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

Contourite drift studies in six seas of Russia have been compiled and analyzed for the first time. The considered sedimentary bodies vary in size, age, and structure, are located at different depths, occur in various environments, and are attributed to several classification types. Our study combines these contourite drifts in two groups. The first includes shallow-water drifts on the shelves of Kara, Pechora, and Baltic seas; the second consists of deep-water drifts on the slopes and in the basins of the Sea of Japan, Sea of Okhotsk, and Caspian Sea. The relationship between the studied sedimentary bodies and bottom currents is considered.

The spatial distribution of picophytoplankton (Pico) abundance and biomass, as well as the distribution of PE-rich and PC-rich picocyanobacteria (PCB), was investigated in the waters of the outer Ob estuary and the adjacent shelf of the Kara Sea in late autumn during the 92nd cruise of the R/V Akademik Mstislav Keldysh. The Pico chlorophyll “a” and its contribution to total chlorophyll “a”, total phytoplankton primary production (PP) and Pico PP were estimated. Pico abundance and biomass varied from 8.18 to 46.3 × 106 cells/L and from 3.74 to 20.48 µg C/L, respectively. Pico was dominated by PCB of the genus Synechococcus, its contribution to the total Pico biomass ranged from 81 to 94

In 1965, the K-27 submarine with an emergency portside reactor with unloaded spent nuclear fuel (SNF) was scuttled in Stepovoy Bay (Novaya Zemlya). The source of radioactivity is nuclear fuel in the port side reactor and fuel carried into the corresponding steam generator. Before scuttling, the power plant elements were sealed with furfural, and the entire compartment was then filled with bitumen. It is believed that when water enters the reactor core, a self-sustaining fission chain reaction (SCR) will occur. However, this process is possible only if the above-mentioned protective barriers are violated. The state of the protective barriers can be judged by the magnitude and variations of the radioactive background on the deck and near the submarine on the seabed. Thus, the most important task of monitoring and preventing an emergency situation on the K-27 is constant monitoring of the environment (ecology) in Stepovoy Bay in general and near the submerged submarine in particular. The article considers the results of research on the submarine and its environment, including an expedition of the R/V Akademik Mstislav Keldysh (cruise 92, 2023), when for the first time the parameters of the radioactive background were measured directly on the submarine’s robust hull near the reactor compartment.

92d cruise of R/V “Akademik Mstislav Keldysh” was organized by Shirshov Institute of Oceanology in a frame of long-term Program “Marine Ecosystems of Siberian Arctic” and was held from 5 October to 7 November 2023. 76 scientists from the institutes of Russian Academy of Sciences, National Research Center Kurchatov Institute, Moscow State University, Moscow Institute of Physics and Technology, VNIRO and MES of Russia participated in the cruise. Coordinated hydrophysical, hydrochemical, biooceanological, geochemical research were carried out in the eastern part of the Kara Sea in the period of seasonal ice formation, as well as in the Ob river estuary and fjords of Novaya Zemlya archipelago. Conditions of largest disposals of radioactive waste in the Kara Sea were evaluated.

Quantification of the vertical distribution of chlorophyll a (Chl-a), as one of characteristics of primary production (PP), is critically important to estimate annual PP in the water column (IPP) using models and remote sensing data. IPP estimation in optically complex and highly variable waters such as the Kara Sea is not a trivial task. In the present study, based on the data obtained during three multidisciplinary cruises to the eastern regions of the Kara Sea in August-October, the differences in the vertical distribution of Chl-a and PP under and without the influence of the river plume (Case II and Case I water types, respectively) are established. In Case I waters in August 2014, under the low values of the diffuse attenuation coefficient (Kd) of downwelling photosynthetically available radiation (PAR) (the median value (Me) K-d = 0.158 m(-1)) and the deep euphotic layer (Z(eu)) (Me = 30 m), the deep chlorophyll maximum (DCM) was well-pronounced. At the end of September 2015 in Case II waters influenced by the river runoff, when Me K-d increased 1.7-fold and Z(eu) decreased 1.3-fold, the DCM was absent. Also, the DCM was not manifested at the end of the growing season, under conditions of extremely low underwater PAR (Me = 0.35 mol quanta m(-2) d(-1)). In the sampling period, the PP maxima were observed at the surface and the DCM did not influence the vertical PP distribution. The depth of the nitracline was directly associated with the distribution of riverine waters and determined the depth and degree of DCM manifestation. The outcomes of the presented study suggest that a decrease in subsurface PAR influenced by the impact of riverine waters and the total decline of incident radiation from August to October determine the vertical distribution of PP and Chl-a.

