The first data on the structural and functional characteristics of planktonic communities of prokaryotes and viruses in the river and lakes of the October Revolution Island of the Severnaya Zemlya archipelago are presented. High values of the abundance, frequency of dividing cells, and specific growth rate indicated a high activity of prokaryotes during a short period of ice absence both in river and lake aquatic ecosystems. During this period (late July), the abundance of viruses exceeded that of prokaryotes in the river and lakes by 9 and 3 times, respectively. The mortality of prokaryotes as a result of viral lysis reached 34

The warming of the Arctic causes increased freshwater input from rivers into the Arctic seas. A high concentration of suspended particulate material entering coastal water via river runoff considerably affects the structure of the viral community and viral infection and mortality of bacteria. The abundance, structure, and activity of planktonic viruses were assessed across zones subjected to varying degrees to Khatanga River runoff. The number of free-living, bacteria-attached, and particle-attached viruses, determined by transmission electron microscope, ranged from 9.6 to 86.3 × 10 5 viruses mL − 1 , 0.3 to 4.9 × 10 5 viruses mL − 1 , and 0.6 to 18.1 × 10 5 viruses mL − 1 , respectively. The relative amount of free-living viruses in the total number of virioplankton varied from 40% in the mixed waters of Khatanga Bay to 94% in seawater of the continental slope. The abundance of free-living viruses was strongly negatively correlated with the abundance of suspended particles 0.3–210 µm in size. The frequency of visibly infected bacterial cells was strongly negatively correlated with the abundance of suspended particles with a size of 0.3–3.0 µm. The viral-mediated mortality of bacteria was 3.7–29.6% (on average 10.3 ± 0.8%) of bacterial production. The large amount of fine suspended particles contained in coastal waters of the western Laptev Sea appears to significantly reduce both the abundance of free-living viruses and ability of viruses to infect bacteria, due to the effective adsorption of viruses by these suspended particles.

The structure and productivity of planktonic microbial communities in the ecosystems of the Siberian Arctic seas are significantly dependent on freshwater input. During the study, we determined the spatial distribution of the abundance, biomass, and production of heterotrophic bacterioplankton in the Western Laptev Sea on the transect from the Khatanga River estuary to the continental slope and assessed the impact of river freshwater discharge. The influence of fresh water on bacterioplankton was restricted mainly to Khatanga Bay (KHAB) and the transitional zone (TZ) and was poorly recognized in the Western shelf (WS) and continental slope (CS) areas. The total bacterial abundance decreased from KHAB to the CS. Particle-attached bacteria constituted on average 63.0% of the total abundance of bacterioplankton in KHAB and 1.0% at the CS. Average bacterial production in the water column was highest in KHAB (10.3 mg C m−3 d−1), decreasing towards the CS (0.7 mg C m−3 d−1). In KHAB and TZ, bacteria were the main component of the planktonic community (44−55%). These results show that at the end of the growing season, bacterial processes prevailed over autotrophic ones and contributed largely to the total biological carbon flux in the coastal ecosystem of the Western Laptev Sea.

We have studied the seasonal dynamics of the structural and functional characteristics of heterotrophic bacterioplankton in the mouth area of the small Il’d River (inflowing into the Rybinsk Reservoir) and sections of bordering water bodies—the rivers themselves and reservoirs during the growing seasons of 2007 and 2009–2011. The abundance, biomass, and production of planktonic bacteria varied greatly, reaching maximum values in the mouth area of the inflow. Statistical processing of the results confirmed the dependence of the number of bacteria on organic matter in spring and early summer and on water temperature in summer. In terms of the abundance and production of bacterioplankton, the mouth area of the inflow is characterized by the manifestation of an edge effect, most pronounced in its frontal zone.

