Vol 64, No 2 (2024)

The paper proposes a theory of similarity of quasi-geostrophic vortices against the background of large-scale flows. This information is useful when planning laboratory and numerical experiments to study mesoscale and submesoscale vortex dynamics of vortices interacting with currents. Special attention is paid to the study of geometric similarity of phenomena. It is revealed that the complete set of dimensionless similarity numbers of baroclinic vortices includes four dimensionless parameters: the dimensionless intensity of the vortex, the geometric similarity of the background flow (the ratio of relative vorticity to the deformation coefficient of the background flow), the coefficient of horizontal elongation of the vortex core and the coefficient of vertical oblateness of the vortex core coinciding with the Burger number. To describe the similarity of barotropic vortices against the background of barotropic flows, the number of necessary dimensionless parameters is reduced by one number — the coefficient of vertical oblateness of the vortex core is eliminated from consideration. When studying axisymmetric vortices or vortex structures close to axisymmetric, another geometric parameter of the vortex is eliminated from consideration — the coefficient of horizontal elongation of the vortex core. As a result, the maximum possible set of similarity parameters includes four dimensionless numbers, and the minimum is two.

Data of suspended matter content, concentrations of dissolved and particulate organic carbon in the Kara Sea water for 2007-2022 are summarized. A difference in the distributions of dissolved (DOC) and particulate (POC) organic carbon in the water column during the autumn (September) and spring-summer (July-August) periods was revealed. An increase of concentrations of dissolved organic carbon was recorded both on the surface and in the water column in the summer. Analysis of the dependences of DOC concentrations on salinity showed that the increase in these concentrations is not associated with an increase in DOC concentrations in river runoff waters. There was also a significant increase in DOC and POC concentrations in phytoplankton bloom zones throughout the water column. Thus, a connection has been identified between seasonal increases of organic matter concentrations in the open part of the sea and the processes of phytoplankton blooming.

Elemental sulfur and its derivatives polysulfides play a key role in the processes of hydrogen sulfide oxidation in anoxic basins. Having low solubility, elemental sulfur is mainly represented by suspended forms. However, in sulfide waters it forms highly soluble polysulfides. This work is devoted to the study of elemental sulfur and polysulfides in the upper part of the Black Sea anoxic zone in 2017–2019 and 2022 at stations located on the continental shelf off the coast of the Caucasus and Crimea. Sampling, filtering and determination of sulfur were carried out under strictly anaerobic conditions in an argon atmosphere.

The concentration of elemental sulfur (together with polysulfides) increases with depth and with an increase in the content of hydrogen sulfide from 0.01 in the area of the redox interface to 0.67 µmol/kg at a depth of 600 m. The fraction of elemental sulfur in the composition of ZVS is 23 ± 5%. The calculation of the polysulfides concentration in equilibrium with suspended sulfur based on thermodynamic data shows that deeper than 20–25 m of the upper boundary of the anoxic zone, their concentration was higher than ZVS and at a depth of 600 m they differed by about 3 times. The predominance of elemental sulfur over sulfide sulfur in the composition of polysulfides in the anoxic zone at depths of 450 and 600 m can be the reason for the fractionation of its isotopic composition by +2.2‰ relative to the sulfur of dissolved sulfide (-41.0‰ VCDT).

Data on the structural and production characteristics of bacterioplankton in the western part of the Kara Sea at the beginning and in the middle of summer are presented. In the region of the St. Anna Trough slope, the average abundance of prokaryotes for the water column was (594–708) × 10³ cells/ml (26.4–36.5 mgC/m³) in June and (247–517) × 10³ cells/ml (12–28 mgC/m³) at the beginning of August. On the transect along Novaya Zemlya, the average abundance of bacterioplankton in the water column was (186–554) × 10³ cells/mL (8.5–30 mgC/m³) within a week after the seasonal ice and (169–443) × 10³ cells/mL (8–21 mgC/m³) in the midsummer. Bacterial specific growth rate did not exceed 1.28 day^–1; its high values were observed in the upper warm water layer, above the halocline, and also at the near-bottom depths. At the beginning of summer, the production of bacterioplankton tended to decrease in the northeast direction. The distribution of prokaryotes abundance was determined by temperature and water saturation with oxygen, possibly as an indirect indicator of past phytoplankton “bloom”.