Cruise 92 of the R/V Akademik Mstislav Keldysh was organized by Shirshov Institute of Oceanology within the long-standing program “Marine Ecosystems of the Siberian Arctic” October 5–November 7, 2023. A total of 76 scientists from institutes of the Russian Academy of Sciences, National Research Center Kurchatov Institute, Moscow State University, Moscow Institute of Physics and Technology, VNIRO, and MES participated in the cruise. Coordinated hydrophysical, hydrochemical, biooceanological, and geochemical research were carried out in the eastern Kara Sea in the period of seasonal ice formation, as well as in the Ob River estuary and fjords of the Novaya Zemlya archipelago. The conditions of largest radioactive waste disposals in the Kara Sea were evaluated.

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

Ongoing warming is leading to the accelerated shrinkage of glaciers located on Arctic islands. Consequently, the influence of glacial meltwater on phytoplankton primary production in Arctic bays becomes critically important in an era of warming. This work studies the spatiotemporal variation of primary production and chlorophyll a concentration in the bays along the eastern coast of the Novaya Zemlya archipelago. Data were collected during nine cruises performed from July to October (2013-2022). The effect of underwater photosynthetically available radiation (PAR) and nutrients on primary production was assessed separately for bays influenced by glacial meltwater (glacial bays) and those without such influence (non-glacial bays). The median value of water column-integrated primary production (IPP) for all bays was 38 mgC m-2 d-1, characterizing them as oligotrophic areas. IPP in non-glacial bays was found to be 2.3-fold and 1.4-fold higher than that in glacial bays during summer and autumn, respectively. Underwater PAR was the main abiotic factor determining IPP during the ice-free period. In the entire bays nutrient concentrations were high, exceeding the limiting values for growth and photosynthesis of phytoplankton. It was concluded that the high turbidity from glacial meltwater runoff leads to decreased underwater PAR and, consequently, to a decline in IPP. This study demonstrates that rapid warming could have a negative impact on the productivity of high Arctic bays and their adjacent areas.

The article is devoted to the water area research at Novaya Zemlya depression of the Kara Sea, where, according to archival data, radioactive waste was flooded. The research concerned observations of the environment state and underwater nuclear and radiation hazardous facilities, such as the nuclear fleet flooded here reactors, vessels loaded with solid radioactive waste and numerous groups of containers. The research was carried out using the technology of route sonar and video shooting based on the use of towed uninhabited underwater vehicles (BNPA) developed at IO RAS. Objects condition monitoring was provided using underwater gammaray REM spectrometers developed at the National Research Center “Kurchatov Institute”. As a result of the research, individual objects have been identified and their location archival data has been clarified. Observations of benthos and the structure of the water column in the burial area were done.

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.