Based on the obtained and all available literature data, it has been shown that the abundance of heterotrophic flagellates in the Kara Sea at the end of the season is relatively stable in different years, while in other Arctic seas (East Siberian and Chukchi Seas) the described values of this indicator are 1.5–2 times higher. The indicators of the abundance of heterotrophic nanoflagellates in the bottom water vary to a much lesser extent than in the water column. The abundance of heterotrophic nanoflagellates in the bottom water of the bays of the Novaya Zemlya archipelago is significantly lower than in the same biotopes in the eastern and western parts of the Kara Sea. Most organisms are free-living forms. About a third of the protozoa are associated with detritus particles. The ratio of the biomass of heterotrophic flagellates to the biomass of bacteria more than doubles in the layer of bottom water and in the upper layer of sediments compared to the water column. For the water column of the Kara Sea, the calculated value of grazing of bacterioplankton by flagellates does not exceed 2% of their biomass. The ratio of the eaten biomass of bacterioplankton to the biomass of consumers in most cases decreases with depth.

The spatial and seasonal distribution of heterotrophic bacterioplankton has been studied in Lake Pleshcheyevo. Over the past 30 years, the abundance of microorganisms has increased threefold. The maximum values have been recorded at littoral stations. The main part of bacterial production is also produced there. Trophic levels have increased, but the ecosystem of the lake withstands the increased load.

A small amount of information about the composition and functioning of prokaryotes and viruses in the Kara, Laptev and East Siberian seas makes it impossible to provide a quantitative assessment of all parameters of the carbon cycle which is necessary to analyze and predict the impact of climate change on the entire Arctic biota. Of particular interest are the areas of the inner and outer shelf of the Arctic seas which are, on the one hand, a zone of mixing of river and sea waters of the Ob, Yenisei, Lena, Indigirka and Kolyma rivers with a large amount of the smallest detrital particles; on the other hand, it is a zone of highly saline marine areas, which determined the task of our study. The modern methods of studying the structural and functional characteristics of microorganisms are used in the work: epifluorescence microscopy and transmission electron microscopy; the growth rate and production of plankton heterotrophic prokaryotes were determined by dilution method in situ and the method involving application of antibiotics, modified for natural habitats. On the shelf of the Siberian Arctic seas, the abundance of free viruses, frequency of visibly infected prokaryotic cells and viral-mediated mortality of prokaryotes values varied within 0.1–13.6 × 106 viruses mL−1, 0.2–5.6% abundance of prokaryotes, 1.4–64.4% production of heterotrophic prokaryotes, respectively. We suggest that the input of a large amount of the smallest organic particles on which a significant amount of viruses is absorbed into the coastal waters of the Siberian seas (including riverine–marine water mixing zone) leads to a decrease in viral infection and mortality of prokaryotes.

In September 2017, studies were conducted in the East Siberian Sea along the transect from a Indigirka delta to the ice edge at the outer shelf margin. The abundances of planktonic prokaryotes (N-PR) and free viruses (N-V), frequency of visibly infected prokaryotic cells (FVIC), and virus-mediated mortality of prokaryotes (VMPR) varied within (0.5-3.4) x 10(6) (on average (1.67 +/- 0.69) x 10(6)) cells/mL; (2.3-10.3) x 10(6) (on average (5.28 +/- 1.91) x 10(6)) viruses/mL; 0.6-2.5 (on average 1.1 +/- 0.4) % of N-PR; and 4.5-22.3 (on average 9.0 +/- 3.8) % of the total prokaryotic production, respectively. The proportions of viruses attached to prokaryotic cells (N-VPR) and suspended particles (N-VP) were 1.9-26.3 (on average 8.0 +/- 5.0) % of N-V and 0.1-65.9 (on average 8.0 +/- 13.3) % of N-V, respectively. High concentrations of detrital and mineral particles, to which a significant number of viruses were attached and, as a result, loss of their activity were recorded in the river-sea water mixing zone. In such a situation, the number of virus attacks on prokaryotes and cases of their infection decreased. There was a negative relationship between the concentration of suspended particles 0.5-5.0 mu m in size and the abundance of infected prokaryotic cells. Thus, we conclude that viruses played a substantial role in controlling the abundance and production of heterotrophic prokaryotic plankton in the low-productive East Siberian Sea at the beginning of autumn. Regional index terms: Russian Federation; East Siberian Sea. Geographic bounding coordinates: 70-78. N; 150-165. E.

