Peculiarities of the primary production process in the Kara Sea at the end of the vegetation season

Research was implemented from September 15 through October 4, 2011 in the Kara Sea along transects located southeastwards Novaya Zemlya, in the St. Anna Trough, the Yenisei River estuary, and the adjacent shelf. The concentration of chlorophyll a was the highest in the photic zone (0.05–2.30 mg/m³, on average, 0.80 ± 0.37 mg/m³). The maximal concentration of Chl a at most of the stations located in the water layer of 7–30 m. Integral primary production in the water column varied from 3.0 to 151.0 mg C/m² per day, on average, 37.2 ± 36.6 mg C/m² per day. The maximal rate of primary production at most of the stations has been observed for the surface layer of the water column. Within the upper mixed water layer, relative primary production was from 31 to 100% (on average, 77 ± 20%). The most productive zone was the waters along Yenisei transect. In the estuary and at the adjacent shelf, primary production was 50 mg C/m² per day, exceeding the range observed for other areas by 1.5–2.0 times. The concentrations of silica and nitrogen together with light regime and water temperature were the major limiting factors affecting the primary production rate in the Kara Sea in autumn.     

Structure of the zooplankton communities in the region of the Ob River’s estuarine frontal zone

The studies were carried out on September 27–30, 2007, in the area of the Ob estuarine frontal zone and over the adjacent inner Kara Sea shelf. Based upon the latitudinal changes in the salinity, the 100 nautical mile wide estuarine frontal zone was marked out. The frontal zone was inhabited by a specific zooplankton community dominated by species that occurred outside the frontal zone in only minor amounts. The biomass of the mesozooplankton averaging 984 mg/m³ in the frontal zone exceeded by 1.5 and 6 times the corresponding values in the inner desalinated area of the estuary and the adjacent areas of the Kara Sea shelf. At the inner southern periphery of the frontal zone, at maximal latitudinal salinity gradients (>2 psu per mile), the maximal development of the mesoplankton with the mean biomass for the water column of 3.1 g/m³ (37 g/m²) and up to 5.8 g/m³ in the subpycnocline layer was observed. The latitudinal extension of the biomass in the maximum zone did not exceed 10 miles. More than 90% of the maximum was composed of herbivorous zooplankton with the strong domination of the copepod Limnocalanus macrurus. The daily consumption within the zooplankton maximum area was estimated at 820 mgC/m² per day. This value exceeds by two orders of magnitude the local primary production. At that level of consumption, the available phytoplankton biomass was consumed by grazers in less than 8 hours (!). A zooplankton aggregation at the southern periphery of the estuarine front exists due to the advection of phytoplankton from the adjacent river zone. The aggregation forms a natural pelagic biofilter where new allochthonous organic matter delivered by the river flow is accumulated and high secondary production is formed on its basis. An anomalously high concentration of planktic predatory Parasagitta elegans with biomass of over 1 g/m³ (46% of the total zooplankton biomass) was associated with the outer northern periphery of the estuarine frontal zone.     

Macrobenthos of the Ob River estuarine zone and of the adjacent regions of the Kara Sea

The composition and distribution of the macrobenthic communities in the Ob estuary and the adjacent Kara Sea shelf were studied during the 54th cruise of the R/V Akademik Mstislav Keldysh. With the transition from the Ob River’s mouth to the open parts of the sea, the gradual changes of the bottom biocenoses included changes in the leading taxa. Along with the increase in the salinity, the freshwater and brackish water taxa are replaced by related forms adapted to dwelling in seawater. The comparison of the original data with the results of the previous investigations revealed considerable spatial and temporal variations of the bottom communities in the studied area. The main environmental factors determining the fauna distribution in the estuarine zone are discussed. The extensive biocenosis dominated by Portlandia aestuariorum in the Ob estuary was found for the first time.     

