Oceanology

The rapid changes in the ice and hydrological regimes of the Barents Sea that occurred in the 2000s-2010s determined the objective of this paper: to quantitatively assess the seasonal and interannual variability of the relative contributions of processes at the air-sea boundary and in the water column to changes in the thermohaline parameters of the upper mixed layer, which ultimately determine the intra-annual and interannual dynamics of the ice cover. The work used hydrological and ice data from the GLORYS12v1 global ocean reanalysis system and data on the heat balance components at the ocean surface from the ERA-5 global atmospheric reanalysis. The study found that the dominant contribution to the formation of intra-annual variability of the mixed layer temperature is made by heat exchange with the atmosphere (more than 50%) and horizontal advection, smoothing out changes in the mixed layer temperature – heating in the warm season, and cooling in the cold season. Intra-annual variability of salinity is controlled by ice melting in the warm season and horizontal advection in the cold season. The increase in salinity due to winter ice formation is significantly less than the freshening caused by summer ice melting. Stable positive trends of varying intensity are observed in the temperature and salinity of the mixed layer. These trends are associated with a gradual reduction in ice cover, which ensures an increase in the effectiveness of positive feedback on a seasonal scale, due to which the heating of the mixed layer by the end of the warm season exceeds the cooling in the subsequent cold season, and freshening in the warm season does not compensate for the salinization in the subsequent cold season.

We investigated the spatial and temporal variability of nutrients (P, Si, N) concentration in the northern part of the water area of the Gulf of Ob, where the zone of interaction between fresh and marine waters is located. The data were obtained in July 2016 and 2019. These years differed significantly: in 2016 we found warm (up to 14°C) and oxygenated (up to 130%) waters, in 2019 the temperature did not exceed 8°C and oxygen saturation 90%. Differences were observed in the content of dissolved silicon and forms of nitrogen, their concentrations were higher in 2019. Significant differences in the content of biogenic elements in water in the studied years from the mean annual values were revealed. Possible factors determining these differences are the seasonality and heterogeneity of river runoff, as well as the time of ice-freeing of the water area, which affects the production and destruction characteristics of the water area.

The materials were obtained in the eastern part of the Kara Sea in September 2015 and 2022 and in the period of season ice formation in the last decade of October 2023 during 63, 89 and 92 cruises of RV “Arademik Mstislav Keldysh”. In the period of ice formation phytoplankton was characterized by poor species composition when dominating dinoflagellates and diatoms. Maximum numbers did not exсeed 3.5×10⁴ cell/L, biomass was 5.0–25.2 mcg/L, which are the lowest values for the whole vegetation period in the Kara Sea. Higher species diversity, 2 to 3 times higher numbers and 5 to 10 times higher biomass of phytoplankton were observed in September. Latitudinal areas with different properties of pelagic environment, algae species complex and quantitative characteristics of phytoplankton were allocated. In all observations area of continental slope and external shelf border were characterized by maximum algae values of numbers and biomass. For inner shallow shelf high variability of environmental conditions and phytoplankton communities were typical. Existence of stable phytoplankton complex and similarity of seasonal suссession processes were established for extensive area of the middle shelf of the Kara Sea desalinated by riverine discharge with surface salinity from 25 to 30 PSU. In September – October “deep” maximum in phytoplankton vertical distribution characteristic for earlier periods of seasonal succession was absent.

For the first time, a case of autumn spawning has been recorded for the smooth lumpsucker Aptocyclus ventricosus. The larvae were caught in Zhitkova Bay (Peter the Great Bay, Sea of Japan). A comparison of the larvae with individuals of similar size, previously described from other habitats, is given, the hatching and spawning times are calculated in the considered hydrological conditions, and possible reasons for the transition of this species from the usual spring spawning in this region to autumn spawning are considered.