The distribution of heterotrophic bacteria, viruses, and heterotrophic nanoflagellates was studied in the shelf waters of the East Siberian Sea along the meridional transect from the Kolyma River delta in September. The production of heterotrophic bacteria and their mortality as a result of protozoan grazing and lysis of viruses were also determined. The patterns of the spatial distribution of microorganisms and viruses varied: the number of bacterioplankton decreased with increasing distance from the Kolyma delta, while, on the contrary, its size and biomass increased to the shelf areas with negative water temperatures. The highest abundance and biomass of virioplankton were recorded at the extreme southern and northern stations of the transect and the abundance and biomass of heterotrophic nanoflagellates increased from the south to the north. The studied shelf waters were characterized by a high concentration of fine-textured detrital particles, and, as a consequence, a relatively large number of viruses attached to detritus. The mortality of heterotrophic bacterioplankton accounted for most of its daily production. Protozoan grazing was the main cause of bacterial mortality that significantly exceeded their mortality due to viral lysis.

The distribution of viruses and their impact on prokaryotes were studied using the epifluorescence and transmission electron microscopy along a transect, located at 130 degrees E from the area adjacent to the Lena River delta next to the shelf area, the continental slope area and the deep-sea regions of the Laptev Sea in September 2015. The abundance of planktonic prokaryotes (N-P) and free viral particles (N-V), the frequency of visibly virus-infected prokaryotic cells (FVIC), and the viral-mediated prokaryotic mortality (VMP) varied within the following ranges: (0.1-3.1) x 10(6) cells mL(-1), (0.1-4.4) x 10(6) viruses mL(-1), 0.3-1.7% of N-P, and 2.1-12.2% of the total mortality of prokaryotes, respectively, and reached maximum values at the inner shelf area. The percentage of viruses attached to prokaryotic cells varied from 2 to 6% of N-V in the surface waters of the outer shelf and the deep-sea areas to 43-46% at the depth of 100-140 m in the continental slope area. The percentage of viruses that were attached to detrital or mineral microparticles varied from less than 1% in the waters of the deep-sea area to 27-50% in the waters of the inner shelf area adjacent to the Lena River delta. A negative correlation was found between the fraction of lysogenic prokaryotes and the FVIC values (r = - 0.60, p < 0.05). In the 20-cm bottom water layer above the sediment the N-P, N-V, FVIC and VMP values were (0.8-3.0) x 10(6) cells mL(-1), (1.6-3.4) x 10(6) viruses mL(-1), 0.5-1.2%, and 3.7-9.5%, respectively. In the surface 2-cm layer of the sediment these parameters were (1.8-4.4) x 10(9) cells cm(-3), (0.8-2.3) x 10(9) viruses cm(-3), 0.2-1.0%, and 1.4-7.8%, respectively. In the bottom water layer the percentage of viral particles attached to prokaryotes and to detrital and mineral particles was 10-29% and 20-79% of N-V, respectively. In the surface sediments the percentage of viral particles attached to prokaryotes was considerably higher compared to the bottom water layer and ranged from 44 to 57% of N-V. The capsid size of the viral particles from the pelagic zone, bottom water layer and surface sediments was on average 71 +/- 14, 74 +/- 7 and 63 +/- 7 nm, respectively. In all these habitats, viruses from the size class of 60-100 nm prevailed. The relatively high abundance of prokaryotes and viruses, as well as the high FVIC values in the freshwater-seawater mixing zone (the highest values were recorded in the waters with a salinity of 9.6 psu) were apparently caused by the influx of considerable amounts of nutrients, and dissolved and particulate organic matter that come with the river runoff. At the same time, the high concentration of small detrital and mineral particles in the freshwaters lead to considerable adsorption of viruses by these particles and caused a decrease in the FVIC in the coastal areas adjacent to the Lena River delta.

