We studied the spatial patterns of the planktonic ecosystems at two Arctic sites strongly affected by Atlantic Inflow (FS, the Fram Strait; and BS, the Barents Sea). A high degree of similarity in the bacterial abundance (mean: 3.1 × 10⁵ cells mL⁻¹ in FS vs. 3.5 × 10⁵ cells mL⁻¹ in BS) was found, while other plankton characteristics were different. Bacterial biomass reached a maximum in BS (3.2–7.9 mg C m⁻³), while viral abundances tended to be higher in FS (2.0–5.7 × 10⁶ particles mL⁻¹). Larger bacterial cells were found in BS, suggesting the presence of different bacterial populations at both locations. The virus-to-bacteria ratio was significantly higher in FS than in BS (13.5 vs. 4.7). Chlorophyll a concentration was extremely low (<0.25 mg m⁻³). The highest zooplankton abundance was in the surface layer (919 individuals m⁻³ in FS vs. 602 ind. m⁻³ in BS). Zooplankton biomass strongly varied (1–39 mg C m⁻³), with the maximum in BS. High proportions of boreal taxa in the total zooplankton abundance indicate the Atlantification of pelagic ecosystems in the Arctic. Plankton indicators are correlated with temperature, salinity, and sampling depth. Strong intercorrelations were found between major plankton groups, suggesting tight links in the studied plankton ecosystems.

In the marine environment, bacteria and viruses play a significant role in carbon fluxes, remineralization processes, and the infection of various organisms. We performed a survey in the northeastern Barents Sea, a region adjacent to the Arctic Ocean, to investigate spatial patterns of microbial plankton, after the main productive period, in October 2020. Two main water masses occurred in the study region—colder Arctic Water and warmer Barents Sea Water, representing transformed Atlantic Water. Multivariate analyses detected patchiness in the horizontal distribution of bacteria and viruses, and their abundances showed no clear association with the water masses. There was an obvious vertical pattern in microbial concentration, with the highest estimates in the upper layers. Surface viral and bacterial abundance varied in a wide range (2.20 × 10⁵–10.7 × 10⁵ cells·mL⁻¹ and 0.86 × 10⁶–14.98 × 10⁶ particles·mL⁻¹, respectively) and were correlated with each other. Bacterioplankton was dominated by small-sized cells (<2 μm, 0.04–0.06 µm³), and the average volume of bacterial cells tended to increase toward the seafloor. The ratio of viral to bacterial abundance (VBR) was 11 ± 1 and did not differ between water masses and depth layers. VBR were higher, compared to summer values, suggesting a strong impact of viruses on bacterioplankton, after the main productive season. Redundancy and correlation analyses showed that inorganic nutrients (nitrate and phosphate) and organic carbon from zooplankton were most responsible for the total variability in the microbial parameters. Water temperature and salinity, also, had a measurable impact, but their influence was lower. Bacterial abundance was lower than in other seasons and regions of the Barents Sea, while viral abundance was comparable, suggesting a stronger viral impact on Arctic marine bacteria in the autumn season.

The state of bacterioplankton was investigated in the northeastern Barents Sea in the autumn period for the first time. The warm transformed Atlantic water and cold Arctic water, as well as the intermediate and bottom waters were delineated in the study area. Small amounts of nutrients and oxygen were recorded in the upper water layer, while the maximum values of the nutrient concentrations were in the bottom layer. The bacterial abundance varied from 220 to 1068 thousands cells/ml, biomass varied from 2.7 to 12.5 mg С/m³, and the mean volume of bacterial cells varied from 0.020 to 0.136 μm³. The horizontal distribution of bacterioplankton demonstrated patchiness, while there was a clear vertical pattern with a decrease of the total bacterial abundance and an increase of the mean bacterial size with depth. Small single cells dominated in the bacterioplankton. The total bacterial abundance and biomass of the bacterioplankton, as well as the number of some bacterial size groups, negatively correlated with the concentration of nutrients (nitrates, phosphates, and silicates) and salinity. The abundance and biomass of the bacteria showed a weak tendency to increase with increasing water temperature and surface chlorophyll a content. A comparison of the obtained data with other Arctic regions and other seasons has been made. The factors determining the distribution patterns of marine bacteria in the Arctic in the autumn are discussed.

New data have been obtained on the distribution of prokaryotic plankton and pelagic viruses in the Fram Strait (the Greenland Sea) during the polar night (late November). Three main types of water masses were present in the study area: surface polar, Atlantic, and transformed Atlantic. The content of mineral biogenic elements increased with depth. The concentration of chlorophyll a had low values (0.07–0.13 mg/m³). The abundance and biomass of prokaryotes varied from 286 000 to 675 000 cells/mL and from 2.7 to 11.7 mg C/m³, respectively. The composition of prokaryotic plankton was dominated by single small cells (more than 97% of the abundance and more than 68% of the biomass). The average cell volume was 0.034–0.096 μm³. The number of viruses varied from 724 000 to 3 920 000 particles/mL, and the biomass varied from 0.040 to 0.216 μg C/m³. Against the background of local maxima in the abundance of prokaryotic plankton in the 0–25 m layer, the highest concentrations of viruses were noted in the surface layer. In the latitudinal distribution (from south to north), an increase in the number of viruses and prokaryotes was revealed. A close relationship between their abundance and biomass with hydrological parameters and phosphate content was established; the role of certain water masses in the vertical distribution of microbes was insignificant. The relatively high abundance of viruses and prokaryotes indicated their significant activity during the polar night.

In the course of conducted studies the influence of climatic fluctuations on the structural and production characteristics of the pelagic zone was determined using both long-term data series on the chl-а content in situ and direct taxonomic study of the pelagic phytocenosis in local areas of the Barents Sea during spring activation. The timing of the onset, dynamics of development and structural organization of spring phytoplankton bloom were determined. A technique was developed for using δ18O isotope ratios to assess individual stoichiometric ratios and primary production of water areas. New information was obtained on the spring pelagic complex of zooplankton in the areas of the Arctic archipelago of Novaya Zemlya. The first unique studies of bacterio- and virioplankton were conducted in previously unstudied coastal areas of the Novaya Zemlya archipelago. Seasonal hydrochemical parameters of the water masses of the Kola Bay are given.

The spatial variability of quantitative indicators of prokaryotes and virus-like particles in the waters of three Arctic fjords was studied in connection with the structure of their dominant water masses in the summer. A comparison of the abundance and biomass of prokaryotes showed that their average value in mesotrophic surface waters (0.60 million cells/ml and 28.64 mg/m3, respectively) exceeded the values in deeper oligotrophic waters – intermediate (0.33 million cells/ml and 16.82 mg/m3) and winter (0.32 million cells/ml and 19.72 mg/m3). The abundance of virus-like particles in different trophic conditions positively correlated with the parameters of prokaryotic communities and in the isolated water masses averaged 1.01 million, 0.09 million and 1.31 million per ml respectively. The factors determining the distribution of the most abundant representatives of the microbial food web and the nature of their interaction are considered.