Vol 63, No 4 (2023)

The Antarctic Circumpolar Current plays the key role in the circulation of the Southern Ocean and affects the distribution of heat by the ocean on the global scale. The study of the dynamics and structure of this current becomes especially important in a changing climate. The current is well revealed by satellite altimetry data, which makes possible to study temporal and spatial variability of its structure at different scales. In these studies, the methods for determining the position of individual fronts of the Antarctic Circumpolar Current based on satellite altimetry data become especially important. In this work, we compare various approaches for detection of front locations. The structure of the northernmost branch of the Antarctic Circumpolar Current, the Subantarctic Front, and its spatial and temporal variability was studied based on satellite altimetry data from 1993 to 2020 and the results of a hydrographical section occupied by the R/V “Akademik Mstislav Keldysh” in the northern part of the Drake Passage in February 2020. We selected a 350 km long part of the TOPEX/Poseidon and Jason-1,2,3 satellite altimeter track from Tierra del Fuego to the south for the analysis of the dynamics of the front. Criteria for determining the position of the northern and main branches of the Subantarctic Front are presented based on satellite altimetry data. A long-term shift of the position of the fronts relative to the previously accepted levels of absolute dynamic topography has been found. It was found that the accuracy of determining the position of fronts using fixed values of dynamic topography may decrease with time, in particular due to changes in the mean sea level. A statistically significant long-term trend of sea level rise in the region of the Subantarctic Front was found. This trend is 4 mm/yr for the northern branch and 2.5 mm/yr for the main branch.

The trend assessment of the dynamic state of the Powell Basin waters in the Weddell Sea was made on the basis of the hydrological data of the 79th cruise of the R/V AkademikMstislavKeldysh (January 16–February 6, 2020) and the World Ocean Database-2018 data for January-February from 1975 to 2020. At each node of the quarter-degree grid, a linear trend was constructed for the calculated values of the maximum buoyancy frequency and the maximum amplitude of the vertical component of the internal wave velocity. It is shown that the southwestern and northwestern parts of the Powell Basin differ significantly in their hydrophysical characteristics. In the northwest of the basin, the linear trend of the maximum buoyancy frequency is negative, the trends of the depths of the maximum values of the Väisälä-Brent frequency and the amplitude of the vertical velocity component are positive. In the southwestern part of the basin, the opposite is true: the trend of the maximum buoyancy frequency is positive, the trends in the depths of the maximum values of the Väisälä-Brunt frequency and the amplitude of the vertical velocity component are negative.

We analyze internal waves based on measurements on cruise 87 of the R/V Akademik Mstislav Keldysh in the Bransfield Strait near Half Moon Island and calculations using a numerical model. The measurements were carried out on January 25, 2022 for four hours using a line of temperature and pressure sensors, along with the measurements with a CTD probe. Temperature fluctuations according to the sensors of the thermal line and the probe showed that the amplitude of internal waves is close to 5 m, sometimes reaching 15 m. According to the results of calculations of the global tidal model TPXO9, irregular semidiurnal tides predominate in the study area. Numerical calculations of the parameters of internal waves show that the baroclinic tide generated on a steep slope breaks up into higher frequency waves.

The work is devoted to the study of the abiotic characteristics in the waters of the Bransfield Strait in January 2022. The cross-section of January 2020 was repeated from the Antarctic Peninsula to the Southern Shetland Islands, as well as new data – 2 cross-sections in the eastern and western parts of the Strait. The scheme of currents in the Strait reflects modern knowledge, absolute values have been measured up to 50 cm/s in the northeast direction and up to 35 cm/s in the southwest. In the studied area, the modified waters of the Sea of Bellingshausen are clearly distinguished (warm and least salted, with reduced values of total alkalinity and all nutrients), the modified waters of the Wedell Sea (colder and salty, with reduced values of silicates and nitrates), as well as Circumpolar Deep Water in layer 200–450 m (warm and salty, with low values of dissolved oxygen and pH and an increased content of phosphates, silicates and nitrates). In the bottom waters of the Strait, there were no serious changes in the structure. The amplitude of the interannual variability of the abiotic characteristics of the waters of the Bransfield Strait is poorly expressed, nevertheless it can serve as a marker of climatic changes in the Southern Ocean.

The spatial distribution of the main primary productivity parameters in the Bransfield Strait in the Antarctic summer presents. The integrated primary production in the strait varied from 435 to 741 mgC/m² per day. The share of primary production in the total production of phyto- and bacterioplankton in the upper 10th layer was 82–91%. Potential photosynthetic capacity (F_v/F_m) was high within the euphotic layer (0.418–0.749) throughout the area. The production parameters in the two main water masses in the strait did not differ. Photosynthetic efficiency (the ratio of the assimilation number and the relative electron transport rate, AN/rETR) varied in different subregions of the study area by almost six times.

A method is proposed for constructing a regional algorithm for calculating the thickness of the euphotic zone from in situ measurements of the vertical profiles of the beam attenuation coefficient under the condition of one-parameter optical properties of water (Case-1 water). Using the proposed methodology, a regional algorithm for determining the thickness of the photic zone was developed based on the results of synchronous measurements of the beam attenuation coefficient and photosynthetically active radiation, performed in January 2022 as part of the 87th cruise of the R/V Akademik Mstislav Keldysh for the northwestern part of the Weddell Sea. For this region, an equation was obtained that makes it possible to estimate the photic zone depth with a relative reconstruction error of 18%.

