The Marginal Ice Zone (MIZ) is a dynamic region where the atmosphere, ocean, and sea ice actively interact, giving rise to frequent eddy formation. Clarifying the processes governing this zone is crucial for both accurate modeling of local circulation and improved prediction of Arctic climate change, particularly sea-ice retreat and ecosystem shifts. This study investigates a mesoscale eddy in the poorly studied northeastern Kara Sea MIZ using a joint analysis of in situ and satellite measurements from summer 2024. We also apply principles from ellipsoidal vortex theory. The eddy's complex evolution is described, and its key parameters are quantified. The eddy was found to contain and transport a substantial volume of freshened cold water, potentially modifying the structure of surrounding waters. Two potential mechanisms for the eddy's formation were proposed, each requiring further investigation through dedicated modeling and observational efforts. Significant differences in the phytoplankton biomass and production rates were identified across the eddy, whereas species composition showed no significant variation. These results highlight the role of mesoscale eddies in freshwater redistribution and biophysical coupling in the MIZ, while underscoring the need to advance theories of eddy dynamics and incorporate these processes into regional and climate models.

Long-term changes in the deep chlorophyll maximum (DCM) and the depth-integrated primary production (PP) in the Black Sea deep basin from 1953 to 2021 were analysed. The changes were compared with alterations in water transparency, nitrate concentration in the deep maximum (NO3-max), its depth and the intensity of winter convection affecting the nutrient supply in the upper layer through the year. A regression between the light attenuation coefficient and the Secchi disk depth was constructed based on field data obtained over the past two decades. The regression differed from that constructed using data before 1989. Both regressions were used to reconstruct long-term changes in the euphotic zone depth. Over 68 years, in the stratified period from April to October, it varied from 21 to 43 m. The annual mean DCM depth ranged from 18 to 55 m and was, in most cases, confined to a depth of 1-2 % of the surface photosynthetically available radiation. The long-term dynamics of annual means of DCM and PP DCM and PP generally corresponded to the change in NO3-max. These variables were significantly correlated with NO3-max, supporting the hypothesis that NO3-max variations altered the nitrate gradient and, consequently, its upward flux into the photic zone, primarily consumed by DCM. At the same time, long-term changes in DCM and PP were also influenced by winter convection, affecting the nutrient supply in the upper layer and the macro-hydrodynamics of the Black Sea throughout the year. After a peak in eutrophication in the mid-1980s and a decline after the mid-1990s, NO3-max began to rise again in the last decade, indicating a new increase in eutrophication. The increasing trends in NO3-max and DCM and an increase in phytoplankton biomass, primarily due to diatoms, support this assumption.

The data obtained from satellite and expedition measurements during the 87th cruise of the R/V Akademik Mstislav Keldysh revealed a significant discrepancy in chlorophyll-a (Chl-a) concentration estimates. Specifically, satellite-derived estimates were found to be underestimated by a factor of more than 25 in the region of an intense phytoplankton bloom in the northwestern part of the Weddell Sea near the Antarctic Peninsula in January 2022. This study aims to identify the reasons for the significant underestimation by satellite and above-water remote sensing data and to examine the related regional bio-optical features, phytoplankton community characteristics, and challenges in atmospheric correction. The in situ Chl-a concentration measured by standard extraction methods (with pheophytin-a correction) was 7.9 mg m(-3), whereas standard satellite bio-optical algorithms produced values of around 0.25 mg m(-3), and above-water remote sensing provided values of about 1.6-2.0 mg m-3. The underestimation was observed in both Level 2 and more rigorously filtered Level 3 satellite ocean color data, suggesting that similar biases might affect other studies relying on satellite-derived Chl-a maps. The extreme underestimation (approximately 25-100-fold) of satellite-derived Chl-a was caused mainly by errors in atmospheric correction. The identified bio-optical reasons leading to the remote underestimation of Chla concentration by a factor of approximately 4 are attributed to relatively low specific light absorption by phytoplankton cells, non-phytoplankton particles, and colored dissolved organic matter compared to global ocean averages. As a result, the depth of the photic zone was approximately twice that expected for the given Chla concentration. Additionally, the study demonstrates that spectral shape and low values of the above-water remote-sensing reflectance are significantly influenced by reduced particulate backscattering fraction and near-zero exponent in the power-law fit to the particulate beam attenuation spectrum. All observed regional biooptical features were significantly influenced by pigment packaging and the prevalence of nano-and microphytoplankton cells, which predominantly formed large colonies (tens to hundreds of microns) and aggregates exceeding 100 mu m.

