The features of the distribution of the 226 Ra, 232 Th, 238 U concentrations in the surface layer of bottom sediments in the northern part of the Laptev Sea are considered. Data are obtained for the bottom sediments of the shelf zone of the sea, the continental slope, and abyssal depths. The concentrations of 226 Ra and 232 Th varied in the range 17–36.3 and 28.7–38.1 Bq/kg, respectively, and the 238 U concentration, within 12.5–38.7 Bq/kg. It is assumed that the enrichment of sediments in 226 Ra and 238 U with depth is due to the reduction of part of the carbonates containing 226 Ra and 238 U, followed by the sorption of 226 Ra from sea water and the sedimentation of hydrolysis forms of 238 U to the bottom. An increase in the 226 Ra and 238 U concentrations in the sediment with depth in the eastern section, as the concentration of inorganic carbon decreases, is consistent with this assumption. In this case, the correlation of the 226 Ra and 238 U concentrations with the concentration of inorganic carbon is negative ( R = –0.98 and –0.94, respectively). The 232 Th concentration correlates with the concentration of organic carbon ( R = 0.85). In the area of the outer shelf and in the Vilkitsky Strait, the 226 Ra, 232 Th, and 238 U concentrations in the surface layer of sediments depend on the fractional composition of the sediment and change with a change in the fractional ratio. The concentrations of 238 U and 232 Th correlate with each other ( R = 0.84) and with the concentration of inorganic carbon ( R = 0.75 and 0.87, respectively).

Inter-annual (2002–2021) variability of the Kara Sea PP and associated environmental factors was assessed by MODIS-Aqua data and model calculations. Warming in the Kara Sea region during the last two decades was characterized by the pronounced positive trend of surface water temperature (T0) and weak positive trend of free-ice area (S) averaged for the growing season (April – October). During the investigated period T0 increased by 3.55°C with a trend of 10% y–1 and S increased by 110×103 km2 with a trend of 1.4% y–1, on average. The values of water column PP (IPP) statistically significant (p 0.05) decreased in the all of the Kara Sea regions (R2 = 0.22 – 0.59). For the entire Kara Sea the IPP declined by 38 mgC m–2 d–1 with a moderate trend of 1.1% y–1 (R2 = 0.37). The growing season averaged value of photosynthetically available radiation (PAR) weak, but statistically significant (p 0.05), decreased in the all of the Kara Sea areas (R2 = 0.20 – 0.31). Also, in the all regions the significant (R2 = 0.24 – 0.38) weak or moderate negative trends of surface chlorophyll a (Chl) were specified. The total annual PP (PPtot) increased insignificantly in accordance with increase of S (0.7% y–1, R2 = 0.08). The most significant decline of IPP was specified for spring (R2 = 0.28). In autumn the statistically significant positive trend of S (R2 = 0.24) was observed. Due to such increase of S, the strongest growth of PPtot was noted in autumn. In the present work was shown that decrease of IPP, resulting from decline of PAR and Chl, was the reason of moderation of PPtot. Weak increase in PPtot was observed in autumn and in the north area of the sea. It should be concluded that during the period of intense warming, the decrease in the IPP of the Kara Sea should affect the productivity of the higher trophic levels of the food web.

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.

Climate-induced variability of phytoplankton size structure influences primary productivity, marine food web dynamics, biosedimentation and exchange of CO 2 between the atmosphere and ocean. Investigation of phytoplankton size structure in the Arctic Ocean is important due to rapid changes in its ecosystems related to increasing temperature and declining sea ice cover. We estimated the contribution of surface micro-, nano- and picophytoplankton to the total carbon biomass, chlorophyll a concentration and primary production in the Kara and Laptev Seas and investigated the relationships of these phytoplankton size groups with environmental factors which determine their spatial variability. Additionally, we compared chlorophyll specific carbon fixation rate, specific growth rate and carbon to chlorophyll ratios among different phytoplankton size groups. The investigation was carried out from August to September 2018. Generally, picophytoplankton was dominant in terms of chlorophyll a and primary production in the whole study area. The spatial variability of phytoplankton size classes was influenced by river discharge and relied mainly on water temperature, salinity and dissolved silicon concentration. Microphytoplankton prevailed across the river runoff region under conditions of low salinity and relatively high water temperature, while picophytoplankton was predominant under conditions of high salinity and low water temperature. Our study is the first to characterize size-fractionated phytoplankton abundance in the Kara and Laptev Seas, and provides a baseline for future assessment of the response of Kara and Laptev Sea ecosystems to climate-induced processes using phytoplankton size structure.