The spatial distribution of the biomass of macro- and mesozoopankton species, as well as the size and sex structure of the species Euphausiasuperba (krill), Salpathompsoni, and Ihlearacovitzai in Bransfield Strait during the Antarctic summer of 2020 were studied. Krill is at the heart of the food chain of the Antarctic ecosystem because numerous populations of seabirds, seals and whales feed on krill. In recent decades, the Antarctic ecosystem has been under significant changes caused by global warming, but the nature and extent of this impact on krill stocks, especially its juveniles in the Bransfield Strait, remains insufficiently studied. In recent decades, the Antarctic ecosystem has been experiencing significant changes associated with global warming, but the nature and extent of this impact on krill stocks, especially its juveniles in the Bransfield Strait, remains insufficiently studied. At the same time, this particular region is a highly productive zone of the Antarctic ecosystem and one of the areas where concentrations of Antarctic krill accumulate. The aim of this study was to study the spatial variability of the structure, numbers and biomass of zooplankton in the Bransfield Strait during the Antarctic summer of 2020. Zooplankton samples in Bransfield Strait were collected with Bongo net in January 2020. The samples were processed on the research vessel in the intervals between sampling according to the standard procedure. At all stations studied, the main zooplankton biomass was by salps (S. thompsoni and I. racovitzai). According to the index of coenotic significance, salps were of primary importance in the zooplanktocenosis, in which E. superba occupied the third place with biomass ranged from 6.3 to 96.3 mg/m³, and krill was meet only in the northwestern deep-water part of the Bransfield Strait. This is due to food competition between salps (S. thompsoni, I. racovitzai) and krill. The presence of krill in the deep-water part of the Bransfield Strait is explained by its introduction from the Bellingshausen Sea due to the prevalence of western winds. The biomass of other zooplankton species in all areas of the strait happened very small in comparison with that of salps and krill and was distributed uniformly. When comparing the ratio of the biomass of the above species obtained during the expedition with the data of the 90s and 2000s, there is a clear trend of an increase in salps and a decrease in E. superba, that is probably associated with the general trend of an increase in water temperature in the Antarctic waters. Thus, the example of Bransfield Strait clearly shows a negative correlation between the biomass of salps and krill.

A review of the composition and structure of deep-sea pelagic ichthyocenes in the Bransfield Strait and the northern part of the Weddell Sea (Powell Basin) in the Atlantic sector of the Southern Ocean is presented. The material was received in the summer of 2020 and 2022 at 26 stations at the layers from 120–0 to 2200–0 m. 901 specimens of juveniles and adult fishes belonging to 19 species from 10 families were collected. Statistical analysis made it possible to identify 4 significantly different regions, in which the dominant ichthyocenes were located in waters of different origin. These ichthyocenes are poor in the number of identified species, but often quite numerous. Relatively shallow-water representatives of the families Bathydraconidae, Channichthyidae, Liparidae, Nototheniidae, endemic to the shelf and slope of Antarctica, dominated in the area of the Antarctic Strait and on the shelf of the Antarctic Peninsula, while in the Bransfield Strait, the abyssal basin of the Powell Basin, as well as in the pelagic zone above deep-sea ridges and trenches, bordering it from the north are meso- and bathypelagic fish from the families Mycthophidae, Bathylagidae, Gonostomatidae, Paralepididae and Scopelarchidae. Near the northern boundary of the Powell Basin, the number of caught mesobatypelagic species and their average abundance were higher than in its central part (11 vs. 7 species and 63.5 vs. 84.1 ind./100 m²). In the central part of the basin, Bathylagus antarcticus dominated, with an average share of 40% in catches and 28.8 ind./100 m², while in its northern part Electrona antarctica dominated, with an average share of 33% in – 27.4 ind./100 m².

Observations were conducted from the Research Vessel “Akademik Mstislav Keldysh” in the Drake Passage, vicinities of the Antarctic Peninsula, Scotia Sea and the northern part of the Weddel Sea during two voyages in January–early March 2020 (the vessel trip number 79) and during the single voyage in January–February 2022 (trip number 87). Data allow to evaluate the number of the most abundant sea mammals in the area near South Shetland Islands, Scotia Sea and Powell Basin. Observation area of 2022 in the Antarctic were located in about 150 km eastwards of the area of 2020. The area under survey of a little bit greater than 343 th. square km in January-February 2020 held approximately 1200 Humpback Whales (Megaptera novaeangliae) and 2800 Fin Whales (Balaenoptera physalus). In 2022 this area held approximately 2500 Fin Whales and again 1200 or more amount of Humpback Whales. Numbers of the Antarctic Fur Seals in each season here assessed approximately 2000-3000 animals. Type of distribution of these two whale species along the vessel track pointed on the absence of food competition either within or between these species during Antarctic summers of 2020 and 2022. It was possible to evaluate the total numbers of the Antarctic Fur Seal (Arctocephalus gazella), however this evaluation appeared to be less precise than the same ones for Humpback Whales and Fin Whales.