The seasonal dynamics of the NE Black Sea phytoplankton follow the following pattern: small diatoms (spring) → coccolithophorid Emiliania huxleyi (late spring–early summer) → large diatoms (summer). Our hypothesis states that nitrogen and phosphorus concentrations regulate the seasonal phytoplankton dynamics. A minimum number of parameters is enough to understand the mechanisms of dominant species change. Based on the concept of intracellular regulation, the following parameters were evaluated: the minimum nitrogen and phosphorus quotas; half-saturation constants for nitrogen and phosphorus uptake; the maximum specific growth rate of the dominant phytoplankton species. Computational experiments on the model show the following: (1) in spring, a species with a high maximum specific growth rate becomes dominant; (2) in late spring and early summer, a species with a low minimum nitrogen quota and a low half-saturation constant for nitrogen uptake is observed; (3) in summer, a low minimum phosphorus quota and a low half-saturation constant for phosphorus uptake allow the species to become dominant.

The modern Arctic is characterized by a decreased ice cover and significant interannual variability. However, the reaction of the High Arctic ecosystem to such changes is still being determined. This study tested the hypothesis that the key drivers of changes in phytoplankton are the position and intensity of Atlantic water (AW) flow. The research was conducted in August 2017 in the northern part of the Barents Sea and in August 2020 in the Nansen Basin. In 2017, the Nansen Basin was ice covered; in 2020, the Nansen Basin had open water up to 83° N. A comparative analysis of phytoplankton composition, dominant species, abundance, and biomass at the boundary of the ice and open water in the marginal ice zone (MIZ) as well as in the open water was carried out. The total biomass of the phytoplankton in the photic layer of MIZ is one and a half orders of magnitude greater than in open water. In 2017, the maximum abundance and biomass of phytoplankton in the MIZ were formed by cold-water diatoms Thalassiosira spp. (T. gravida, T. rotula, T. hyalina, T. nordenskioeldii), associated with first-year ice. They were confined to the northern shelf of the Barents Sea. The large diatom Porosira glacialis grew intensively in the MIZ of the Nansen Basin under the influence of Atlantic waters. A seasonal thermocline, above which the concentrations of silicon and nitrogen were close to zero, and deep maxima of phytoplankton abundance and biomass were recorded in the open water. Atlantic species—haptophyte Phaeocystis pouchettii and large diatom Eucampia groenlandica—formed these maxima. P. pouchettii were observed in the Nansen Basin in the Atlantic water (AW) flow (2020); E. groenlandica demonstrated a high biomass (4848 mg m−3, 179.5 mg C m−3) in the Franz Victoria trench (2017). Such high biomass of this species in the northern Barents Sea shelf has not been observed before. The variability of the phytoplankton composition and biomass in the Franz Victoria trench and in the Nansen Basin is related to the intensity of the AW, which comes from the Frame Strait as the Atlantic Water Boundary Current.

In August 2020, during a dramatical summer retreat of sea ice in the Nansen Basin, a study of phytoplankton was conducted on the transect from two northern stations in the marginal ice zone (MIZ) (north of 83° N m and east of 38° E) through the open water to the southern station located in the Franz Victoria Trench. The presence of melted polar surface waters (mPSW), polar surface waters (PSW), and Atlantic waters (AW) were characteristic of the MIZ. There are only two water masses in open water, namely PSW and AW, at the southernmost station; the contribution of AW was minimal. In the MIZ, first-year and multiyear ice species and Atlantic species were noted; Atlantic species and first-year ice species were in open water, and only ice flora was at the southernmost station. The maximum phytoplankton biomass (30 g · m−3) was recorded at the northernmost station of the MIZ, and 99% of the phytoplankton consisted of a large diatom Porosira glacialis. Intensive growth of this species occurred on the subsurface halocline separating mPSW from PSW. A thermocline was formed in open water south of the MIZ towards the Franz Victoria Trench. A strong stratification decreases vertical nutrient fluxes, so phytoplankton biomass decreases significantly. Phytoplankton formed the maximum biomass in the thermocline. When moving south, biomass decreased and its minimum values were observed at the southernmost station where the influence of AW is minimal or completely absent. A transition from the silicon-limited state of phytoplankton (MIZ area) to nitrogen-limited (open water) was noted.