Structure of the phytoplankton communities and primary production in the Ob River estuary and over the adjacent Kara Sea shelf

The material was collected in the Ob River estuary and over the adjacent shallow Kara Sea shelf between 71°14′0 and 75°33′0N at the end of September 2007. Latitudinal zoning in the phytoplankton distribution was demonstrated; this zoning was determined by the changes in the salinity and concentration of nutrients. Characteristic of the phytocenosis in the southern desalinated zone composed of freshwater species of diatom and green algae were the high population density (1.5 × 10⁶ cells/l), biomass (210 μgC/l), chlorophyll concentration (4.5 μg/l), and uniform distribution in the water column. High primary production (∼40 μgC/l/day) was recorded in the upper 1.5-m layer. The estuarine frontal zone located to the north contained a halocline at a depth of 3–5 m. Freshwater species with low population density (2.5 × 10⁵ cells/l), biomass (24 μgC/l), and chlorophyll concentration (1.5 μg/l) dominated above the halocline. Marine diatom algae, dinoflagellates, and autotrophic flagellates formed a considerable part of the phytocenosis below the halocline; the community characteristics were twofold lower as compared with the upper layer. The maximal values of the primary production (∼10 μgC/l per day) were recorded in the upper 1.5-m layer. The phytocenosis in the seaward zone was formed by marine alga species and was considerably poorer as compared with the frontal zone. The assimilation numbers at the end of the vegetation season in the overall studied area were low, amounting to 0.4–1.0 μgC/μgChl/h in the upper layer and 0.03–0.1 μgC/μgChl/h under the pycnocline.     

Chlorophyll,primary production,and organic carbon fluxes in the Kara Sea

New maps of the mean monthly distribution of chlorophyll and the primary production in the Kara Sea were compiled using joint processing of CZCS (1978–1986), SeaWiFS (1998–2005), and MODIS (2002–2006) satellite data and field measurements. The annual primary production of phytoplankton is estimated at 22.3 × 10⁶ t of C per year or 70 mg of C/m² per day. The results of the calculations of the organic carbon budget in the Kara Sea are presented.     

Microbial processes of the carbon and sulfur cycles in the Kara Sea

The results of microbiological, biogeochemical, and isotope geochemical studies in the Kara Sea are described. The samples for these studies were obtained during the 54th voyage of the research vessel Akademik Mstislav Keldysh in September 2007. The studied area covered the northern, central, and southwestern parts of the Kara Sea and the Gulf of Ob. The quantitative characteristics of the total bacterial population and the activity of the microbial processes in the water column and bottom sediments were obtained. The total population of the bacterioplankton (BP) varied from 250000 cells/ml in the northern water area to 3000000 cells/ml in the Gulf of Ob. The BP population depended on the content of the water suspension. The net BP production was minimal in the central water area, amounting to 0.15–0.2 μg C/(l day), and maximal (0.5–0.75 μg C/(l day)) in the Gulf of Ob. The organic material at the majority of the stations in the Ob transect predominantly contained light carbon isotopes (−28.0 to −30.18‰) of terrigenous origin. The methane content in the surface water layer varied from 0.18 to 2.0 μl CH₄/l, and the methane oxidation rate changed in the range of 0.1–100 nl CH₄/(l day). The methane concentration in the upper sediment layer varied from 30 to 300 μl CH₄/dm³; the rate of the methanogenesis was 44 to 500 nl CH₄/(dm³ day) and that of the methane oxidation, 30 to 2000 nl CH₄/(dm³ day). The rate of the sulfate reduction varied from 4 to 184 μg S/(dm³ day).     

Structure and Productivity of the Phytocenosis in the Southwestern Kara Sea in Early Spring

Results of plankton biota studies in the southwestern Kara are presented. The spatial distribution of hydrochemical and hydrophysical parameters related to structural and functional characteristics of phytoplankton in the surface water is considered. The chlorophyll a concentration varied in the surface layer of the Kara Sea from 0.08 to 3.22 mg m^–3 (mean value 0.62 mg m^–3). Primary production varied from 0 to 1.92 mg C m^–3 day^–1 (the mean value of 0.42 mg C m^–3 day^–1) in the ice-covered water areas and was greater by a factor of four, ranging from 1.01 to 3.46 mg C m^–3 day^–1 (the mean value of 1.79 mg C m^–3 day^–1) in ice-free areas. In this case, the total algal biomass varied from 0.8 to 110.7 mg C m^–3 (mean value 10.6 mg C m^–3). It is shown that in the study period, waters from the western Kara Sea were more productive than the estuarine water areas of the Ob and Yenisei rivers. The activity of phototrophic phytoplankton in river waters was almost completely absent. It is established that the contents of nutrients and iron were higher than the threshold for limitation of phytoplankton development. The experiments showed that the production activity of phototrophic algae is restrained by light deficit beneath the ice.     

