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.

Among the Arctic seas, the largest volume of river runoff (~45% of the total river-water inflow into the Arctic Ocean) enters the Siberian Kara Sea.The viral communities of the Kara Sea are important for the functioning of the marine ecosystem.Studies of virus-prokaryote interactions on the Kara Sea shelf have been conducted only in spring and autumn.Here, we investigated the abundance of free viruses, viruses attached to prokaryotes, and pico-sized detrital particles; the morphology (shape and size) of the viruses, viral infection and virus-mediated mortality of prokaryotes in early summer, i.e., during a seasonal ice melting period and maximum inflow of river-water volumes with high concentrations of dissolved and suspended organic carbon.Seawater samples for microbial analyses were collected across the Kara Sea shelf zone on board the Norilskiy Nickel as a research platform from June 29 to July 15, 2018.Abundances of prokaryotes (range (0.6-25.3) × 10 5 cells mL -1 ) and free viruses (range (10-117) × 10 5 viruses mL -1 ) were correlated (r = 0.63, p = 0.005) with an average virus: prokaryote ratio of 23.9±5.3.The abundance of free viruses and viral-mediated mortality of prokaryotes were significantly higher in early summer than in early spring and autumn.Free viruses with a capsid diameter of 16-304 nm were recorded in the examined water samples.Waters in the Kara Sea shelf contained high concentrations of suspended organic particles 0.25-4.0µm in size (range (0.6-25.3) × 10 5 particles mL -1).The proportions of free viruses, viruses attached to prokaryotes, and viruses attached to pico-sized detrital particles were 89.8±6.0%,2.2±0.6% and 8.0±1.3%, respectively, of the total virioplankton abundance (on average (61.5±6.2) × 10 5 viruses mL -1). Viruses smaller than 60 nm clearly dominated at all studied sites.The majority of free viruses were not tailed.We estimated that an average of 1.4% (range 0.4-3.5%) of the prokaryote community was visibly infected by viruses, suggesting that a significant proportion of prokaryotic secondary production,

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.

The Kara Sea receives ~ 1/3 of total freshwater discharge to the Arctic Ocean, mainly from the large Ob and Yenisei rivers. The Ob-Yenisei plume covers wide area in the central part of the Kara Sea during ice-free season (June–October) and accumulates ~ 1000 km 3 of freshwater volume. In late autumn, the Kara Sea becomes covered by ice, which hinders in situ measurements at this area. As a result, the fate of the Ob-Yenisei plume below sea ice during winter and spring remains unclear. In this study, we report multiple in situ measurements performed in the Kara Sea shortly before and during ice-covered season. We demonstrate that late autumn convection in the plume shortly before ice formation significantly reduces friction between the plume and the subjacent sea. The subsequent formation of solid sea ice coverage isolates the plume from wind forcing. These two factors precondition the Ob-Yenisei plume to form an intense buoyancy-driven coastal current below sea ice. As a result, the plume advects eastward to the Laptev Sea through the Vilkitsky Strait during several months in November-February. Eventually, by late winter this huge freshwater volume disappears from the Kara Sea and contributes to freshwater content of the Laptev Sea. The obtained result improves our understanding of freshwater balance of the Kara and Laptev seas, as well as provides an important insight into the large-scale freshwater transport in the Eurasian Arctic, which remain largely unknown during ice-covered season.

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.

As a result of research conducted in July 2018 in the southwestern part of the Kara Sea, it was revealed that during the period of the beginning of the seasonal ice shift, the pelagic phytocen was at the stage of ice/marginal bloom, which was realized within a single spatiotemporal field. Distinctive features of the organization of the community of pelagic microalgae were a characteristic set of dominants and a mosaic structure of the pelagial. The mosaic pattern of marine phytoplankton is determined by the interaction between physical environmental factors and the biological activity of microalgae populations. In this case, the pelagic phytocene was a network of short-lived basins limited by local frontal sections with the predominant development of individual species/species complexes against the background of the functioning of the basic cluster of early spring cryophilic diatoms.