Within the framework of the program “Marine Ecosystems of the Siberian Arctic”, bioaccumulation of heavy metals and arsenic by zooplankton organisms of different size fractions in two areas of the Kara Sea (the north-eastern part and the Blagopoluchie Bay) was investigated. For the first time, near the southern edge of the Severnaya Zemlya ice sheet, concentrations of eleven trace elements were determined in the sized fractions of the most abundant group of copepods Calanus spp. The highest concentrations of Fe, Mn, Co, Ni, V, Cu, Pb, Zn and Cd were found in small fraction (0.5–1 mm) compared to the middle (1–5 mm) and coarse (>5 mm) ones. A spatial distribution of trace elements in copepods, amphipods, euphausiids, and pteropods along transects in both areas was considered. The environmental factors, the totality of which can indirectly affect the bioaccumulation of trace elements, were assessed.

New data on the stability of marine systems and the manifestation of the Holocene thermal optimum as a modern interglacial in the eastern part of the North Atlantic Subpolar Gyre were obtained. Interpretation of micropaleontological data from the AMK‑325 sediment core and reconstructed summer sea paleotemperatures revealed the peculiarities of the development of the local environment in the last 10.8 thousand years. The rapid transition in the Early Holocene 10.1–9.3 thousand years ago to a warm state almost up to the modern level occurred in accordance with regional and global paleoclimatic archives. The subsequent strongly pronounced cold interval 8.2–7.3 thousand years ago is consistent with the global cooling “8.2 ka”, but was much longer. The establishment of optimal conditions with an increase in water temperature by 1.5–2°C above modern values was noted in the second half of the Middle Holocene 6.8–4.9 thousand years ago, later than the global Holocene optimum.

According to a set of geophysical methods, the equatorial Ninetyeast Ridge and neighboring areas of the Central (to the west) and Wharton (to the east) Basins have been studied. Detailed map of the magnetic anomalies (∆Ta) for this Ninetyeast Ridge segment were created for the first time due to generalization of magnetic data of 5 cruises. The mapped area of 136.6 thousand km² is located near the Site 216 DSDP. For the first time, it was found that the linear magnetic anomalies C31r and C32n.1n do not terminate front the Ninetyeast Ridge, but continue here from the basins. The calculated interval spreading rate for the ridge is about 45 mm/year. In the Wharton Basin, the N-S Fracture Zone 90°E, along where linear magnetic anomalies ruptured and shifted, has been clarified. Magnetic modeling for the seamount on the eastern slope of the Ninetyeast Ridge allowed assuming that this volcanic structure has deep root. It could be formed as a result of repeated younger ("secondary") magmatism after the formation of the main volcanic massif of the ridge itself. Interpretation of magnetic survey data, heat flow measurements and earthquake mechanisms indicate the presence of modern tectonic activity within the studied section of the ridge.

The results of the spring trawl-acoustic surveys indicated that the estimates of the backscatter coefficient (density) for walleye pollack measured in the deep-water areas of the Okhotsk Sea are significantly lower than those calculated based on trawl catches. The discrepancies are attributed to the physiological state of the recorded aggregations. After spawning, walleye pollock individuals have minimal indicators of gonad development and swim bladder filling, which contributes to a weakening of their backscatter properties. Relationships between the target strength of post-spawning walleye pollock individuals and their zoological length are investigated in natural environmental conditions (in situ). The regression equation for target strength (TS) of post-spawning walleye pollock in the Okhotsk Sea, normalized to 20log(L) with an average length (L) takes the form: TS = 20log(L) – 71.5 (at 38 kHz). The derived target strength values for post-spawning walleye pollock are 5.5 dB lower than those for feeding walleye pollock, and 2.5 dB lower than those for pre-spawning and spawning walleye pollock. The application of refined target strength will enhance the accuracy and reliability of acoustic assessments of the walleye pollock abundance in the Okhotsk Sea during the spring season.

Brief results of expedition investigations of the southeastern part of the Baltic Sea and the eastern part of the Gulf of Finland during the 56^th cruise of the R/V Akademik Boris Petrov (2024) are presented. The expedition was dedicated to continuation of longstanding oceanological researches conducted by the IO RAS. An integrated geophysical, geological, hydrological, hydrophysical, hydrochemical and hydrobiological investigations were carried out. New data on the state and dynamic of natural systems and bottom landscapes of the Baltic Sea were obtained.