The seasonal variability of primary production in the Laptev Sea has been studied and its annual values have been determined using the data of a MODIS-Aqua scanner (mean for 2002–2018). Regional-specific algorithms of primary production and chlorophyll are used for this purpose for the first time. Two regions of the Laptev Sea—northwestern and southeastern—are distinguished on the basis of the primary production long-term averaged over many years. Seasonal variations in the primary production in the water column of the northwestern region are characterized by the maximum in June (245 mg C/m 2 per day). The maximal primary production in the southeastern region and over the entire area of the Laptev Sea (273–282 mg C/m 2 per day and 256–281 mg C/m 2 per day, respectively) is recorded from May to July. Daily and annual primary production in the southeastern region are 1.9 and 3 times higher, respectively, than in the northwestern region. The primary production mean for the Laptev Sea is 125 mg C/m 2 per day and the total annual primary production is 8 × 10 12 g C.

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.

Studies of seasonal variability of the East Siberian Sea (ESS) water column primary production (IPP) and evaluation of its total annual value (PPtot) were performed using MODIS-Aqua data (2002–2018). Region-specific primary production and chlorophyll algorithms were used for the first time to achieve that result. Northeastern and Southwestern regions were distinguished in ESS based on multiyear average daily primary production. Seasonal variations in IPP in the Northeastern region were characterized by the maximum in June (273 mgC m–2 d–1). The maximum of water column primary production value in the Southwestern region was recorded in May (311 mgC m–2 d–1). The maximum of IPP for the entire ESS was registered in June (273 mgC m–2 d–1). Values of daily primary production and PPtot in the Southwestern region were, respectively, 1.8 and 2 fold higher than in the Northeastern region. The multiyear averaged value of ESS IPP was equal to 91 mgC m–2 d–1 and PPtot value was equal to 9 TgC. Productivity of the Siberian seas decreased eastward.

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.

Studies of seasonal variability of Laptev Sea water column primary production and evaluation of its annual values were performed using MODIS-Aqua data (20022018). To reach that result regional-specific primary production and chlorophyll algorithms were used for the first time. Based on multiyear averaged daily primary production Northwestern and Southeastern regions were distinguished in the Laptev Sea. Seasonal variations in water column primary production in the Northwestern region were characterized by the maximum in June (245 mgC m‑2 d‑1). In the Southeastern region and for all Laptev Sea the maximum water column primary production values were denoted from May to July, 273282 mgC m‑2 d‑1 and 256281 mgC m‑2 d‑1, respectively. Daily primary production and annual values of total primary production in the Southeastern region were, respectively, 1.9 and 3 fold higher than in the Northwestern region. Multiyear averaged value of Laptev Sea water column primary production was equal to 125 mgC m‑2 d‑1 and total annual primary production was equal to 8 1012 gC.