The vast majority of marine upwellings are widely examined because they are detected at the water surface. At the same time, there are poorly studied so-called "hidden" upwellings, where significant vertical water movements occur at depth. In the north-eastern Black Sea, high-frequency observations using a moored CTD and current meter profiler revealed hidden upwelling and downwelling beneath the seasonal thermocline over the continental slope. Anomalies in the depth of the indicative isopycnal sigma theta = 14.5 kg m(-3) relative to the annual mean values varied from -16 m (rising isopycnal) to +22 m (deepening). On average, the relative changes were -8.5 % and +7.2 %, respectively. In the layer 40-70 m below the photic zone, the turbulent diffusion coefficient (Kt) varied from 0.35 to 53 x 10(-5) m(2) s(-1). The diffusive nitrate flux (F-dif) depended on Kt but more strongly on gradient of nitrate (N-grad). However, variations in F-dif were smoothed by the inverse changes in Kt (increasing during downwelling) and N-grad (increasing during upwelling), yielding a nearly phase-independent upward flux. Changes in the advective nitrate flux (F-adv) were driven by equally short upwelling/downwelling phases (similar to 5.5 days). The mean F-adv and F-dif were 0.68 and 0.22 mmol N m(-2) day(-1), respectively. Estimated monthly means of new primary production ranged from 30 to 153 mg C m(-2) day(-1) with an average f-ratio of 0.31. The only visible biological response was the redistribution of the chlorophyll-a below the photic zone at high Kt. Such an impact means that, in addition to the positive effect on F-dif, and, therefore on new PP in the water column, high Kt can limit growth of deep phytoplankton assemblages.

The features of the empirical bio-optical algorithm operation in the waters around the Antarctic Peninsula are analyzed based on a comparison of calibrated data from the shipborne flow fluorimeter and satellite data from the OLCI radiometer on Senti nel-3A and Sentinel-3B satellites during the Antarctic summers of January-February 2020 and 2022. It is shown that the standard OC4 bio-optical algorithm significantly underestimates satellite estimates of Chl-a concentration from ~1.5 to ~9 times (on aver age by a factor of ~3.1). The known regional OC4-SO algorithm provides acceptable errors of Chl-a concentration estimates and can be used for studies related to the analysis of Chl-a concentration in the waters around the Antarctic Peninsula. The developed in this work new regional algorithm OC4-AP has significantly lower error in comparison with the known standard and regional algorithms. It can be used if it is necessary to obtain a remote estimate of the concentration of Chl-a, as close as possible to the accumulated world experience in determining this value by standard extract spectrophotometric and fluorimetric methods. The observed underestimation of satellite estimates of Chl-a concentration using the standard empirical bio-optical OC4 algorithm can be attributed to at least three reasons typical for the studied water area: low relative CDOM content, high phycoerythrin content, and stronger effect of pigment packing in phytoplankton cells compared to the average values in the World Ocean.

Brief results of oceanological studies in the south-eastern Baltic Sea and Gulf of Finland on cruise 54 of the R/V Akademik Sergey Vavilov (2022) are presented. Hydrological, hydro- and geochemical, hydrobiological, geoecological, geological, and geophysical studies were carried out. New data on the state and dynamics of the Baltic Sea natural complexes under conditions of increasing anthropogenic pressure and climate change were obtained.

Studies were carried out of the seasonal dynamics of quantitative parameters of picoplankton: the total number of bacteria, the number of bacteria with an active electron transport chain and their share in bacterioplankton, the number of autotrophic picoplankton, the content of chlorophyll a and its picofraction, as well as temperature and hydrochemical regimes in the waters of three stations with different levels biogenic pollution on the Moscow River. The highest values of all studied parameters were observed for waters at st. NKS is the zone of direct influence of the Kuryanovsky treatment facilities. A bimodal type of seasonal dynamics of picoplankton was observed: for autotrophic picoplankton, maximum abundance was observed at the end of April and June, for bacterioplankton – in July and October.

