The material was obtained during cruise 92 of the R/V Akademik Mstislav Keldysh in the area with a latitudinal extent of 72°32.0′ to 73°14.5′ N, including the Ob River estuary and adjacent shallow shelf on October 12–13 in the period preceding seasonal ice formation. The estuarine front was well pronounced in terms of salinity (gradients 0.6 PSU/km) and temperature fields and had a quasi-meridional direction. Sharp boundaries in nutrient distribution typical of earlier seasons were not observed. The abundance and biomass of phytoplankton varied from 22 × 103 to 75 × 103 cells/L and 7.3 to 19.0 mg/m3, respectively, which were two to three orders of magnitude lower than the values obtained in July and September. The abundance (30–78

The Black Sea is a semi-enclosed basin subject to major anthropogenic pressures, including marine litter and plastic pollution. Due to numerous large rivers draining into the basin and a population settled along the coast, the region could accumulate significant amounts of floating litter over time. Until now, only limited field data were available, and litter quantities and distribution remained unknown. In this study, floating marine macro litter (FMML) was assessed at the regional Black Sea scale for the first time, showing relatively high litter densities across the basin that reached a weighted mean of 81.5 items/km(2). Monitoring data revealed an accumulation of floating items offshore in the eastern part of the basin, resembling on a small scale a "garbage patch', where litter items were trapped, showing elevated densities in comparison to their surrounding areas. Most of these items were made of plastic materials (ca. 96%) and included large numbers of plastic and poly-styrene fragments of small size ranges (2.5-10 cm). Harmonised field data collection through consistent and regular monitoring programmes across the region is essential to establish baselines and thresholds for large scale assessment at international level.

The research is based on the materials collected during cruise 66 of R/V Akademik Mstislav Keldysh in the Yenisei estuary and over the adjacent Kara Sea shelf in the latitudinal range from 71°50.5′ to 75°55.0′ N. The studies were performed from July 25 to 27, 2016, three weeks after the end of the flood period. Two hundred fifty species of plankton algae were recorded. Well-pronounced latitudinal zoning in phytoplankton communities structure associated with changes in the hydrophysical and hydochemical conditions of the pelagic environmental was determined. The maximum species diversity was formed by diatoms, green and blue-green algae in the southern desalinated part of the estuary (<1 PSU) and by diatoms and dinoflagellates in the offshore areas. The highest algae abundance and biomass, 2.7–2.9 × 106 cell/L and 1.1–1.5 g/m3, respectively, were recorded in the inner desalinated part of the estuary, where freshwater diatoms of the genus Aulacoseira dominated. The distribution of freshwater algae northwards was bounded by the 12–15 PSU surface isohaline (74°20′ N). The most pronounced decline in phytoplankton abundance and biomass was found in the outer part of the estuary north of ~73° N. In the middle shelf to the north of the Yenisei estuary, the maximum abundance (up to 740 × 103 cell/L) and biomass (up to 240 mg/m3) of phytoplankton were recorded at the depths of 30–40 m with low illumination and high nutrient concentration, which was also typical of the Ob region in spring–summer.

The vertical distribution of phytoplankton primary production (PP) and chlorophyll a (Chl) was studied based on the data carried out in August–September 2015, 2017, and 2018. The PP maximum was located at the surface or within the 0–5 m subsurface layer. The subsurface chlorophyll maximum (SCM) was recorded at 39% stations on the outer shelf and in the vicinity of the continental slope. The SCM was not detected along the northward transect (130° E) from the Lena River delta. As in other areas of the World Ocean, the SCM was located below the upper mixed layer (UML), in the nitracline, near the boundary of the euphotic zone (1% of photosynthetically active radiation). Generally, the SCM was not accompanied by an additional PP maximum. The Chl concentration at the SCM did not exceed 1 mg m–3. PP produced within the UML and SCM contributed 72 and 23%, respectively, to the integrated primary production (IPP) of the water column. Our results suggest that the influence of the SCM on IPP was insufficient due to low Chl concentration and PP colimitation by the low light and temperature at these depths.

