Between 2010 and 2021, a pronounced warming was observed in the mid-depth water layer (similar to 100-700 m) in the northeastern Black Sea attributed to the influence of Bosphorus-derived waters. During this period, the magnitude of positive temperature anomalies in Bosphorus intrusions along the southern limb of the Rim Current and in the northeastern part of the sea significantly exceeded the magnitude of anomalies in the intrusions detected in 2005-2009. Since 2014, there has been a notable increase in the number of Bosphorus lenses observed in the northeastern Black Sea. This, in conjunction with the rising temperatures within these lenses, has resulted in alterations in the thermohaline structure of the mid-depth water column.

Most dramatic environmental changes in the Arctic seas occur in spring, with timing of ice melt, increase of water temperature and food concentration decisive for the reproduction and recruitment success of the large copepods. In the Kara Sea, most of the studies on zooplankton community were performed at the end of the productive season. The main goal of this study is to gain knowledge on distribution, demography, grazing and reproduction of the dominant zooplankton species in the Kara Sea in early spring soon after ice retreat. Zooplankton were collected, feeding and reproduction rates studied along the retreating ice edge over the Kara Sea shelf and in the St. Anna Trough from 22 to June 30, 2021. All studied copepod species, despite different overwintering and life cycle strategies, responded similarly to the timing of ice retreat. An earlier ice melt led to higher abundance and an advanced demographical structure of the populations of dominant copepods, Calanus glacialis, C. finmarchicus, C. hyperboreus and Metridia longa. Egg production of C. glacialis and C. finmarchicus was high in the areas where the ice has recently melted. Spawning of C. finmarchicus females and presence of early developmental stages over the Kara Sea shelf suggests successful local recruitment of this basically boreal species. Daily rations of three Calanus species were high irrespective of phytoplankton concentration which indicates considerable exploitation of alternative food sources during spring post-bloom situations. Fairly low grazing impact on phytoplankton suggests that copepods community does not control spring development of phytoplankton and most part of the newly produced organic matter would be available for benthos.

The digital elevation model (DEM) of the Kara Sea bottom was created on the basis of multiscale navigation charts from the Department of Navigation and Oceanography of the Ministry of Defense of the Russian Federation. The errors in the created DEM and publicly available IBCAO v3 DEM were calculated using data obtained during expeditions from 2007 to 2022 in the Kara Sea. The accuracy of these two DEMs was assessed using descriptive statistical methods, demonstrating significantly better seabed relief detail in the created DEM.

The material was obtained during cruise 92 of the R/V Akademik Mstislav Keldysh in the area with a latitudinal extent of 72°32.0′ to 73°14.5′ N, including the Ob River estuary and adjacent shallow shelf on October 12–13 in the period preceding seasonal ice formation. The estuarine front was well pronounced in terms of salinity (gradients 0.6 PSU/km) and temperature fields and had a quasi-meridional direction. Sharp boundaries in nutrient distribution typical of earlier seasons were not observed. The abundance and biomass of phytoplankton varied from 22 × 103 to 75 × 103 cells/L and 7.3 to 19.0 mg/m3, respectively, which were two to three orders of magnitude lower than the values obtained in July and September. The abundance (30–78

Research on the structure and trophic characteristics of zooplankton communities of the East Siberian Sea (ESS) was performed within the program “Marine Ecosystems of the Siberian Arctic” during cruise 69 of the R/V Akademik Mstislav Keldysh . The material was collected on two quasimeridional transects conducted in the latitudinal range ~71°00′–75°30′ N from the inner shelf adjacent to the Indigirka and Kolyma outfalls to the outer shelf during September 5–9, 2017. The list of species and larger taxa, as well as their biomass and trophic characteristics, are presented, and the peculiarities of cross-shelf distribution in relation to the conditions of the pelagic environment are described. The obtained estimates of the mezoplankton biomass in the ESS are on the whole within the range typical of the rest of the Siberian Seas in summer–fall. It has been established that mezoplankton communities of the western and eastern ESS significantly differ in quantitative characteristics, the role of dominant species and taxonomic groups in biomass, grazing impact of mesoplankton on the phytoplankton biomass and production, and features of the cross-shelf distribution of these characteristics. The boundary between the communities with different structural functional characteristics is located approximately at 160°–163° E. A conclusion is made that analysis of climatic and seasonal changes in ESS mezoplankton should be examined with allowance for the principal differences between pelagic biotopes and communities of the western and eastern parts of the basin.