Исследования сезонной изменчивости первичной продукции в столбе воды моря Лаптевых и оценка ее годовых величин выполнены по данным сканера MODIS-Aqua, осредненным за 2002–2018 гг. Для этого впервые использованы региональные модели первичной продукции и хлорофилла. В море Лаптевых выделены два контрастных по продуктивности района: Северо-западный и Юго-восточный. Сезонные изменения первичной продукции в столбе воды в Северо-западном районе характеризовались максимумом в июне (245 мгС/м 2 в день). В Юго-восточном районе и для всей акватории моря Лаптевых максимальные (273–282 мгС/м 2 в день и 256–281 мгС/м 2 в день, соответственно) значения первичной продукции в столбе воды были зарегистрированы с мая по июль. Интенсивность первичного продуцирования и годовая первичная продукция в Юго-восточном районе были, соответственно, в 1.9 и 3 раза выше, чем в Северо-западном районе. Среднее для моря значение первичной продукции в столбе воды составило 125 мгС/м 2 в день, а ее годовая величина равнялась 8 × 10 12 ґС.

По материалам трех экспедиций, проведенных в море Лаптевых в августе–сентябре 2015, 2017 и 2018 гг., исследовано пространственное распределение продукционных показателей фитопланктона. Первичная продукция в столбе воды (ИПП) на разрезе от устья реки Лены была в 2.8 раза выше, чем на разрезе от устья реки Хатанги, что было обусловлено уровнем освещенности и условиями минерального питания. На разрезах через континентальный склон рост содержания хлорофилла в слое фотосинтеза (Хл фс ) происходил благодаря образованию глубинных максимумов. Увеличение ИПП и Хл фс происходило в районе континентального склона. В августе–сентябре зарегистрированы средние величины ИПП < 100 мгС/м 2 , что свидетельствует об олиготрофии моря Лаптевых в конце лета и в начале осени.

Spatial variability of primary production (PP) was study on vast area of East Siberian Sea in autumn 2017. Water column PP (IPP) value was equal to 28±13 mgC m-2 day-1 on average that testify ultraoligotrophic conditions. IPP was limited by low incident and underwater photosynthetically available radiation and nitrate concentration. Ammonium concentration partly compensates lack of dissolved nitrogen.

The spatial variability of phytoplankton primary production characteristics has been studied along transects between the Shetland Islands and Iceland (transect I) and along 59.5° N (transect II) from June 30 to July 16, 2013. It has been shown that the surface chlorophyll a concentration (Chl0) varied within more than two orders of magnitude from 0.07 to 6.67 mg/m3 along transect I and from 0.02 to 3.63 mg/m3 along transect II. The water column primary production (PPint) changed by a factor of 3.8 from 273 to 1040 and by a factor of 5.6 from 68 to 379 mgC/m2 per day along transects I and II, respectively. It has been established that the spatial variability of Chl0 and PPint was consistent with the distribution of the main surface flows and thermohaline fronts. This conclusion was made based on a reliable positive correlation between Chl0 and the zonal potential temperature gradient (R = 0.43, p < 0.01, N = 65). Phytoplankton assimilation activity along transect II depends on the nutrient concentration. This is confirmed by the reliable positive correlation of the assimilation number with the phosphate concentrations (R = 0.58, p < 0.05, N = 76) and dissolved silicon (R = 0.51, p < 0.05, N = 76).

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.

Пространственная изменчивость первичной продукции в столбе воды (ИПП) и продукционное районирование Карского моря выполнены впервые с использованием региональных моделей, разработанных и верифицированных на основе базы данных, созданной по материалам экспедиций в Карское море в конце августа -начале октября, и спутниковых данных MODIS-Aqua.Придерживаясь описательной концепции и частного подхода, мы выделили шесть продукционных районов, отличающихся по средним многолетним (2002)(2003)(2004)(2005)(2006)(2007)(2008)(2009)(2010)(2011)(2012)(2013)(2014)(2015) значениям ИПП: Эстуарный район (эстуарии Оби и Енисея) со среднегодовым значением солёности на поверхности ≤10 psu; Внутренний (<100 м) и Внешний (100-200 м) шельфы, круглый год находящиеся под влиянием речного стока; Юго-западный район, на который это влияние распространяется в значительно меньшей степени; Северо-западный (жёлоб Св.Анны) и Северо-восточный (жёлоб Воронина) районы с прилегающими акваториям.Средние многолетние значения ИПП