper mixed layer (UML), in the nitracline, near the boundary of the euphotic zone (1% of photosynthetically active radiation). Generally, the SCM was not accompanied by an additional PP maximum. The Chl concentration at the SCM did not exceed 1 mg m–3. PP produced within the UML and SCM contributed 72 and 23%, respectively, to the integrated primary production (IPP) of the water column. Our results suggest that the influence of the SCM on IPP was insufficient due to low Chl concentration and PP colimitation by the low light and temperature at these depths.

Seasonal variations in primary production (PP) in the Kara Sea are underresearched. Previous studies only collected data during autumn or in late summer. However, the middle of summer is close to the beginning of the growing season, when PP can contribute significantly to annual water column integrated primary production (IPP). In addition, differences can be expected in the spatial and vertical distribution of phytoplankton communities in this period. This gap in midsummer data was addressed within the framework of a multidisciplinary research cruise by the R/V “Akademik Mstislav Keldysh” (from 15 July to 18 August 2016). High values of IPP (> 200 mgC m−2 day−1) and surface chlorophyll a (Chl a) concentration (Chl0 > 1 mg m−3) were associated with the Ob–Yenisey river plume, located in the central part of the Kara Sea. Beyond the influence of the plume, in the western and southwestern regions of the Kara Sea, well-pronounced subsurface chlorophyll maxima (SCM) were observed. In some cases, the Chl a concentration in SCM exceeded Chl0 by two orders of magnitude. SCM were often accompanied by subsurface PP maxima (SPM). At stations where SCM was pronounced, IPP values reached 500–800 mgC m−2 day−1, and > 30 % of IPP was accounted for by SPM-integrated PP. Thus, in the middle of summer in the Kara Sea, IPP was linked with the chlorophyll-specific phytoplankton biomass and depended on the strength of the SCM.

Based on satellite and in situ data, we quantitatively estimated the penetration of PAR (photosynthetically available solar radiation in the visible spectral range of 400–700 nm) into waters of the central Barents Sea in the summer seasons of 2014–2016. The effects of cloudiness and coccolithophore blooms on the incidence and penetration of PAR was examined. These blooms occur in the Barents Sea almost every year. We estimated the impact of visible solar radiation on the SST against a background of incoming warmer waters of the Norwegian Current and established that the PAR impact is the most pronounced under clear sky conditions in July and August.

The depth-integrated model (Ψ-Mod) and depth-resolved Kara Sea model (KDRSM) of primary production in the water column were verified using field (2013–2015) and satellite (MODIS-Aqua scanner, 2007, 2011, 2013–2015) observations. The KSDRM and Ψ-Mod over- or underestimate the values of integrated primary production (IPP) in autumn by a factor of 2 and 2.5 with shipboard data as input parameters; the rootmean-square difference (RMSD) was 0.29 and 0.39, respectively. In summer, the efficiency of Ψ-Mod decreased by a factor of 1.5 (RMSD = 0.57), while the predictive capacity of the KSDRM remained the same (RMSD = 0.31). In the Laptev Sea in autumn, the KSDRM performed better than Ψ-Mod (the RMSD was 0.24 and 0.41, respectively). There was no sufficient decrease in the predictive skill of either algorithm when MODIS-Aqua data were used as input parameters. Thus, Ψ-Mod, being a simple and precise algorithm, can be recommended for evaluating the annual IPP in the Kara Sea and for studying its long-term variability using satellite data.