The East Siberian Sea is an area of high biogeochemical activity caused by multiple factors, such as an influence of river runoff, Atlantic, and Pacific waters, formation and melting of sea ice, and internal circulation. Extensive amounts of carbon are accumulated in sub-sea permafrost within the Arctic shelf in East Siberia. Thawing permafrost and resulting microbial decomposition of frozen carbon pool is a potential feedback process affected global climate dynamics. Several studies of dissolved organic carbon (DOC) distribution in the East Siberian Sea have demonstrated untypical for other Arctic shelf seas pronounced non-conservative DOC behavior. Using seawater samples from the 69th cruise of R/V Akademik Mstislav Keldysh conducted in the Laptev and East Siberian seas in September 2017, this study examines the distribution of fluorescent dissolved organic matter (FDOM) along the shelf-crossing transects including the areas affected by the Khatanga, Lena, Indigirka, and Kolyma river runoff. The set of 137 excitation-emission matrices (EEMs), analyzed with Parallel Factor analysis (PARAFAC), was described by a 4-component model demonstrating Tucker’s congruence coefficient above the 0.95 threshold. Spectral characteristics of the resulting components allowed identifying them as well-known humic-like A and C, protein-like B/T, and marine humic-like M fluorophores. Component C1 (A fluorophore) showed a good correlation with salinity for different river plume influenced regions. As well as chromophoric DOM (CDOM) absorption, it can be used as marker of input of terrestrial DOM to the Arctic Ocean. A distinctive feature of the East Siberian Sea shelf waters in comparison with the Laptev Sea is the higher content of the C2 and C4 components, exhibiting protein-like and marine humic-like fluorescence. Component C3 (C fluorophore) which is usually identified as terrestrial-derived material, was found to be produced locally on the East Siberian Sea shelf and in the continental slope region of the Khatanga transect. Destruction of dead algae cells, production of marine biota and dissolved organic matter (DOM) reworking are considered as possible autochthonous FDOM sources in the Siberian shelf seas.

During cruise 81 of the R/V Akademik Mstislav Keldysh comprehensive studies of the Kara Sea were carried out over sill separating the central spur of the St. Anna Trough and the Novaya Zemlya depression. For the first time, the water exchange parameters between the shelf southwestern and deep northern parts of the Kara Sea have been evaluated. The meridional component of the geostrophic current velocity calculated from CTD data was used to identify both the northern and southern jets reaching the bottom. Two southern streams are responsible for water transfer from the deep-water part of the Kara Sea to the shelf zone. In the eastern part of the transect, transfer of river-runoff-freshened waters to the deep-water area of the Kara Sea was noted.

Research on the structure of phytoplankton communities of the East Siberian Sea has been carried out during the 69th cruise of the research vessel Akademik Mstislav Keldysh within the framework of the Program “Ecosystems of the Siberian Arctic Seas”. The materials were obtained from 5th to 9th September at two quasi-meridional transects from inner shelf areas adjacent to the Indigirka and Kolyma mouths towards the outer shelf. The list of dominant algae groups and species, estimates of abundance, biomass and features of the vertical distribution of phytoplankton are given for pelagic biotopes with different environmental conditions. The significant quantitative deficiency of phytoplankton in the East Siberian Sea in the summer season in comparison with the Laptev Sea is determined, which is especially pronounced in the eastern area of the basin. The algae abundance along the Indigirka transect varies in the range of 11.9 to 66.3 × 103 cell/L, and the biomass from 16.4 to 339.1 mg/m3. The corresponding values for the Kolyma transect are 7.7–90.3 × 103 cell/L and 9.4–42.1 mg/m3. The maximum values along both transects are recorded in the area of the outer border of the river plume with increasing surface salinity up to 21–25 PSU. The material obtained makes it possible to reveal significant differences in the structure of phytoplankton communities of western versus eastern areas of the East Siberian Sea, which confirms the well pronounced zonal heterogeneity of the basin.