Cyclonic eddies often increase the primary productivity of marine ecosystems. However, the study of their influence on the taxonomic structure and productivity of plankton is complex due to the short-term and mesoscale nature of the action of eddies. In a laboratory bioassay experiment, we simulated two mechanisms of eddy's effect on the deep phytoplankton maximum: an increase in the upward flow of deep nutrients and illumination. Doubling of nutrient additions compared to control increased chlorophyll's specific growth rate (SGR) 1.7-fold over 12 days while doubling the light intensity increased SGR by 3.5 times. During the first 4 days of the experiment, at the exponential growth stage, SGR of carbon biomass was maximum with the simultaneous doubling of nutrients and light (0.44 day-1). It was statistically significantly higher than at increased nutrients but not light. These results suggest that nutrient deficiencies were less crucial for phytoplankton growth than light. The increase in the phytoplankton biomass was mainly due to the growth of a large-celled diatom Pseudosolenia calcar-avis. It showed the highest SGR (1.15-1.28 day-1) at increased nutrients and/or light, resulting in high wet biomass of 2-3 g m-3 over 4 days. The ecological meaning of the obtained results is as follows. First, in the eddy, where the deep phytoplankton maximum is located close to the nutricline, its rise to more illuminated layers results in higher phytoplankton growth than the shortening distance to the nutricline. Second, increases in the upward flow of nutrients and light intensity, separate or simultaneous, stimulate the rapid reproduction of large-celled diatoms that increase the carbon-to-chlorophyll ratio by 2-4 times. Third, exposure to a typical Black Sea mesoscale cyclonic eddy can lead to phytoplankton blooms in the thermocline within a few days.

The parameters of primary productivity were studied in different seasons at the carbon polygon: the rate of primary production, the concentration of chlorophyll a, the assimilation number (AN), the rate of electron transport in photosystem II of phytoplankton (rETR), and the photosynthetic efficiency (AN/rETR). The direct correlation between the values of AN and rETR allows using the coefficient of photosynthetic efficiency to calculate primary production in mass units of carbon to assess the carbon flux from the atmosphere. This coefficient has seasonal specificity.

Environmental monitoring of the Russian sectors of the Baltic Sea was continued through three expeditions in 2023: cruise 55 of the R/V Akademik Sergei Vavilov (April) and cruises 54 and 55 of the R/V Akademik Boris Petrov (November and December, respectively). Seasonal quantitative estimates of hydrological–hydrochemical and hydrobiological parameters and carbon dioxide fluxes at the water–air interface were obtained, and the geological and geomorphological conditions of the study areas were refined. The influence of the Neva River on the eastern Gulf of Finland was stable in all seasons and could be traced at a distance of up to 30 nautical miles from Neva Bay. The Southeastern Baltic bottom layer experienced extensive hypoxia and mosaic euxinia. Significant aeration of bottom waters occurred in December due to the combined influence of seasonal convection and downwelling resulting from a series of storms. In both study areas, high values of integral primary production were obtained in April, which were two orders of magnitude higher than the December values. The concentration of chlorophyll a, accordingly, differed by an order of magnitude. The Gulf of Finland was an emitter of carbon dioxide into the atmosphere, especially in April, while the Southeastern Baltic absorbed carbon dioxide in April and was a zone of weak emissions in November and December.

Investigating variability in phytoplankton primary productivity as a key component of the “biological pump” is critical to quantifying flux in the marine environment. We hypothesized that under certain hydrological conditions, changes in phytoplankton productivity are greater with changes in photosynthetic efficiency (the ratio of primary production (P P ) to the rate of electron transport in the phytoplankton photosystem, P P /ETR) than with changes in chlorophyll content. This study showed that increase of P P during sharp changes in hydrological parameters in the temporary frontal South-East Baltic (SEB) is achieved by increasing the efficiency of photosynthesis, i.e., the degree of use of light energy captured by chlorophyll a (Chl a). In the Gulf of Finland (GF), an increase in P P followed an increase in salinity from the Neva mouth to the sea and controls chlorophyll contents with low variability in photosynthetic efficiency. For SEB and GF, measurements of parameters of phytoplankton productivity and chlorophyll a content in late autumn (November) are carried out. The first stage of carbon flow (in biological pump), expressed in terms of primary production, was higher in the SEB than in the GF

The spatial distribution of the main primary productivity parameters in the Bransfield Strait in the Antarctic summer presents. The integrated primary production in the strait varied from 435 to 741 mgC/m2 per day. The share of primary production in the total production of phyto- and bacterioplankton in the upper 10th layer was 82–91%. Potential photosynthetic capacity (Fv/Fm) was high within the euphotic layer (0.418–0.749) throughout the area. The production parameters in the two main water masses in the strait did not differ. Photosynthetic efficiency (the ratio of the assimilation number and the relative electron transport rate, AN/rETR) varied in different subregions of the study area by almost six times.