— For the first time, the species composition of heterotrophic nanoflagellates, their quantitative characteristics, and abundance and biomass of their main food objects—bacteria and picophytoplankton, were determined in the water column and sediments of the Laptev Sea. In early autumn, an increase in the number and biomass of planktonic flagellates was recorded on a meridional transect from the Lena River delta to the deep-sea zone in the area of the internal shelf impacted by freshwater input from the Lena River. The pattern of the vertical distribution of planktonic flagellates differed in different parts of the sea, but everywhere their largest number was confined to either the surface or near-bottom layers. The quantitative parameters of planktonic flagellates correlated positively with the temperature, bacterial abundance, and biomass, and negatively correlated with water salinity. The abundance and biomass of heterotrophic flagellates in surface sediments was three to four orders of magnitude higher than in the water column and exceeded those of planktonic flagellates at shallow sites under 1 m 2 . A total of 28 species and forms of heterotrophic flagellates have been identified. Representatives of two taxonomic groups—Kinetoplastea and Stramenopiles, the most tolerant to the salinity—dominated in the community. The species richness of the flagellates decreased with water depth, it was minimum in less saline coastal waters, and it increased in the deep-sea area.

В водной толще и донных отложениях моря Лаптевых впервые изучен видовой состав гетеротрофных нанофлагеллят, определены их количественные характеристики, а также численность и биомасса основных пищевых объектов — бактерий и пиководорослей. На меридиональном разрезе от приустьевого участка р. Лены до глубоководной зоны моря в начале осени увеличение численности и биомассы планктонных гетеротрофных жгутиконосцев отмечено в прибрежных районах, испытывающих влияние речного стока. Количественные показатели планктонных гетеротрофных нанофлагеллят положительно коррелировали с температурой воды, численностью и биомассой бактерий и отрицательно — с соленостью. В поверхностном слое донных отложений численность и биомасса гетеротрофных нанофлагеллят были на 3–4 порядка выше, чем в водной толще, и на шельфе в пересчете на 1 м 2 превышали таковые в воде. Всего идентифицировано 28 видов и форм гетеротрофных жгутиконосцев. В их сообществе преобладали представители двух таксономических групп — Kinetoplastea и Stramenopiles. Видовое богатство гетеротрофных жгутиконосцев возрастало в направлении от прибрежных районов к морским и снижалось с глубиной.

Here we present our research results on the virioplankton of the shelf zone of the Kara Sea. This is the first study of viruses and bacteria done in the Kara Sea in early spring when viruses can play an important role in the abundance and production control of the bacterioplankton. All other studies cover July September, that is, when the Kara Sea is completely ice-free and the community of viruses and bacteria is in an absolutely different stage of development. The samples were collected and processed on board of the vessel "Norilskiy Nickel" (project Aker ACS 650) between March 29 and April 8, 2016 on the regular cruise Murmansk - Dudinka. The abundance of bacteria (N-B) and viruses (N-V) located both underneath the ice and in the open waters of the Kara Sea and the Yenisei River estuary varied within a range of (0.075-0.429) x 10(6) cells x ml(-1) and (0.30-2.11) x 10(6) viral particles x I, respectively, and the N-V/N-B ratio varied from 2.6 to 15.4. A positive correlation was found between N-B and N-V = 0.70, p 0.05). Viruses with capsid diameter of 16-297 nm were registered in the water samples examined. The transmission electron microscopy data show that the fraction of the virus-infected bacterial cells is about 0.2-2.3% of total N-B. The mean viral-induced mortality of the bacteria calculated from all the sampling stations in the Kara Sea and the Yenisei River estuary was 7.8 +/- 1.2% (with a range from 1.4 to 20.2%) of the daily bacterioplankton production. Our results show that viruses play an important role in the abundance and production of bacterioplankton control in early spring.

The experiments were performed in microcosms situated on a control site and on a site regulated by beavers. Two cladoceran species of different sizes were placed in microcosms in different proportions. It was found that the vital activity products of Castor fiber L. promote an increase in the concentration of total phosphorus (P), a decrease in the N/P ratio in water, and an increase in the chlorophyll a concentration and the abundance and biomass of bacterioplankton. In such conditions, the abundance and biomass of the large species Daphnia (Ctenodaphnia) magna Straus was increased to the greatest extent when compared with background values when it was placed in combination with Ceriodaphnia dubia Richard or without it. Moreover, in the case of the combination of two crustacean species in microcosms, the abundance of C. dubia was lower when compared with the control site, while the abundance of D. magna was higher. The results of a bioassay showed that the productivity of C. dubia decreases in waters where D. magna is most abundant. We concluded that the formation of zooplankton in beaver ponds with specific features determined by the presence of large cladoceran species is provided by changes in the quantitative and qualitative parameters of food resources. This contributes to the massive distribution of large D. magna and its competitive relations with small C. dubia. According to the results of a bioassay, the vital activity products of D. magna can inhibit the fecundity of C. dubia.