The interest of researchers in macrophytes in the seventies and eighties of the last century was associated with the need to obtain technologically important products, and by the end of the last century, technologies for cultivating several important species of algae were created and put into practice. Currently, the problem of climate change requires new approaches to find ways to transfer inorganic carbon from the atmosphere to the ocean floor. One of the ways to sequester inorganic carbon seems to be the creation of systems based on marine macrophytes. This should stimulate research into natural populations of macrophytes and the organization of cultivation methods according to a new optimization criterion, namely the optimal transfer of atmospheric carbon to the ocean floor.

The possibility of creating a bio-optical model for calculating parameters of underwater light fields under conditions of intense coccolithophore bloom in the Black Sea is shown. Hydrolight software is used for bio-optical modeling. The influence of the parameters of the bio-optical model both on the remote sensing reflectance spectrum and on the concentration of coccolithophores is demonstrated. The coccolithophore concentration is calculated using a regional algorithm. The optimal modeling parameters yield the remote sensing reflectance spectrum which nearly coincides with measured one. A decrease in the thickness of the coccolithophore layer relative to the optimal one leads to a comparable decrease in the concentration of coccolithophores, at the same time its increase does not play such a significant role. Estimates of the concentration of coccolithophores change little with variations in the concentration of chlorophyll, but they are more sensitive to changes in the amount of colored dissolved organic matter. The created bio-optical model made it possible to study the accuracy of chlorophyll estimation algorithms under conditions of intensive coccolithophore bloom.

Primary production (PP) and the chlorophyll-a concentration (chl-a) in the European Arctic in the summer of 2020–2021, where continued climatic warming and increased “Atlantification” accelerate the sea ice losses, are discussed. The maximum integrated PP and the total chl-a content were observed in the marginal ice zone (MIZ) of the Barents Sea under weakened stratification of the water column and reached 1109 mgC m–2 day–1 and 118 mg m–2. Near the ice edge in the Nansen Basin, the main part of PP formed in the upper mixed layer and did not exceed 469 mgC m–2 day–1; the chl-a content reached 56 mg m–2. The early and late stages of phytoplankton bloom in the MIZ were characterized by the leading role of picophytoplankton in carbon fixation. Large centric diatoms, microphytoplankton, were recorded to dominate in the MIZ at the stage of peak bloom in 2020 under the dense ice cover of the Nansen Basin. A similar phenomenon was observed earlier only in the Arctic shelf seas and was not recorded in the high-latitude basins of the Arctic Ocean. With the sparse ice cover of the Nansen Basin in 2021, the main primary producers were pico- and nanophytoplankton. The low variability of assimilation numbers (1.7 ± 0.3 mgC mg chl-a–1 h–1) at all bloom stages indicates indirectly the acclimatization of different species of phytoplankton to the environmental changes. The ecological flexibility of the primary production link of the MIZ ecosystems in the studied seas of the European Arctic during the period of climate changes is confirmed.

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.

Seasonal dynamics, including the Pseudo-nitzschia delicatissima (PDG) group's vertical and spatial distribution, were analysed based on field data collected in the Black Sea from 1948 to 2020. The main annual bloom occurred in February-March when the abundance in the upper mixed layer could reach 5 x 10(6) cells m(-3). Bloom was most intense in the centre of the sea and on the outer shelf waters. In April, the bloom subsided, and in summer, only small peaks could be found in the deep-sea waters. On the contrary, high cell abundance was observed in the shelf areas in summer and autumn. Three periods with different levels of biomass were distinguished in the centre of the deep-water basin: highest (February-March), moderate (April-June), and lowest (July-December). Nutrient levels varied during these three periods, contributing to different growth-promoting phases. Six experiments with natural phytoplankton conducted in April-June showed that dissolved nitrate (N) and phosphate (P) were growth-limiting factors for PDG. Very low dissolved silicate (DSi) in October likely limited growth, as evidenced by the algae's lack of response to N and P additions. In February-March, dissolved inorganic nutrient concentrations were highest, with dissolved inorganic nitrogen (DIN), P, and DSi above 2.0 mu M, 0.2 mu M, and 2.5 mu M, respectively. During the moderate growth period, DIN (< 1.2 mu M) and P (< 0.15 mu M) were expected to significantly limit the growth of PDG, while DSi was too high (> 3.5 mu M) to have such an effect. In the second half of the year, DSi fell below 2.0 mu M, which could severely inhibit cell growth in addition to DIN and P. Thus, after June, low DIN, P, and DSi combined resulted in the lowest PDG biomass.

