№ 2 (2024)

The paper examines seasonal variability in the spatial distribution and magnitude of horizontal gradients of temperature, salinity and density in large-scale surface frontal zones in the North Atlantic Ocean. Monthly average temperature and salinity data at the 0.5 m horizon from the ORAS5 oceanic reanalysis (1958–2021) are used. High gradients of temperature exceeding 2 °C/100 km, those of salinity exceeding 1 PSU/100 km, and those of density exceeding 1 kg·m^–3/100 km were observed in the subpolar and temperate regions in fronts along large-scale currents carrying warm salty waters from the southern latitudes (Gulf Stream, North Atlantic Current) and cold waters with low salinity from the Arctic regions (Labrador Current, East Greenland Current). These fronts occur throughout the year. High salinity and density gradients are also observed in the tropical summer in the front at the edge of the Amazon River plume, resulting from seasonal river flow. In these five frontal zones, areas were identified for which quantitative estimates of seasonal variability of gradients are provided. In the subpolar and temperate latitudes, maximum temperature gradients are observed in winter. Warming up of water in the summer season is accompanied by a decrease in gradients. The greatest range of seasonal variability of temperature gradients was noted in the frontal zones of the Gulf Stream and the East Greenland Current. In summer, in the fronts of subpolar regions, salinity gradients increase due to the melting of Arctic and continental ice and an increase in the influx of waters with low salinity. In the frontal zone of the East Greenland Current, as well as at the boundary of the Amazon River plume, the highest range of seasonal changes in salinity and density gradients was noted. In these areas, the contribution of salinity to seasonal changes in density at the ocean surface increases.

This paper studies the characteristics of storm waves in Laspi Bay (Crimean Peninsula) using the numerical hydrodynamic model SWASH with a spatial resolution of 5 m. The wave reanalysis data obtained from the spectral model SWAN were set as boundary conditions. The fields of significant wave heights and wave current velocities in the bay were analyzed for storms of various regime conditions. It was established that the maximum values in the bay could reach 2.5–3.0 m, 4.0–4.5 m, 5.0–5.5 m and 6.0–6.5 m during storms that are possible once a year, once every 5, 10 and 25 years, respectively. An increase in wave velocities to 1.5–3.0 m/s occurred near the coast at depths of less than 10 m during storms that are possible once every 25 years. The influence of the protective breakwater, built in the 1980s, on the waves was local and manifested itself in the formation of a shadow zone on its downwind side. The possible influence of storm waves on the reduction of bottom vegetation in Laspi Bay was discussed. An analysis of the wave load on the bottom of the bay showed that during periods of extreme storms in its waters, the slopes most susceptible to the effects of waves were in the depth range from 2 to 12 m where the kinetic energy density increased to 500–2000 J/m³. At the same time, the density could reach 3000–4500 J/m³ in the western end of the bay. The energy load values were low in the middle part of the bay. Therefore, the disappearance of bottom vegetation here could be not due to storm impact, but an increase in water turbidity caused by anthropogenic factors. The obtained results are of great practical importance for the safety of navigation, engineering and exploitation of coastal infrastructure.

The paper studies the dynamics of biogenic element (mineral (phosphate) and total phosphorus and ammonium) content based on the results of annual monitoring surveys of water in the eastern Gulf of Finland conducted in 2020–2022. Information on the horizontal and vertical distribution of the indicators was analysed, so samples were taken in the surface, bottom and middle (at deep-water stations) layers of water. The content of elements was determined by the spectrophotometric method. The results are compared and analysed by median values. During the study period, phosphate phosphorus concentrations in the absolute majority of cases did not exceed the maximum permissible concentration (0.15 mg/dm³), total phosphorus concentrations on average corresponded to the mesotrophic status, although there were cases of its concentration increase to values characteristic of the eutrophic status of a water body. Namely, in 2020, the concentrations amounted up to 0.091 mg P/dm³ in the bottom and surface water layers in June (mainly at the coastal stations) and in September (mainly in the bottom layer at the central offshore stations). In summer 2021, the concentrations reached 0.147 mg P/dm³ (surface layer) and 0.171 mg P/dm³ (bottom layer) at the coastal stations and 0.163 mg P/dm³ at the central station. Ammonia nitrogen concentrations were mainly within the MPC (0.5 mg/dm³). In June 2021, local areas along the southern and northern shores of the Gulf of Finland with relatively high levels of ammonia nitrogen (up to 0.285 mg/dm³) in surface and bottom water layers were identified. In general, despite the high anthropogenic load, concentrations of mineral phosphorus and ammonium in the waters of the Gulf of Finland were within the MPC, with exceedances recorded rarely, usually in Neva Bay, Koporye Bay and near the coast of the Kurortny district. Elevated concentrations of total phosphorus at the central stations can apparently be explained by transport of the substance from the western part of the Gulf and diffusion from bottom sediments. On average, higher concentrations of total phosphorus were found in bottom water layers than in surface water layers. In general, biogenic element concentrations correspond to the mesotrophic status of the water body.