The sources and quantities of marine litter and plastic pollution in the Black Sea are yet unknown. It is important to identify the main pathways in order to enable mitigation strategies to reduce the input of plastic waste to the marine environment. In this sense, rivers in this region are expected to play an important role in transporting mismanaged waste to the sea, but data on this matter is still very limited. This study presents a first compilation of riverine floating macro litter data collected in rivers flowing into the Black Sea. Visual observations provided indicative information on the most frequent litter items and rates of riverine litter fluxes in ten rivers from Ukraine, Russia, Georgia and Turkey. Top items presented an 83.7% of plastics, including cover / packaging, bottles, pieces and bags as main contributors. Riverine litter fluxes were variable, showing median values between 4 and 75 items/hour in the different rivers, and maximum values up to 700 items/hour in the individual monitoring sessions. The establishment of future monitoring programmes require the implementation of harmonized approaches and consistent frequency in the data collection to improve the representativeness of results, enabling appropriate comparable assessments of riverine litter inputs in the Black Sea.

Samples were collected in the area of the St. Anna Trough in the northern Kara Sea in September 2015 and July–August 2016. Based on salinity and temperature distribution, four areas have been distinguished: the shelf adjacent to the St. Anna Trough, the shelf edge, and the upper and lower parts of the continental slope. The variability of the structure of phytoplankton communities has been analyzed in view of environmental conditions in the distinguished areas. It is shown that changes in the species structure of phytoplankton are more associated with seasonal reorganization in the community than with the effect of river runoff and waters of the Barents Sea and Atlantic. In the shelf area, the presence of Barents Sea winter water led to an increase in the number of species of early spring diatoms and an increase in the abundance of early and late spring diatoms. An increase in the abundance and biomass of phytoplankton and changes in their vertical distribution in mid-July–early August occurred against the effect of Barents Sea and Atlantic waters on the area and desalination of the upper layer under the effect of river runoff in early September.

We proposed a new parameter regulating distribution of the species – the daily average absorbed energy of PAR by a unit of phytoplankton cell volume, determined by the shape, size and concentration of pigments. We calculated this parameter for some dominant diatom species of the northeastern part of the Black Sea and normalize it by the average daily PAR for periods of dominance. We established that for species successfully compete for an ecological niche this parameter should be approximately at the same level in different seasons.

We proposed hypothesis that describes the mechanism of formation and disintegration of cell chains, according to which the number of cells in a chain is determined by the difference in the rates of cell division and the separation of chains into individual cells, and the maximum number of cells in the chain is achieved at the highest division rate. Experiments showed that the addition of phosphorus stimulates the formation of longer chains. We developed a simple mathematical model of the dynamics of the number of cells in cell chains.

— The studies were conducted along the transect (11 stations) from the inner part of Khatanga Bay in the south to the Laptev Sea continental slope in the north in September 17—20, 2017. Four biotopes with different parameters of the pelagic environment, composition, quantitative characteristics and vertical distribution of phytoplankton have been distinguished: the inner part of the Khatanga Bay, estuarine frontal zone, western shelf of the Laptev Sea and continental slope area. The inner part of Khatanga Bay and the continental slope are characterized by the highest values of phytoplankton abundance and biomass, which reach 1 × 10 6 cell/L and 160 mg/m 3 , respectively. The formation of the maximum at a depth of 45 m is typical for the phytoplankton vertical distribution in the continental slope area. Algal biomass in the maximum reaches 400 mg/m 3 which is the highest value for the transect. Well-pronounced latitudinal zoning in the structure of phytoplankton communities has been determined in the western part of the Laptev Sea similar to that in other regions of the Arctic seas under a strong impact of Siberian rivers discharge.