The distribution of structural and functional characteristics of virioplankton in the north of the Ob River estuary and the adjacent Kara Sea shelf (between latitudes 71°44′44″ N and 73°45′24″ N) was studied with consideration of the spatial variations in the number (N B) and productivity (P B) of bacteria and water properties (temperature, salinity, density) by analyzing samples taken in September 2013. The number of plankton viruses (N V), the occurrence of visible infected bacteria cells, virus-induced mortality of bacteria, and virioplankton production in the studied region varied within (214−2917) × 103 particles/mL, 0.3−5.6% of NB, 2.2−64.4% of P B, and (6−17248) × 103 particles/(mL day), respectively. These parameters were the highest in water layers with a temperature of +7.3–7.5°C, salinity of 3.75−5.41 psu, and conventional density (στ) of 2.846−4.144. The number of bacterioplankton was (614−822) × 103 cells/mL, and the N V/N B ratio was 1.1−4.5. A large amount of virus particles were attached to bacterial cells and suspended matter. The data testify to the considerable role of viruses in controlling the number and production of heterotrophic bacterioplankton in the interaction zone of river and sea waters.

The structural and functional properties of the bacterioplankton in two small rivers and the Sheksna River that run through the large industrial city of Cherepovets were studied from April to October 2009–2011. The three-year average numbers and biomass of planktonic bacteria in small rivers were 18.7–18.8 million cells/mL and 3.5–4.9 g/m3, respectively; these values in the Sheksna River’s littoral zone were 15.8 million cells/mL and 2.2 g/m3. The bacterioplankton production in the small water courses exceeded that in the Sheksna River by a factor of 1.8–2.2. The contribution of bacterial filaments to the total biomass and the total bacterioplankton production increases in small polluted rivers.

Experiments in microcosms have demonstrated that beaver vital activity products (BVAPs) promote an increase in concentrations of total nitrogen (N) and total phosphorus (P), a decrease in the N/P value in water, and an increase in the abundance and biomass of bacterioplankton. Under such conditions, the abundance and biomass of small Ceriodaphnia dubia Richard and large Daphnia (Ctenodaphnia) magna Straus, which live separately, increase. The coexistence of these cladocerans in microcosms under the BVAP influences results in a high increase in the abundance and biomass of D. magna; in similar experiments without the influence of BVAP, Ceriodaphnia dubia becomes more abundant. The results of bioassay demonstrate that the number of newborns of Ceriodaphnia dubia decreases in water where Daphnia magna is numerous owing to BVAPs. It is suggested that the vital activity products of large representatives of the genus Daphnia inhibit the fecundity of small species of Cladocera. This fact, along with the high competitiveness of large cladoceran species under conditions of a high level of nutritive base, determine the formation of zooplankton communities in beaver ponds which are characterized by a high abundance and biomass and low uniformity

The paper considers the concentrations and functional characteristics of viruses, bacteria, and heterotrophic nanoflagellates determined for the first time in the Laptev Sea in August-September, 2014. The abundance of bacteria, viruses, and heterotrophic nanoflagellates varied from 110.1 × 103 to 828.4 × 103 cells/mL, from 384.2 × 103 to 2932.8 × 103 particles/mL, and from 108 to 651 cells/mL, respectively. The daily bacterioplankton production varied from 4.2 × 103 to 381.7 × 103 cells/mL, with an average of 117.6 × 103 cells/mL. Electron transmission microscopy has for the first time shown that the frequency of visibly infected bacterial cells varied from 0.2 to 2.0% (0.8% on average) of NB. The average virus-induced mortality of bacteria was 6.3% of bacterioplankton production, with variations ranging from 1.4 to 16.9%. Grazing on bacteria by heterotrophic nanoflagellates contributed more to bacteria mortality than virus-induced bacterial lysis. By grazing on bacteria, heterotrophic nanoflagellates consumed large quantities of viruses located on the surface and inside bacterial cells.