Cadmium is a highly toxic metal actively migrating in the system water–suspended sediments–bottom sediments. The paper aims to study the Cd content in the water and bottom sediments of the Sea of Azov in 1991–2020 and to evaluate the process of sedimentation self-purification of waters. The data on Cd distribution showed that from 1991 to 2009 its concentration decreased slowly in the water of the open part of the sea and in Taganrog Bay with an increase in 2010–2016. Cd concentration in the Sea of Azov water did not exceed the maximum permissible concentration (10 µg/L) for marine waters of fisheries. Levels of Cd contamination in bottom sediments were assessed by comparison with the soil contamination criteria according to the Dutch List. The Cd content in the bottom sediments had been decreasing until 2010 followed by its increase in the open sea and in Taganrog Bay. The Cd content exceeded its clarke value throughout the study period. Cd elimination from the waters of the open sea was 0.9–6.0 tons/year, that from the waters of Taganrog Bay was 0.5–2.4 tons/year. These estimates of Cd fluxes into the bottom sediments can characterize sedimentation self-purification of waters. The period of sedimentation turnover of Cd in the open sea and Taganrog Bay at different Cd concentrations in water during the study period averaged 70 and 13.7 years, respectively, taking into account the differences in the volume of the studied water areas. Dependence of the coefficient of Cd accumulation by bottom sediments on its concentration in water showed that the increased intensity of sedimentation self-purification of waters at low Cd concentrations in water was provided by high concentrating ability of the bottom sediments associated with their granulometric composition. In the Sea of Azov, clay and silt sediments (fraction 0.01 mm) make up over 70 %. With increasing degree of Cd contamination of waters, the accumulation coefficient value decreased and accordingly the contribution of sedimentation processes to water self-purification decreased. The assimilation capacity of the bottom sediments with respect to Cd amounted to 3.8 t/year in the open Sea of Azov and 0.7 t/year in Taganrog Bay.

The scallop Flexopecten glaber ponticus (Bucquoy, Dautzenberg & Dollfus, 1889), which is endemic to the Black Sea, can be classified as a mollusk species potentially cultivable in the coastal waters of Crimea. Recent data indicate emerging trends in the scallop population recovery off the Crimean coast. The scallop settles in large quantities into nursery cages together with the giant oyster Crassostrea gigas (Thunberg, 1793), which suggests the scallop can be reared in suspended culture due to its availability and ease of collection. We studied the seasonal dynamics of allometric growth and weight increase of the Black time, a growth model is presented that adequately describes the linear growth of the mollusk. The linear relationship between shell height and age of the scallop and the exponential relationship between the total live weight and shell height were found. It was shown that the commercial quality indices of F. glaber ponticus – meat yield, condition index and gonadosomatic index – vary with season. The maximum values of the condition index and meat yield were noted in April, 63.40 and 33.01%, respectively. The gonadosomatic index increased from January to June (from 6.8 to 13.14%) and decreased from July to November, which trends are associated with the gametogenesis and spawning of the mollusk. The percentage of dry matter in soft tissues was 16.5%. We propose the cultivation duration (2.5–3 years) and optimal timing for harvesting marketable Black Sea scallop as a promising mariculture species. Winter and spring can be the best period for collection of the Black Sea scallop of marketable size.