Notable changes in the Arctic ecosystem driven by increased atmospheric temperature and ice cover reduction were observed in the last decades. Ongoing environmental shifts affect freshwater discharge to the Arctic Ocean, and alter Arctic land-ocean fluxes. The monitoring of DOC distribution and CDOM optical properties is of great interest both from the point of view of validation of remote sensing models, and for studying organic carbon transformation and dynamics. In this study we report the DOC concentrations and CDOM optical characteristics in the mixing zones of the Ob, Yenisei, Khatanga, Lena, Kolyma, and Indigirka rivers. Water sampling was performed in August–October 2015 and 2017. The DOC was determined by high-temperature combustion, and absorption coefficients and spectroscopic indices were calculated using the seawater absorbance obtained with spectrophotometric measurements. Kara and Laptev mixing zones were characterized by conservative DOC behavior, while the East Siberian sea waters showed nonconservative DOC distribution. Dominant DOM sources are discussed. The absorption coefficient aCDOM (350) in the East Siberian Sea was two-fold lower compared to Kara and Laptev seawaters. For the first time we report the DOC content in the Khatanga River of 802.6 µM based on the DOC in the Khatanga estuary.

The research is based on the materials collected during cruise 66 of R/V Akademik Mstislav Keldysh in the Yenisei estuary and over the adjacent Kara Sea shelf in the latitudinal range from 71°50.5′ to 75°55.0′ N. The studies were performed from July 25 to 27, 2016, three weeks after the end of the flood period. Two hundred fifty species of plankton algae were recorded. Well-pronounced latitudinal zoning in phytoplankton communities structure associated with changes in the hydrophysical and hydochemical conditions of the pelagic environmental was determined. The maximum species diversity was formed by diatoms, green and blue-green algae in the southern desalinated part of the estuary (<1 PSU) and by diatoms and dinoflagellates in the offshore areas. The highest algae abundance and biomass, 2.7–2.9 × 106 cell/L and 1.1–1.5 g/m3, respectively, were recorded in the inner desalinated part of the estuary, where freshwater diatoms of the genus Aulacoseira dominated. The distribution of freshwater algae northwards was bounded by the 12–15 PSU surface isohaline (74°20′ N). The most pronounced decline in phytoplankton abundance and biomass was found in the outer part of the estuary north of ~73° N. In the middle shelf to the north of the Yenisei estuary, the maximum abundance (up to 740 × 103 cell/L) and biomass (up to 240 mg/m3) of phytoplankton were recorded at the depths of 30–40 m with low illumination and high nutrient concentration, which was also typical of the Ob region in spring–summer.

Исследования выполнены на разрезе из 11 станций от внутренней части Хатангского залива на юге до области континентального склона на севере с 17 по 20 сентября 2017 г. Выделены четыре биотопа, характеризующихся различными свойствами пелагической среды, составом, количественными характеристиками и вертикальным распределением фитопланктона: внутренняя часть Хатангского залива, эстуарная фронтальная зона, западный шельф моря Лаптевых и область континентального склона. Внутренняя часть Хатангского эстуария и область континентального склона характеризовались наиболее высокими численностью и биомассой фитопланктона, которые достигали 1×10 6 кл./л и 160 мг/м 3 соответственно. Для вертикального распределения фитопланктона в области склона было характерно формирование максимума на глубине 45 м, где биомасса водорослей достигала максимальных значений для всего разреза — 400 мг/м 3 . В структуре сообществ фитопланктона восточной части моря Лаптевых выявлена широтная зональность, подобная той, которая формируется в других районах Арктических морей под влиянием мощного стока Сибирских рек.

— The studies were conducted along the transect (11 stations) from the inner part of Khatanga Bay in the south to the Laptev Sea continental slope in the north in September 17—20, 2017. Four biotopes with different parameters of the pelagic environment, composition, quantitative characteristics and vertical distribution of phytoplankton have been distinguished: the inner part of the Khatanga Bay, estuarine frontal zone, western shelf of the Laptev Sea and continental slope area. The inner part of Khatanga Bay and the continental slope are characterized by the highest values of phytoplankton abundance and biomass, which reach 1 × 10 6 cell/L and 160 mg/m 3 , respectively. The formation of the maximum at a depth of 45 m is typical for the phytoplankton vertical distribution in the continental slope area. Algal biomass in the maximum reaches 400 mg/m 3 which is the highest value for the transect. Well-pronounced latitudinal zoning in the structure of phytoplankton communities has been determined in the western part of the Laptev Sea similar to that in other regions of the Arctic seas under a strong impact of Siberian rivers discharge.