A characteristic feature of the eastern Kara Sea, which is separated from its central part by the shallow Central Kara Rise, is a later ice breakup, a shorter ice-free period, and lower water temperature. Zooplankton in this part of the sea is virtually unstudied. The first data on the composition, spatial distribution of zooplankton, age structure and feeding of the dominant species, and their grazing impact on phytoplankton were obtained in the cruise 125 of the R/V Professor Shtokman (September 2013) and cruise 63 of the R/V Akademik Mstislav Keldysh (September 2015) to the eastern Kara Sea. 2015 was a typical year in terms of the ice regime for this region, while in 2013, seasonal ice coverage lasted longer than usual. Zooplankton biomass (17–112 mg dry weight m–3), to which copepods Calanus glacialis contributed the main portion, were close to the values in the central Kara Sea in September. The seasonal development of the C. glacialis population began much later than in the central Kara Sea, but development of Metridia longa was not influenced by the specifics of the ice regime and hydrophysical conditions in the eastern Kara Sea. The interannual differences in the timing of ice retreat had almost no effect on the zooplankton abundance, seasonal development of the dominant copepod populations, feeding activity, and grazing impact on phytoplankton.

Hydrochemical studies of watercourses and the water area of Blagopoluchia bay (Novaya Zemlya, Arctic, Russia) have been carried out. The concentrations of nutrients in rivers and streams are higher than those in the water area of Blagopoluchia bay. It is shown that the concentration of silicon in constantly flowing rivers is 1–13 μM, the concentration of NO 3 — 0.5–8, for small and temporary streams these values are higher and are in the range of 18–46 μM Si, 1–11 μM NO 3 – . The influence of streams and rivers flowing into Blagopoluchia Bay on the water area of the bay is local and extends to 1 km from the mouth, and does not influence the Kara Sea nutrient content.

Organic carbon of fecal pellets and small-sized (≤ 1mm) zooplankton was measured using a high- temperature combustion method. The method was adapted for measurements with Shimadzu TOC-VCPH analyzer equipped with a manual injection module. The range of organic carbon values was 30–10 000 ng ind–1. The samples for analysis were collected during the cruises of the R/V “Akademik Mstislav Keldysh” in the Kara Sea in 2019–2021. Relationship between body carbon (C, µg ind–1) and prosome length (L, mm) of zooplankton describes by the equation С = 4.24 L1.84, r2 = 0.85, n = 46. Carbon of field collected fecal pellet varied from 9.4 to 102.9 µg С mm–3. The carbon to chlorophyll a ratio of fecal pellets differed by more than two orders of magnitude. The highest values (542.1 and 726.1) were obtained in the bays of the Novaya Zemlya Archipelago, the lowest (3.2–5.6) on the Kara sea shelf in June 2021 soon after ice melting. The perspectives of implication of high temperature combustion method for studies of zooplankton in situ feeding and to estimate the contribution of fecal pellets to POC vertical flux are discussed.

Cruise 89 of the R/V Akademik Mstislav Keldysh was organized by the Shirshov Institute of Oceanology in the framework of the Program "Marine Ecosystems of the Siberian Arctic" and was conducted from September 19 to October 10, 2022. Sixty scientists from institutes of the Russian Academy of Sciences, Moscow State University, Moscow Institute of Physics and Technology, VNIRO, and MES participated in the cruise. Coordinated hydrophysical, hydrochemical, biooceanological, and geochemical research and assessments of greenhouse gas concentrations in the near-surface atmosphere were carried out over the shelf and continental slope of the poorly studied eastern Kara Sea. The ecosystem of Blagopoluchiya Bay (Novaya Zemlya) and the conditions of radioactive waste disposal there were also objectives of the research.

In the estuaries of large Siberian rivers, ice coverage and the timing of ice retreat have varied in recent decades under the ongoing climate change. The seasonal development and functioning of the mesozooplankton community depend to a great extent on the timing of ice retreat. In the Arctic estuaries, the response of zooplankton to the timing of ice melt remains unclear. An earlier ice retreat was suggested to result in an advanced seasonal development of zooplankton, and higher biomass and feeding rates. Zooplankton composition, biomass, demography and grazing (assessed with the gut fluorescent approach) were studied in the Ob Estuary in July 2019 ("typical" ice retreat time). The obtained results were compared with the published data for July 2016 (ice retreat three weeks earlier). Zooplankton biomass in 2019 was considerably lower than in 2016, while species composition was similar; dominant populations were at an earlier stage of development. Herbivorous feeding of the dominant copepod, Limnocalanus macrurus, was also lower in 2019. The consequences of an earlier ice melt and increased temperature on seasonal population dynamics of the dominant brackish-water species are discussed. Our findings demonstrate that zooplankton communities in the Arctic estuaries are highly sensitive to the environmental changes associated with early sea-ice reduction.