Among the Arctic seas, the largest volume of river runoff (~45% of the total river-water inflow into the Arctic Ocean) enters the Siberian Kara Sea. The viral communities of the Kara Sea are important for the functioning of the marine ecosystem. Studies of virus–prokaryote interactions on the Kara Sea shelf have been conducted only in spring and autumn. Here, we investigated the abundance of free viruses, viruses attached to prokaryotes, and pico-sized detrital particles; the morphology (shape and size) of the viruses, viral infection and virus-mediated mortality of prokaryotes in early summer, i.e ., during a seasonal ice melting period and maximum inflow of river-water volumes with high concentrations of dissolved and suspended organic carbon. Seawater samples for microbial analyses were collected across the Kara Sea shelf zone on board the Norilskiy Nickel as a research platform from June 29 to July 15, 2018. Abundances of prokaryotes (range (0.6–25.3) × 10 5 cells mL −1 ) and free viruses (range (10–117) × 10 5 viruses mL −1 ) were correlated (r = 0.63, p = 0.005) with an average virus: prokaryote ratio of 23.9 ± 5.3. The abundance of free viruses and viral-mediated mortality of prokaryotes were significantly higher in early summer than in early spring and autumn. Free viruses with a capsid diameter of 16–304 nm were recorded in the examined water samples. Waters in the Kara Sea shelf contained high concentrations of suspended organic particles 0.25–4.0 µm in size (range (0.6–25.3) × 10 5 particles mL −1 ). The proportions of free viruses, viruses attached to prokaryotes, and viruses attached to pico-sized detrital particles were 89.8 ± 6.0%, 2.2 ± 0.6% and 8.0 ± 1.3%, respectively, of the total virioplankton abundance (on average (61.5 ± 6.2) × 10 5 viruses mL −1 ). Viruses smaller than 60 nm clearly dominated at all studied sites. The majority of free viruses were not tailed. We estimated that an average of 1.4% (range 0.4–3.5%) of the prokaryote community was visibly infected by viruses, suggesting that a significant proportion of prokaryotic secondary production, 11.4% on average (range 4.0–34.0%), was lost due to viral lysis. There was a negative correlation between the abundance of pico-sized detrital particles and the frequency of visibly infected prokaryotic cells: r = −0.67, p = 0.0008.

Knowledge of the features of the structure and productivity of the Arctic communities of marine planktonic algae is necessary to identify possible changes in the pelagic ecosystem functioning under the changing climate condition of the Kara Sea. This study shows that the species diversity, abundance of phytoplankton, and production activity of algae are at a maximum at the beginning of summer during a seasonal ice melting period. The studies were carried out in the southwestern Kara Sea and in the estuarine zone of the Ob and Yenisei rivers from 29 June to 15 July 2018. The concentrations of nutrients and dissolved organic carbon were determined. The optical properties of chromophoric dissolved organic matter, species composition, abundance and biomass of all size groups of phototrophic and heterotrophic phytoplankton, and parameters of primary production and potential photosynthetic capacity were considered. Statistical data analysis showed that the leading factors influencing changes in the abundance of phytoplankton and its productivity are the content of silicates and salinity. At the same time, the production potential of algae is realized as short-lived and small phytoplankton assemblages differed in number taxa and diversity, with an equally rapid decrease in photosynthetic activity. Such changes affect the Marine Zone to a greater extent and the Estuarine Zone to a lesser extent.

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.

Phaeocystis pouchetii (Hariot) Lagerheim, 1893 regularly dominates phytoplankton blooms in higher latitudes spanning from the English Channel to the Arctic. Through zooplankton grazing and microbial activity, it is considered to be a key resource for the entire marine food web, but the actual relevance of biomass transfer to higher trophic levels is still under discussion. Cell physiology and algal nutritional state are suggested to be major factors controlling the observed variability in zooplankton grazing. However, no data have so far yielded insights into the metabolic state of Phaeocystis populations that would allow testing this hypothesis. Therefore, endometabolic markers of different growth phases were determined in laboratory batch cultures using comparative metabolomics and quantified in different phytoplankton blooms in the field. Metabolites, produced during exponential, early and late stationary growth of P. pouchetii , were profiled using gas chromatography-mass spectrometry. Then, metabolites were characterized that correlate with the growth phases using multivariate statistical analysis. Free amino acids characterized the exponential growth, whereas the early stationary phase was correlated with sugar alcohols, mono- and disaccharides. In the late stationary phase, free fatty acids, sterols and terpenes increased. These marker metabolites were then traced in Phaeocystis blooms during a cruise in the Barents Sea and North Norwegian fjords. About 50 endometabolites of P. pouchetii were detected in natural phytoplankton communities. Mannitol, scyllo -inositol, 24-methylcholesta-5,22-dien-3β-ol, and several free fatty acids were characteristic for Phaeocystis -dominated blooms but showed variability between them. Distinct metabolic profiles were detected in the nutrient-depleted community in the inner Porsangerfjord (< 0.5 µM NO 3 − , < 0.1 µM PO 4 3− ), with high relative amounts of free mono- and disaccharides indicative for a limited culture. This study thereby shows how the variable physiology of phytoplankton can alter the metabolic landscape of entire plankton communities.