In this work, potential (by the fluorescence method) and realized (by the radiocarbon method) phytoplankton primary production and the production of heterotrophic bacteria were determined in the euphotic zone of the seaward part of the Gdansk Bay of the Baltic Sea in summer and autumn. The maximum quantum efficiency of photosystem II of phytoplankton (Fv/Fm) varied significantly in depth in June (0.33–0.59), but was uniformly very high in October (0.68–0.71), which indicates a potentially more active state of early autumn phytoplankton. Integral primary production in October was almost two times higher than in June (633 and 375 mg C/m2 per day, respectively), while bacterial production was one-and-a-half times lower (239 and 371 mg C/m2 per day, respectively). In the upper tenth layer of the water column, autotrophic production (phytoplankton) only slightly exceeded heterotrophic (bacterial) production in June (63

—The occurrence and distribution of fin whales and humpback whales along the route of the R/V Akademik Mstislav Keldysh in the Atlantic sector of Antarctica in January and February 2022 were studied. The features of the whales’ distribution were considered depending on the depth of the water area, the abundance and age of krill, and the Chl-a concentration in the water. The highest occurrence of fin whales was found in the deep waters of the Powell Basin and the Orkney Trench, where the highest concentrations of adult krill were recorded. The highest occurrence of humpback whales was observed in shallow Bransfield Strait, where small and young krill predominated. The dependence of the occurrence of whales on the concentration of chlorophyll was not confirmed.

River runoff is an important source of nutrients as well as suspended and dissolved organic matter that in coastal zones and on the shelf are transformed due to local production cycles. River runoff affects the hydrological regime, salinity, temperature, and irradiance in river–seawater mixing zone. Our study focuses on the response of phytoplankton to the impact of small Caucasian rivers in the Northeastern (NE) Black Sea, as one of the most sensitive components of marine ecosystems with respect to the changes in abiotic factors. The leading role of marine species of diatoms, dinoflagellates, and coccolithophores in the structure and functioning when impacted by runoff from small rivers is demonstrated in comparison to the freshwater community. Variability of the taxonomic composition and quantitative and productive characteristics of marine phytoplankton communities impacted by small rivers were comparable to or exceed the seasonal and interannual variability on the NE Black Sea shelf. This indicates the significant role of runoff from small Caucasian rivers in maintaining of a high production level of phytoplankton overall and of the coccolithophore Emiliania huxleyi in particular in the coastal zone.

Cyclonic eddies in the ocean often increase primary production and phytoplankton biomass. The main mecha-nism is the rise of the nutricline in the cyclone's core, which determines the intensification of the ascending flow of deep nutrients into the photic layer. However, some cyclones suppress primary productivity, and the natural processes underlying them are little studied and are of great fundamental interest. In August 2021, a vast cyclonic zone with two mesoscale cyclones occupied the Black Sea deep-water basin. A powerful quasi-tropical atmospheric vortex amplified one of the cyclones, which increased the Ekman pumping and led to strong sea surface cooling by 4 degrees C. However, the inflow of thermocline water into the euphotic layer and higher nutricline gradients did not lead to elevated depth-integrated concentration of chlorophyll a (Chl) and phytoplankton biomass. Moreover, the surface Chl and the average primary production was less than in the outside waters. The proposed Stratification-Lock hypothesis explains this phenomenon. A combination of several factors led to its appearance. The drastic water cooling, followed by calm weather and vigorous summer heating, led to strong thermal stratification, dampening turbulent mixing in the near-surface layer. At the same time, the velocity and shear of current in and below the thermocline were low in the entire cyclonic zone, which prevented substantial diapycnal mixing generated at depth. As a result, a 'stratification lock' appeared in the entire water column, suppressing nutrients' upward flux. The proposed hypothesis is consistent with the effect of thermal stratification on primary productivity revealed in the entire cyclonic zone. The higher was the thermal stratification the lower was the depth-integrated total phytoplankton biomass and primary production. Also, diatoms developed in the upper layer in waters with weaker thermal stratification. The described stage lasted two weeks and was inter-rupted by strong wind mixing, which destroyed the 'stratification lock' and caused an increase in Chl at the sea surface. This stage of the cyclone life cycle occurs under a strictly defined combination of hydro-meteorological conditions, suggesting a relatively rare occurrence. Nevertheless, some evidence of the similar effect of a cyclone on primary productivity can be found in other studies, indicating that this natural mechanism, consisting of a complex weather-current-eddy combination, can operate in various marine areas.

Brief results of oceanological studies of the southeastern part of the Baltic Sea and the Gulf of Finland during 54th cruise of the R/V Akademik Sergei Vavilov (2022) are presented. Hydrology, hydro- and geochemistry, hydrobiology, geoecology, geology and geophysics investigations were carried out. New data on the state and dynamics of the natural complexes of the Baltic Sea under the conditions of increasing anthropogenic pressure and climate change were obtained.