The differences and similarity of the chemical composition (ions, trace elements, and polyaromatic hydrocarbons (PAHs)) of the near-water atmospheric aerosol collected in summer 2021 along the expedition routes of the R/V Akademik Mstislav Keldysh in the region of the Kara Sea (the second stage of cruise 83, from June 18 to July 8, 2021), in the Barents Sea, and in the Norwegian-Greenland Basin (cruise 84, from July 24 to August 26) are revealed.

Glaciers retreat linked with rapid Arctic warming changes bioproductivity in the bays and coastal areas of the Arctic Ocean (AO). The impact of different types of glaciers shrinking on water column primary production (IPP) is widely discussed. This work studied the influence of the land-terminating glacier on IPP and chlorophyll a concentration (Chlph) in Blagopoluchiya Bay (Novaya Zemlya archipelago). It is known that meltwater runoff from land-terminating glaciers enriches coastal waters with inorganic suspended matter and nutrients. The aim of the present study was to establish the main environmental factor determining IPP in the bay influenced by the glacier. During summer and autumn, IPP in the bay was low. The medians of IPP were 69 and 26 mgC m−2 day−1, respectively, which was 1.7- and 1.9-fold lower than in open waters of the Kara Sea in the vicinity of the bay. Generally, IPP increased from the inner part, close to the glacier, to the outer part of the bay. In summer, IPP and Chlph depend on both nutrient concentrations in the photosynthetic layer and the parameters of water transparency. On the contrary, in autumn, IPP and Chlph depend only on the optical properties of the subsurface layer. It can be concluded that low IPP in Blagopoluchiya Bay is determined mainly by unfavorable conditions of underwater PAR formed by meltwater discharge from the glacier. Thus, the fast melting of the land-terminating glaciers linked with climate warming can lead to a decrease in the biological productivity of the AO bays.

Within the program “Ecosystems of the Siberian Arctic Seas,” carried out by Shirshov Institute of Oceanology, Russian Academy of Sciences since 2007, studies of the water structure and spatial variability of the parameters of the carbonate system have been performed, and the intensity and direction of the carbon dioxide flux over the continental slope of the Laptev Sea and in the Vilkitsky Strait in September 2018 have been calculated. The presence of several main water masses that govern the water structure in the study area is shown. A strong spatial variability of the parameters of the carbonate system of seawater, determined by complexes of physical and chemical–biological processes, has been revealed. The intensity and direction of the carbon dioxide flux at the water–atmosphere boundary were calculated, which range from –12 to 4 mmol m–2 day–1. It was revealed that the investigated area of the outer shelf and continental slope of the Laptev Sea is an emitter of carbon dioxide into the atmosphere as of September 2018. Conversely, the area of the Vilkitsky Strait, is a CO2 sink zone.

The article presents the results of studying specific organomineral formations—cryoconite sediments—samples of which were taken on the surface of the Witte Glacier (Medvezhiy Bay, Novaya Zemlya) on cruise 76 of the R/V Akademik Mstislav Keldysh . The specific activities of 137 Cs, 241 Am, 207 Bi, 210 Pb, and 7 Be were measured in 14 samples. Analysis of the materials suggests that cryoconite contains radioactive contamination deposited in the glacier in the period from September 1957 to December 1962 as a result of atmospheric nuclear explosions at the Sukhoy Nos test site, located 100 km from the glacier. Extremely high Bi, Ag, Sb, Mo, Pb, Sn contents and a strong correlation with anthropogenic radionuclides may indicate the technogenic emission of these elements during the destructive impact of nuclear explosions on bedrock, the metallogenic specialization of which corresponds to rich polymetallic mineralization.