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 numbers, biomass, and production of bacterioplankton were determined in the Russian Sector of the Gdansk Basin (Baltic Sea) in 2007–2009. Significant spatial and temporal variations were determined. During the year, bacterial activity increased with increasing water temperature and higher availability of organic substrates. The lowest bacterial production (0.01–31.63 mg C m⁻³ day⁻¹) was observed in late winter and late autumn, while the highest (0.17–341.70 mg C m⁻³ day⁻¹) occurred in spring and summer. Since bacterial numbers and biomass were found to depend on the weather conditions and the terrigenous inflow, significant variations were observed from year to year. The highest and lowest numbers and biomass of bacterioplankton determined in summer were 0.09–1.10 × 10⁶ cells mL⁻¹ and 2–22 mg C m⁻³ for July 2007 and 1.96–11.23 × 10⁶ cells mL⁻¹ and 23–123 mg C m^–3 for July 2009. The values of these parameters were the highest along the coast and decreased towards the open sea.

The seasonally ice-covered marine region of the European Arctic has experienced warming and sea ice loss in the last two decades. During expeditions in August 2020 and 2021, new data on size-fractioned primary production (PP), chlorophyll a concentration, phytoplankton biomass and composition and carbon fixation rates in the dark were obtained in the marginal ice zone (MIZ) of the Barents Sea, Nansen Basin and Greenland Sea to better understand the response of Arctic ecosystems to ongoing climate changes. Four different situations were observed in the study region: (i) a bloom of the large-cell diatom Podosira glacialis, whose biomass was trapped in a strong halocline at the edge of a dense ice cover; (ii) a bloom of the chain-like colonies of Thalassiosira diatoms on the shelf in mixed waters in fields of shallow ice that could be supported by “fresh” elements in the polynya condition, as well as by terrestrial run-off and drifting ices; at the late stage, this bloom was accompanied by intensive growth of Phaeocystis pouchetti; (iii) dominance of small-cell phytoplankton under weakened stratification and the significant influence of the Atlantic water, depleted of microelements and silicates; (iv) dominance of dinoflagellates of eutrophic water in the contact zone between the water masses of Arctic origin and Atlantic origin in clear water under conditions of increased light intensity. The >10 µm phytoplankton cell size group increased its relative contribution to PP as a response to stratification, light and nutrient load associated with sea ice conditions. Small phytoplankton with sizes < 2 µm formed the basis of total PP in the MIZ regardless of the state of the sea ice.

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.

Many studies related to the influence of eddies on the primary production chain of marine ecosystems have been conducted; however, this effect tends to be regionally specific, especially in coastal dynamic waters. In the Baltic Sea, mesoscale and submesoscale eddies are a ubiquitous feature of water circulation during summer, when diazotrophic cyanobacteria blooms occur in surface waters due to the excess of phosphorus in seawater. Climatic change may increase the frequency and duration of these negative events in the marine ecosystem. We examined the taxonomic composition, abundance, and primary production of phytoplankton in the southeastern Baltic Sea during the occurrence of the packed eddy system at the end of the abnormally warm summer of 2018. Massive cyanobacteria growth was observed in the plume of the eutrophic Vistula Lagoon in the Gulf of Gdansk. The only species diazotrophic Dolichospermum flos-aquae ((Bornet & Flahault) P. Wacklin, L. Hoffmann & Komárek, 2009) vegetated along the western coast of the Sambia Peninsula. Its colonies reached the highest biomass nearby the dumping site of the Amber Mining Plant in Yantarny, Kaliningrad Region, Russia. The cyanobacteria colonies dispersed in the outgoing jet of a relatively warm eddy dipole. Chrysochromulina spp. (Lackey, 1939) was dominant in these nitrogen-rich waters. In contrast, cryptophyte species dominated in the relatively cold waters of the dipole anticyclone that resulted in a fourfold decline in primary production. The decrease in the number of mobile phytoplankton species was revealed within the “old” eddies near the northern coast of the Sambia Peninsula and the Curonian Spit. Meanwhile, species of the spring–autumn complex Coscinodiscus granii (Gough, 1905), Peridiniella catenate ((Levander) Balech, 1977) and other developed in the community. This implies that the appearance of eddies can cause phytoplankton succession in the coastal area. The mechanism of their influence was similar to the action of other physical factors perturbing a relatively stationary environment. Capture of cyanobacterial colonies by eddies led to an improvement of the ecological situation in the area, as cyanobacteria transported their biomass outside the coastal area. However, the opposite direction processes obviously were the deterioration of light conditions, increased water turbidity, and organic matter concentration.