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.

This data article contains data on the photosynthetic activity, shares of the main algae groups in the total phytoplankton biomass and the main environmental forces in the Kara Sea. The data were collected from 17 to 29 of September, 2011 in the following different areas: the Kara Sea Shelf with and without river runoff influence, the shelf edge and continental slope. The photosynthetic activity parameters include the relative electron transport rate (rETR) values, the maximum quantum efficiency of PSII (Fv/Fm) and the PB/rETR ratio as photosynthetic performance. The main environmental forces include the average day incident light, salinity and water temperature in the upper mixed layer and dissolved nutrients. The data presented in this article are associated with the research article entitled “Assessment of phytoplankton photosynthetic efficiency based on measurement of fluorescence parameters and radiocarbon uptake in the Kara Sea” (Mosharov et al., 2019). The related research article examines the relationships between biological parameters and with environmental characteristics such as temperature, salinity, incident photosynthetically active radiation (PAR) and nutrient concentration.

We studied vertical distribution of bacteria and viruses in different layers of the Arctic sea ice drilled at the North Pole. The sampled multi-year ice was characterized by uneven vertical distribution of bacterial abundance. This characteristic varied within the range of 8±1.2 ×103 to 95±2.6 ×103 cells ml-1. The average bacterial abundance was 28±2.9×103 cells ml-1. The layers with the maximal bacterial abundance were located in the intermediate and lower layers of the ice cores. The morphological composition of bacterial population of the multi-year ice was dominated by the cocci cells (63-92%). Bacterial population of the multi-year ice consisted of rather large cells with the mean cell volume of 0.14 μm3. Bacterial biomass varied from 0.5 to 5 mg C m-3 with the mean value 1.57±0.2 mg C m-3. The vertical distribution of bacterial biomass generally followed the pattern of bacterial abundance distribution. The maximal viral abundance was also located in the upper, intermediate and lower layers of the ice. The ratio of viral to bacterial abundance varied from 0.6 to 28, with the mean value 12.5. The average total number of phages attached to bacteria was 6.2 ×103 viral particles ml-1. The fraction of viruses attached to bacteria varied from 0.5 to 20.7%, with the mean value 7.1% of the total viral abundance. The number of bacterial cells with viral particles attached to them varied from 0.7 to 32×103 cells ml-1, with the mean value 8×103 cells ml-1. The number of viral particles located within bacterial cells varied from 2 to 21 particles per a bacterial cell. The minimum and maximum numbers of viruses located on bacterial cells varied from 1 to 6 viral particles per a bacterial cell. The mean viral capsid size varied from 42 to 83 nm. The frequency of visibly infected bacterial cells (FVIC) calculated for the upper, intermediate and lower layers of the ice was 0.92, 1.23 and 0.8% of the total viral abundance, respectively. The overall frequency of infected cells (FIC) calculated for the same layers was 6.3, 8.4 and 0.8% of the viral numbers, respectively, while the viral-mediated mortality of bacteria (VMB) was 7.1, 9.8 and 6.1 %, respectively. The average number of viral particles located within bacterial cells was 7.3 particles per a cell. Our data show that during the study period the rate of viral infection of bacterial cells and the viral-mediated mortality of bacterial cells in the multy-year ice of the North Pole were relatively low.