A modified regional algorithm to quantify the coccolithophore concentration in the northeastern part of the Black Sea under conditions of intense bloom is presented. To modify the algorithm, the data of in situ measurements of coccolithophore Emiliania huxleyi abundance performed in June 2017 and 2022 (when the maximum values were 9 × 106 and 13 × 106 Cells L−1, respectively), as well as the data from hydro-optical and satellite measurements, were used. In addition, the ratio between the number of detached coccoliths and coccolithophore cells was taken into account. Based on the expanded array of in situ data, the optimal values of the regional algorithm parameters were obtained. The modified algorithm makes it possible to obtain more accurate results in areas of high coccolithophore concentrations and takes into account the contribution of coccoliths. To test the sensitivity of the algorithm to variations in bio-optical characteristics, model calculations were performed using Hydrolight software. The updated algorithm is significantly less sensitive to variations in chlorophyll concentration and CDOM absorption coefficient than its previous version.

In the northeastern part of the Black Sea, the biological carbon pump is represented by both organic and carbonate pumps. The organic carbon pump consists of small-cell diatoms (mainly Pseudo-nitzschia pseudodelicatissima) and large-cell diatoms (Pseudosolenia calcar-avis and Proboscia alata). The carbonate pump is represented by only one species of cococcolithophore, Emiliania huxleyi. These species form intense blooms that require characteristic hydrological and hydrochemical conditions. The seasonal dynamics of the biological carbon pump is as follows: organic pump (spring) → carbonate pump (late spring and early summer) → organic pump (summer and autumn). An exception is the long-term dynamics of carbon concentration, and no significant carbon growth trends have been identified. During the intensification of the work of the carbonate pump, partial concentrations of carbon in water, increased relative to the atmosphere, and an increased influence of the organic pump on high partial pressure are released. In late spring and early summer, CO2 is released in the Black Sea, as a result, absorption increases in summer. The carbonate pump arises with a greater arrival at sea.

The phytoplankton structure in ecologically diverse areas of the sea and the role of marine invasive species in the formation of the community were studied on the meridional and latitudinal trans-Caspian sections in the autumn period of 2008, 2009, and 2012. It was established that the transformation of the phytoplankton community continues in the Caspian Sea, associated with the entry of the Black Sea diatoms Chaetoceros peruvianus, Cerataulina pelagica, and Pseudo-nitzschia seriata into their composition. During the autumn bloom of phytoplankton in November 2008, the number of invasive species reached 1.3-2.3 × 10 cells/L, C. peruvianus and P. seriata to the total weight biomass of the community reached 49-50%. The highest abundance of invasive species was recorded in the shelf areas of the Middle Caspian Sea in its eastern ( C. peruvianus , 2.3 × 10 cells/L), western ( P. seriata , 1.4 × 10 cells/L), and northern ( C. pelagica , 1.3 × 10 cells/L) parts. The north boundary of the distribution of these species in the Caspian Sea was the 5.0 isohaline, which coincided with the highly productive frontal zone (4.0-6.0) of the marginal filter of the Volga River. For the first time, it was shown that the C. peruvianus diatom was a part of the phytoplankton of the upper productive 25-30-meter water layer both in the Middle and in the Southern Caspian during the periods preceding the autumn bloom of phytoplankton (September-October). During that time the most considerable abundance of this species (1.6 × 10cells/L) was recorded in the shallow areas on the Apsheron Sill and the eastern part of the Middle Caspian in the zone of coastal wind upwelling.

The effect of the anticyclonic eddy on the morphological structure, abundance (NB) and biomass of bacterioplankton (BB) in the deep phytoplankton maximum (DPM) was experimentally studied on natural phytoplankton in the Black Sea. Characteristic processes were simulated: the death of phytoplankton as a result of deepening DPM below the photic zone and a simultaneous increase in nutrient concentrations. In the experiment, primary production was terminated. Two cases were simulated: Regular and Deep DPM sinking with medium to high daily nutrient additions. Deep and Regular cases showed similar dynamics without significant differences. The NB increased during the first 4 days by 3.5 times to the maximum of 1.12 × 106 ml−1 cells. BB increased by 11.8 times to 42.5 mg C m−3. At the same time, the bacterial morphological structure changed towards the dominance of large rods in the BB (up to 100%), resembling copiotrophic bacteria. The sharp increase in BB cannot be explained by utilizing the organic matter released from dead eukaryotic phytoplankton biomass since it was low (4.0 mg C m−3). We assume that the enrichment of the environment with biogenic elements resulting from anticyclonic deepening can promote the development of large, fast-growing copiotrophic forms of bacteria. This growth can be achieved through a more complete use of the already available labile organic matter rather than the new one released from the dying DPM.