The study was carried out in the Ob estuary and the adjacent shelf of the Kara Sea in July 2016. For the first time data on the species composition, abundance and distribution as well as on feeding of zooplankton were obtained during the period of intensive river runoff. The biomass of zooplankton in terms of wet weight (without jelly animals and chaetognaths) varied over a wide range from 40 to 1880 mg/m3. The biomass was dominaned by the Limnocalanus macrurus population, represented by all age stages, and Pseudocalanus spp. (an average 60% and 17% of biomass, respectively). Maxima of zooplankton biomass were associated with the boundary of the distribution of freshwater and with the southern periphery of the estuary frontal zone (EFZ). The short-term temporal variation of the abundance of zooplankton was observed when the transect was repeated after 2 days: the biomass at the southern periphery of the EFZ decreased from 1900 to 250 mg/m3. The observed changes is supposed to be caused by the shift of the southern boundary of the EFZ under changing in the wind regime. At high phytoplankton biomass in the estuary (>7 μg Chl-a/L), the daily ingestion of Limnocalanus macrurus was 5.77, 4.46, 2.59 and 1.4 μg C/day in CVI, CV, CIV and CIII, respectively. Energy intake by younger copepodite stages not only covered the metabolic requirements, but also enabled the copepods to growth. At lower Chl-a concentration on the shelf (<2 μg/L), the mean values were lower (1.56 μg C/day and 1.17 μg C/day in CVI and CV, respectively) and hardly met the metabolic demands. The zooplankton grazing impact estimated with the gut fluorescence method varied from 0.2 to 7.3% of phytoplankton standing stock and from 2.2 to 30% of primary production. The results are discussed in relation to the specific hydrophysical conditions in the summer period and in comparison with the data obtained for the autumn season.

Based on the material obtained in the spring–summer season of 2016, the composition and quantitative distribution of phytoplankton in the Ob estuary and over the Kara Sea shelf has been estimated. The latitudinal extent of the region exceeded 5° (~560 km), encompassing areas with different river discharge impact, which formed latitudinal zoning in the distribution of hydrophysical and hydrochemical conditions. These environmental properties determined the zoning in the spatial structure of phytoplankton communities. The estuarine biotope with a salinity <1 PSU and high concentrations of nitrate, phosphate, and silica was inhabited by a freshwater phytocenosis stable in species composition with high-quantitative characteristics and dominated by Aulacoseira species. The most spatial and seasonal variability of phytoplankton is characteristic of the area of intense interaction of fresh and marine waters in the outer part of the Ob estuary and over the adjacent shallow shelf, where freshwater algae species are replaced by marine species. A sharp biotopic boundary separates the inner shallow shelf with depths of approximately 30–35 m and outer shelf of the depth from 150 to 200 m. In the area of the outer shelf the lower boundary of the euphotic zone descends to 25–35 m. At theses depths plankton algae actively vegetate, and form subsurface phytoplankton maximum under conditions of sufficient nutrient concentration.

Phytoplankton community and its distribution were investigated in the south part of the Polar Front in the eastern Barents Sea in October 2014. Analysis of the spatial differences in the phytoplankton structure was performed in connection with changes of the temperature, salinity and biogenic regime. At the end of the growing season in the phytoplankton community was dominated by destruction processes and the concentration of nutrients in the upper mixed layer was higher than the limiting level. Coccolithophores ( Emiliania huxleyi and Discosphaera cf. tubifer ) dominanted over investigated area. The maximum values of abundance and biomass of coccolithophores reached 90.4 mln.cell/m 3 and 30.8 mgC/m 3 , drawing up 82% of the total number and 93% of the total biomass of phytoplankton. Influence of transformed the waters of Atlantic origin was observed in the western part of the investigated area. The number of species in the phytoplankton community here was 1.5–2 times lower than in the eastern part of the occupied mostly by Barents Sea water. In the eastern part of the presence of large dinoflagellates Neoceratium spp. ( Ceratium spp.) and Dinophysis spp., lower values of chlorophyll a concentration, a higher proportion of pheophytin in the amount of pigment chlorophyll + pheophytin, the high content of ammonia in the upper mixed layer showed that in this area the phytoplankton was at a later seasonal succession stage than the western part.