Проведен анализ вертикального распределения бактерий и вирусов в разных слоях арктического морского льда, отобранного на Северном полюсе. Керны двухлетнего льда характеризовались неравномерным вертикальным распределением бактериальной численности, варьирующей в пределах от 8±1.2×103 до 95±2.6×103 клеток/мл. Слои с максимальным обилием бактерий были расположены в средней и нижней частях кернов. Бактериальная биомасса варьировала от 0.5 до 5 мгС/м3. Соотношение численности вирусов и бактерий варьировало в пределах 0.6–28 при среднем значении 12.5. Среднее общее количество фагов, прикрепленных к бактериям, составляло 6.2×103 вирусных частиц/мл. Количество вирусных частиц, находящихся в бактериальных клетках, менялось от 2 до 21 частицы на одну бактериальную клетку. Частота видимых инфицированных бактериальных клеток, рассчитанная для верхних, средних и нижних слоев льда, составляла 0.92, 1.23 и 0.8% от общей численности бактерий соответственно. Частота инфицированных клеток бактерий, рассчитанная для тех же слоев, составляла соответственно 6.3, 8.4 и 0.8% от числа бактерий, тогда как вирусиндуцированная смертность бактерий составляла 7.1, 9.8 и 6.1% соответственно. Наши данные показывают, что в период исследований уровень вирусной инфекции и вирусиндуцированная смертность бактерий во льдах Северного полюса в целом были относительно низкими.

Spatial distribution of relative electron transport rate (rETR) values, the maximum quantum efficiency of PSII (F-v/F-m), and biomass-specific primary production (P-B, mg C (mg Chl a)(-1) h(-1)) were described for the eastern part of the Kara Sea in the autumn (September). A characteristic feature of this period was a noticeable decrease in the length of the day and the elevation angle of the sun, leading to a significant decrease in surface photo-synthetically active radiation (PAR). Despite low light in the euphotic zone, the phytoplankton maximum quantum efficiency of PSII (F-v/F-m) was high (0.5-0.7) in the upper 30-m layer of the water column, which indicates a potentially active state of phytoplankton. At the same time, the main productive activity of phyto-plankton was linked to the surface 0-3 m and was closely related to the daily incident PAR and the share of diatoms in the total phytoplankton biomass. Despite a decrease in light level and following a reduction in the values for productive characteristics of the phytoplankton community, diatoms continued to play a major role in primary productivity in the eastern Kara Sea at the end of the vegetative season. Comparative analysis of data obtained by two different techniques - fluorescence measurements and experimental carbon fixation estimations - demonstrated a close relationship between rETR and P-B. However, surface values of P-B changed more strongly than rETR, which may, apparently, reflect different efficiency in the use of absorbed light energy in the synthesis processes. Photosynthetic efficiency, reflecting the extent of use of the light energy caught in processes of organic matter synthesis, can be expressed through the ratio between P-B and rETR values. The P-B/rETR ratio increased in areas characterized by drastic gradients of hydrophysical conditions: over the external coastal shelf, central shelf and lower continental slope of the St Anna Trough.

The paper concerns the pelagic microbial community of the southern part of the St.Anna trough in the late vegetation period. in 2007 and 2011. The abundance, biomass and production of bacterioplankton, as well as the abundance and species composition of heterotrophic flagellates and ciliates were analyzed. The abundance and bacterial production values observed in the trough were significantly lower than the values typical of the surface brackish water layer above the Kara Sea shelf. The bacterioplankton abundance values of the upper warm layer in 2007 were several times lower than in 2011 (10-60*103 cells/ml and 24-147*103 cells/ml, respectively). In the deeper water layers, this difference diminished. The interannual differences in the values of bacterioplankton production were also most pronounced in the upper 60-meter layer: 0-0.17 and 0.05-0.87 mgC/m3 per day in 2007 and 2011, respectively. The distribution of the abundance of heterotrophic flagellates and ciliates is significantly influenced by the contour current along the slope of the trench. In the area of its influence in 2007, high values of the number of flagellates (154 ± 29 cells/ml) were observed throughout the water column, whereas in 2011 maximal values were confined to the upper 50-meter layer, exceeding almost twice the abundance at neighboring stations (810 ± 102 and 481 ± 61 cells/ml, respectively).The results of multifactor analysis showed that the number of heterotrophic flagellates, as well as the number of protozoan species decreased with depth. However in the euphotic layer there was a difference between two years of observations: in 2007, the abundance of consumers depended on the state of primary producers, whereas in 2011 the main factor affecting their abundance was the number of bacteria. Analysis of the species composition of heterotrophic flagellates suggests that the complex vertical hydrophysical structure of the water column does not affect substantially the microbial community of the region. However the influence of main currents of the area under consideration may be traced regarding the distribution of the abundance and species composition of heterotrophic microorganisms.