Research regarding the conditions and processes of paleo- and modern sedimentation with an assessment of particles and pollution fluxes, with determination of the rates of biogeochemical processes and regional paleoclimatic reconstructions in the contact area of cold Polar and warm Atlantic water masses under the influence of cold (seep) and hot (hydrothermal) fluids were carried out during the expedition in the Norwegian-Greenland Sea Area and the Barents Sea. The consequences of atlantification are recorded in all components of the geosystems of the Barents Sea and the high-latitude Arctic as well as the reverse effect of the Arctic amplification on the geosystems of the northern North Atlantic occurring both at the present time and in the Pleistocene and Holocene.

Studies of the annual dynamics of phytoplankton in the NE Black Sea at two stations on the shelf and the continental slope were conducted in 2016, 2017, and 2019. The species composition of phytoplankton has not undergone significant changes compared to previous decades. The coccolithophore Emiliania huxleyi, small flagellates, and diatoms determined the abundance of phytoplankton; and diatoms, coccolithophores, and dinoflagellates determined the total biomass. The annual dynamics of the satellite-derived chlorophyll-a showed peaks in spring and autumn, and sometimes in summer. During the stratified water column period, strong winds in most cases led to a detectible increase in chlorophyll-a. The annual dynamics of phytoplankton followed the pattern: small diatoms (spring) → coccolithophores (late spring, early summer) → large diatoms (summer, autumn). Such a pattern was typical for the previous decades. Coccolithophores dominated in weak SE winds, diatoms in NE winds. The combined effect of sustained offshore wind and strong current can cause diatom blooms during stratified water, even if the wind velocity is moderate.

Long-term (2004-2020) studies showed yearly summer/autumn blooms in the NE Black Sea dominated by large (cell volume> 5000 mu m(3)) diatoms (Pseudosolenia calcar-avis and Proboscia alata). This phenomenon is characterized by high (>250 W m(-2) photosynthetically active radiation, PAR) insolation, and low phosphorus concentrations (to analytical zero). These diatoms contained >100 chloroplasts per cell, which at low irradiance are evenly distributed throughout the cell. As light increases (to 1000 mu mol photons m(-2) s(-1) PAR), chloroplasts aggregate within 20 min, usually to the center of the cell. In consequence, the light absorption coefficient is decreased by >3 fold. At elevated photon flux density (PFD), P. calcar-avis also shows a "conveyor" of chloroplasts moving from the aggregate to the cell periphery and back. This mechanism enables a continuous fine-tuning of the cells' ability to absorb light, likely also facilitating photo-damage repair. This rapid photoacclimationmechanism allows large diatoms to minimize photodamage at high PFD and acclimate well to low PFD. We hypothesize that competitive success of large diatoms in conditions of high light gradients is aided by this short-term rapid photoacclimation enhancing growth rate while minimizing chloroplast repair costs, aided by the ability of large cells to accumulate nutrients for chloroplast synthesis.

Unique data were obtained on the abnormally high level of phytoplankton biomass (30 g m–3) in the marginal ice zone of the deep-water High Arctic (the Nansen Basin, 83° N) during observations onboard a ship in the summer of 2020. The changes in species composition and the increase in abundance of plankton phytocenoses were determined by the complex hydrographic structure formed due to the interaction between the warm Atlantic and cold Arctic water masses and confirmed the important role of the North Atlantic Current in the European sector of the Arctic Basin. For the first time, a reliable relationship was established between the phytoplankton productivity and the halocline depth (an indicator of rising Atlantic water enriched with nutrients, primarily nitrogen). It was shown for the first time that the large cryopelagic centric diatom Porosira glacialis (= Podosira hormoides var. glacialis) can be a species forming phytoplankton bloom in the marginal ice zone of the High Arctic.