Estimates of vertical turbulent diffusion coefficient (K_t) in the Black Sea pycnohalocline have been obtained from data of simultaneous observations of seawater temperature, salinity, density, and horizontal current velocity, obtained in the northeastern part of the Black Sea during 2013–2014 with a moored Aqualog profiler. A Munk and Andersson (1948) type parameterization, adapted for the Black Sea environment, is proposed for calculating K_t. Strong short-period (several days) variability of turbulent exchange is revealed, induced by vertical shear variations of the current velocity.

The paper analyzes quasiperiodic upwellings and downwellings on the shelf and upper part of continental slope of the northeastern Black Sea. It is shown that these processes are related to changes in intensity and direction of alongshore current and the following geostrophic adjustment of the density field. The source of such changes is the meandering of the Black Sea Rim Current (RC). It leads to a quasiperiodic change in direction of the alongshore current, from northwestern (cyclonic RC meander) to southeastern (anticyclonic RC meander, or eddy). These cycles, or phases, have an average duration of about 10 days. During the northwestern phase, the permanent Black Sea pycnohalocline (hereafter pycnocline) and seasonal thermocline descend, their thickness increases, and so does the thickness of the upper mixed layer (UML). During the southeastern phase, both the pycnocline and seasonal thermocline ascend and become thinner, along with the UML, which also becomes thinner. In both phases, isopycnals in the pycnocline and isotherms in the thermocline demonstrate quasi-in-phase vertical oscillations, which have a good correlation with the speed and direction of the alongshore current. These correlations allow estimation of the magnitude of upwellings and downwellings in the shelf–slope area of the northeastern Black Sea using data series of current velocity profiles.

Purpose. The paper is aimed at assessing salinity and temperature variability in the upper 300-meter layer of the northeastern part of the Black Sea based on the analysis of the archival and modern expeditionary data. Methods and Results. The data on the cross-sections “coast – sea center” (with the length of 70–110 nautical miles) performed from 1999 to 2009, as well as the results of regular ship monitoring in the shelf-slope zone of the northeastern part of the Black Sea carried out in 2010–2020 were used. It was found that salinity was progressively increasing in the upper 200-meter layer during the last decade. Salinity increase, on the average, constituted annually about 0.05–0.06 PSU. An increase of temperature was also observed below the layer of temperature minimum (core of the cold intermediate layer). In particular, the lower 8.7 °C isotherm rose annually, on the average, by 11 m from its annual average depth 242 m in 2010 up to 121 m in 2020. Salinity growth led to the corresponding changes in water density that resulted in elevation of the lower boundary of the oxygen-containing layer (potential density is 15.8) from the depth of 143 m in 2010 to 124 m in 2020. Conclusions. Climatic changes have led to a noticeable salinity increase in the upper 200-meter layer of the northeastern Black Sea, as well as to a temperature increase in the layers situated below the temperature minimum layer. Though the measurements were carried out in a certain area of the shelf-slope zone, there are reasons to assume that the observed dynamics can be attributed to the entire Black Sea. Physical reasons for the observed changes require a detailed research.

The biogeochemistry of waters is an essential regulator of phytoplankton dynamics, determining the level of species bloom and the change in dominants. This paper investigated the seasonal dynamics of phytoplankton and the nutrient concentrations and their ratios in the northeastern Black Sea in 2017–2021. Two taxonomic groups, diatoms and coccolithophores, determine the seasonal dynamics and significantly contribute to the total phytoplankton biomass. Coccolithophores formed blooms in early June annually, except in 2020. Large diatoms dominated in summer with a biomass exceeding 1000 mg m⁻³ annually, except in 2019. During the blooms of these taxonomic groups, their contribution to the total phytoplankton biomass exceeded 90%. Each group has characteristic biogeochemical niches in the nitrogen and phosphorus concentration coordinates. The position of the seasonal thermocline regulates the biogeochemistry of the water. With a high-lying and sharp gradient thermocline (the average for five years is 6.87 m), low nitrogen concentrations and a nitrogen-to-phosphorus ratio below the Redfield ratio are created in the upper mixed layer. These conditions are optimal for the dominance of coccolithophores. When the thermocline is deepened (the average for five years is 17.96 m), the phosphorus concentration decreases significantly and the ratio of nitrogen to phosphorus is significantly higher than the Redfield ratio, and these conditions lead to the dominance of large diatoms. The results of experimental studies with nitrogen and phosphorus additives in the natural phytoplankton population confirm the above statements. The addition of phosphorus leads to the increased role of coccolithophores in the total phytoplankton biomass, the addition of nitrogen alone contributes to the growth of large diatoms, and the combined addition of phosphorus and nitrogen in a ratio close to the Redfield ratio leads to the dominance of small diatoms.

The consequences of extremely heavy rains in the coastal zone near Gelendzhik on July 6, 2012 are discussed with defining the sources of the solid material supply to the seashore, estimating their...