Most likely, the non-indigenous snow crab opilio, Chionoecetes opilio, entered the Kara Sea from the Barents Sea, both due to the migration of adults and with currents at the larval stage. At the moment, all bottom stages, including mature individuals and a large number of pelagic larvae are present in the Kara Sea. However, the origin of the larvae has not yet been clarified. The larvae that hatched in the Kara Sea should be at an earlier stage of development compared to the Barents Sea larvae that got here due, to later development of phytoplankton and, accordingly, later hatching.

The larvae of the snow crab Chionoecetes opilio and the spider crab H. araneus were collected in the central and southwestern parts of the Kara Sea in July — early August 2019 by the Bongo zooplankton net 60 cm in diameter. It was established that the larvae were unevenly distributed across the Kara Sea. The main concentrations were found on the border with the Barents Sea in the St. Anna Trough (up to 860 ind./m²), and relatively high concentrations of larvae were noted in the southwestern part, where their abundance at the stations varied from 18 to 302 ind./m². In the zone of the Ob-Yenisei plume, crab larvae were absent or their abundance was minimal. Using molecular genetic methods, the species identity of 361 larvae (344 C. opilio and 17 H. araneus) was reliably determined, and measurements of a number of morphological structures were made for 401 larvae. Significant differences in size at the zoea stage II between C. opilio and H. araneus were established. In most of the Kara Sea in July 2019, in zooplankton samples, crab larvae were represented by zoea I C. opilio with rare specimens of zoea I H. araneus of the Kara Sea origin. Only in the southwestern part, on the border with the Barents Sea, the presence of zoea II C. opilio and H. araneus was observed in samples with an increase in the proportion of the latter species in catches, which probably originate from the Barents Sea.

In order to explain the spatial nature of the distribution of eelgrass in the inner bays of Posyet Bay, numerical experiments were performed to simulate hydrodynamics and morphodynamic processes. The hydrodynamics was modeled using the Delft3D Flow model. The spectral wave model SWAN (Simulating WAves Near shore) was used to calculate the wind wave parameters (propagation direction, wave length and height, near-bottom orbital velocities). Alteration of coastal zone profile under the impact waves and the hydrodynamic of tidal and wind currents was performed using the Delft3-MOR morphodynamics module included in the Delft3D software package. The results of numerical experiments have shown that the dynamics of the coastal bottom topography is determined by sediment transport forced by combined action of waves and currents. Comparison of the modeling results with information on the distribution of Zostera of previous studies and field surveys data showed a high agreement with model data on the spatial distribution of erosion and accumulation zones in the coastal zone.

The upper layer of bottom sediments in the northeastern part of the Kara Sea is studied. It is shown that the composition of sediments is dominated by silt, the proportion of sand increases towards the shore. The content of organic carbon (average value — 1.1% wt.) is close to the average content in sediments of the Kara Sea, increasing at deep-sea stations. The revealed linear relationship between the content of organic carbon and the surface area of sediment shows that sorption on the particles surface is the predominant accumulation form of organic matter (OM) in sediments. The average n-alkanes concentration in the analysed samples is 1.1 μg/g of sediment, the distribution spectrum is dominated by terrigenous origin odd high-molecular homologues.

Results of the comprehensive studies of hydrological and chemical structure of the Middle and Northern Caspian Sea carried out aboard r/v “Issledovatel Kaspiya” in September 2022 are presented. It is shown, despite the continuing decrease in sea level, there is no aeration of the deep layers, the hydrogen sulfide layer persists with a tendency to increase the concentration of hydrogen sulfide in the bottom layer. There is an increase of the oxygen minimum zone. The changes of the Caspian Sea ecosystem have led to a weakening of the removal of nutrients from the photic layer into the bottom layers and, as a consequence, a decrease in the concentration of silicon in the bottom layer. The results obtained will allow to evaluate the trends of ongoing changes.