The data of the sea reflectance coefficient were obtained during the cruise of the R/V “Akademik Ioffe” (10 August–8 September 2021) in the Kara Sea beyond the Arctic Circle. A total of 28 measurements of sea reflectance were performed in different conditions ranging from estuarine zones to open ocean. In addition, at 10 stations, water samples were taken, and chlorophyll-a concentrations were determined using the fluorometric method. In situ reflectance data were compared to satellite data (MODIS Aqua/Terra, Sentinel OLCI 3A/B, VIIRS SNPP) obtained within 24 h before and after the measurement. It was shown that, in general, an overestimation of remote sensing reflectance is observed in short-wave channels (412, 443, 469 nm for MODIS; 410, 412.5, 442.5 for OLCI) and underestimation in long-wave channels (708.75, 753.75 nm for OLCI). The obtained in situ chlorophyll-a concentrations were compared with the results of standard models (GIOP, OC2, OC3, OC4), and regional algorithms (semi-analytical MHI, empirical K17) were applied to the in situ and remotely sensed reflectances. In the cases of GIOP and OC4, unrealistic concentrations of chlorophyll-a were obtained. Regional algorithms developed earlier, specifically for the Kara Sea, showed good correlation (0.6 and 0.76) with in situ measurements. Despite the approximately 20% overestimation of chlorophyll-a by both regional algorithms, they can be used to obtain chlorophyll-a concentrations in the Kara Sea in a concentration range of <1 mg/m³.

This work was aimed at estimation of an input of small and medium rivers of Sochi-Adler area into the standing stock of dissolved organic carbon (DOC) in the coastal zone of the Black Sea. Presented data were acquired in May 2011 and May 2012 in the coastal region of the Greater Sochi. Studied area included river mouths of the Mzymta, Kudepsta, Sochi and Bitkha rivers, where mean DOC concentrations were 69.22 μM, 174.33 μM, 171.04 μM and 4678.31 μM respectively, while the background seawater concentration of the DOC (two kilometers away from the river mouth) was about 220 μM. The Mzymta, Kudepsta, Sochi and Bitkha rivers significantly differ in chemical composition, including the amount of carried organic matter, mainly due to the difference in height of their water-producing areas, different contribution of feeding sources and different anthropogenic impact.

This study was based on data obtained during the cruise 63 of the R/V Akademik Mstislav Keldysh in September 2015. We examine the features of the hydrochemical structure of the Lena River–Laptev Sea mixing zone. The studies were carried out in the eastern part of the Laptev Sea on a quasi-meridional transect, with the length of 710 km along 130°30' E. The distribution of dissolved oxygen, pH, alkalinity, major nutrients (phosphates, silicon, nitrate, and ammonium nitrogen), and chlorophyll a was analyzed. The relationship between the main hydrochemical parameters and chlorophyll a was considered. The distribution of hydrochemical parameters is similar to the data of previous studies: significant removal of silicon by waters of the Lena River (66.6 μM) with a moderate nitrate content (3.87 μM) and low concentration of mineral phosphorus (0.08 μM) in the area of strong influence of river runoff (salinity of 3.012 PSU). The presence of a near-bottom stagnant water zone in the part of research area affected by river runoff was detected. It is shown that the influx of mineral forms of nitrogen and phosphorus into the marine region of the Lena River delta occurs mostly as a result of nutrient recycling rather than due to the continental runoff.

River runoff is the main source of nutrients and man-made pollutants that affects functioning of biological communities. About a thousand small and medium rivers flow into the Black Sea, and while their contribution in total sea balance is significant, there are no systematic observations on that matter. This work presents the studies carried out in 2006–2010 in coastal area of the north-eastern Black Sea. Attention is focused on studies in 2009 near the Greater Sochi region, since two major local rivers, Kudepsta and Mzymta, significantly differ in composition, which allows tracing the inflow of each individual river and estimating their fractional contribution to formation of surface water structure. Influence of river runoff on hydrochemical balance of nearshore water is demonstrated, with estimations of amount of dissolved carbon, alkalinity, silicate, dissolved oxygen and pH. Development of the river plume is traced, along with its reaction on wind direction and shelf circulations.

The results of the analysis of the variability of the concentration of dissolved oxygen and chlorophyll-a (Chl-a), as well as the values of apparent oxygen utilization (AOU), in a 5-mile transect perpendicular to the coastline of Golubaya Bay (Gelendzhik), in April–November 2012, are presented. It is shown that the maximum oxygen content during the study period in most cases is observed in the thermocline layer or immediately below it, and the minimal values of AOU (negative values reflecting the maximal photosynthetic oxygen release) are above the thermocline. The maximum content of Chl-a is most often at a depth of 10 to 20 m; i.e., in spring and summer, for the most part, it is located within the thermocline, and in autumn, above it. It is shown that the oxygen content in the upper water layer is largely determined by the solubility of the gas in the sea water at the given temperature and salinity. Biological processes of oxygen release and consumption (production and destruction of organic matter) have a lesser effect on its seasonal dynamics in the upper layer. In the water layer under the thermocline, the AOU can serve as an indicator of destructive processes.

From 1995 to the present, the level of the Caspian Sea decreased by almost 3 m, which significantly affects the morphometry and the ecological state of shallow water areas. First of all, this concerns the shallow Northern Caspian and, especially, the Volga Delta, which over the years has moved 10–20 km forward and grown by about 3000 km² in area. Changes in the morphometric characteristics of the Volga Delta inevitably affect the hydrodynamic, hydrophysical, hydrochemical, and hydrobiological characteristics of river water in the foredelta. From May 18 to 20, 2021, complex hydrophysical, hydrochemical, and hydrobiological studies of river water were carried out in the southwestern part of the Volga Delta. A total of 32 stations were completed, at which water surface temperature, electrical conductivity, turbidity, and chlorophyll a and oxygen concentrations were measured and samples were taken for further analysis in the laboratory. They showed a complex picture of the distribution of the studied parameters, both along the watercourses and across them. Only high spatial resolution optical and infrared Landsat-8 OLI/TIRS satellite images, obtained on May 20, 2021, almost simultaneously with the ongoing studies in the Volga Delta, helped the spatial distribution of these parameters to be understood. As a result of the analysis of the images, it was possible for the first time to identify a horizontal fine structure of watercourses, which is an alternation of clear immiscible narrow jets with significantly different characteristics of temperature, suspended matter, turbidity, and chlorophyll a.