Based on the material obtained in the spring–summer season of 2016, the composition and quantitative distribution of phytoplankton in the Ob estuary and over the Kara Sea shelf has been estimated. The latitudinal extent of the region exceeded 5° (~560 km), encompassing areas with different river discharge impact, which formed latitudinal zoning in the distribution of hydrophysical and hydrochemical conditions. These environmental properties determined the zoning in the spatial structure of phytoplankton communities. The estuarine biotope with a salinity <1 PSU and high concentrations of nitrate, phosphate, and silica was inhabited by a freshwater phytocenosis stable in species composition with high-quantitative characteristics and dominated by Aulacoseira species. The most spatial and seasonal variability of phytoplankton is characteristic of the area of intense interaction of fresh and marine waters in the outer part of the Ob estuary and over the adjacent shallow shelf, where freshwater algae species are replaced by marine species. A sharp biotopic boundary separates the inner shallow shelf with depths of approximately 30–35 m and outer shelf of the depth from 150 to 200 m. In the area of the outer shelf the lower boundary of the euphotic zone descends to 25–35 m. At theses depths plankton algae actively vegetate, and form subsurface phytoplankton maximum under conditions of sufficient nutrient concentration.

The main ecological strategies of phytoplankton community structure of the north-eastern part of the Black Sea were revealed. In the spring period, small cell size of diatoms predominate, when the concentrations of elements of mineral nutrition are high, and the light fluxes are far below the maxima (R-strategists). At the end of the spring – early summer the main role is played by coccolithophores, which are tolerant to limited content of mineral nutrition elements in the environment (S-strategists). In summer prevail large cell species of diatom under high insolation conditions and periodic wind-wave mixing of the upper water layer (C'-strategists).

The International Maritime Organization (IMO) in the convention adopted in 2004 imposes stringent requirements on the quality of seawater used in the testing of ballast water management systems (BWMS). They concern both the abundance of plankton organisms of two size groups, 10–50 μm and more than 50 μm, and the taxonomic composition (at least five species of three taxonomic types). Marine phytoplankton has a wide variety of sizes and morphological forms of cells, which makes it difficult to apply the imperative size adopted by the IMO. It is proposed to formalize the size criterion by calculating an equivalent spherical diameter. The real test of the BWMS in 2017 set the task of assessing the compliance of natural water with these quality standards. According to the results of annual monitoring in the northeastern part of the Black Sea, it has been shown that the species diversity of phytoplankton in the size group of 10–50 μm always corresponds to the necessary requirements, but its abundance is two orders of magnitude lower than required. In this case, the simultaneous presence of representatives of three different systematic groups in the initial water is not always observed. This poses the task of modifying the hydrobiological parameters of ballast water by the addition of cultivated species and the formation of a new community with predetermined properties. In this work, we used an intense culture of green algae, which made it possible to increase the abundance of cells to the level corresponding to IMO requirements and also to add a representative of another taxonomic group. Taxonomic diversity in the size group above 50 μm is sufficient and is provided by species diversity of zooplankton; however, the contribution of these organisms to the total population is not large (no more than 3%). The necessary abundance of representatives of this size group was ensured by the cultivation of large dinoflagellates.

The following hypothesis is tested in the paper: increasing the sulfur concentration to the oceanic level has contributed to the taxonomic diversity of phytoplankton. Since the Black Sea phytoplankton is adapted to sulfur concentrations equal to half the oceanic level, increasing the concentration of this element to the oceanic level should not lead to a decrease in the production properties of the dominant species. In the experimental conditions, the effect of increasing concentrations of nitrogen, phosphorus and sulfur on the biomass concentration of diatom Leptocylindrus danicus and Chaetoceros sp. in the stationary phase of a bath culture. It is shown that an increase in the sulfur concentration leads to a significant decrease in the production properties of diatoms. Consequently, at the level of centric diatoms, the proposed hypothesis c’ not be accepted.