Structure of the macrozoobenthic communities in the southwestern Kara Sea

The structure and distribution of the macrobenthic communities were studied in the southwestern Kara Sea. The material was collected in Baidaratskaya Bay in July 2007 and in a section running westward of the Yamal Peninsula in September 2007. The depths of the sampling stations ranged from 5 to 25 m in the Baidaratskaya Bay area and between 16 and 46 m in the Yamal section. A total of 212 benthic invertebrate species were recorded. In both areas, Bivalvia was the group with the highest biomass (54.88 g/m² in the Yamal section and 59.71 g/m² in the Baidaratskaya Bay area), while polychaetes were the group with the highest number of species (45 in the Yamal section and 64 the Baidaratskaya Bay area). Three major macrozoobenthic communities were recognized: the Astarte borealis community (20–46 m, the deepest sampling stations in both areas); the “medium-depth” community (10–20 m, extremely mosaic, usually dominated by Serripes groenlandicus); and the Nephtys longosetosa community (depth smaller than 10 m, characterized by low biomass and the absence of large bivalves and echinoderms). The western Yamal shallow-water communities were shown to be generally similar to those of Baidaratskaya Bay. The comparison of these results with those of the benthos censuses performed in 1927–1945, 1975, and 1993 showed that the benthic communities in the southwestern Kara Sea remained relatively stable during the second half of the 20th century and the early 21st century.     

Optical characteristics of the Kara Sea derived from shipborne and satellite data

Results of optical studies of the Kara Sea waters are considered. The data of ship borne measurements are compared with satellite observations. The maximum values of the beam attenuation coefficient, the chlorophyll, and the yellow substance fluorescence were observed in the Ob gulf and in the coastal waters west of the Yamal Peninsula. The minimum values were observed in the central and western parts of the Kara Sea. Frontal zones with sharp changes of the parameters involved were observed. The distribution of the seawater’s optical characteristics was closely related to the hydrological structure of the Kara Sea’s waters. The data of the shipborne measurements were consistent with the satellite observations.     

Input of organic carbon as determinant of nutrient fluxes,light climate and productivity in the Ob and Yenisey estuaries

Spectral light attenuation profiles and concentrations of total and dissolved carbon (C), nutrients and chlorophyll a (Chla) were studied along transects running from the river mouth to the Kara Sea during late summer 2003 for the Yenisey and fall 2005 for the Ob estuaries. Earth Observation data were used to generate composite images of water color and Chla distribution over the estuaries and the Kara Sea to reveal the spatial impact of the river efflux in terms of optical properties.     

Seasonal variation of the satellite-derived phytoplankton primary production in the Kara Sea

Seasonal variation of the integrated primary production (IPP) and surface chlorophyll (Chl₀) in different regions of the Kara Sea was studied from satellite data obtained by the MODIS-Aqua colour scanner and averaged for 2003–2015. The minimum variation of Chl₀ concentration during the growing season (from April to October) was 1.5 times in southwestern region and 2 times in the northern region of the sea. It was found that the Chl₀ concentration increased slightly in all regions by the end of the growing season. The maximum IPP value recorded in June coincided with the peak level of photosynthetically active radiation (PAR) and maximum river discharge. The IPP value varied in a wider range compared with the Chl₀ concentration. The ratio of the maximum and minimum monthly average IPP values varied from 8.9 times in Southwestern region to 11.7 times in the Northern region of the sea. The average increase in the Chl₀ concentration was 1.7 times (from 0.78 mg/m³ in April to 1.29 mg/m³ in October). The IPP value varied by a factor of 10.7 (from 26 mg C/m2 per day in October to 279 mg C/m² per day in June). The article also discusses the influence of water column stratification, the concentration of nutrients, the PAR level, and river discharge on the seasonal IPP dynamics in the Kara Sea.     

The upper desalinated layer in the Kara Sea

An area of about 40000 km² of desalinated upper layer waters with a salinity of less than 25 psu was found during cruise 54 of the R/V Akademik Mstislav Keldysh in the southwestern part of the Kara Sea (September 2007). Close to the boundary of this region located near the eastern coast of Novaya Zemlya, the salinity was less than 16 psu. The thickness of the desalinated layer was about 10 m. The results of the chemical analysis revealed that the observed desalination of the sea water was produced, first of all, by the Yenisei River, while the contribution of the Ob River’s waters was secondary. However, the most desalinated region near the eastern coast of Novaya Zemlya was separated from the Ob-Yenisei estuary and corresponded to a quasi-isolated lens. It is likely that the formation of this lens, as well as the major part of the desalinated upper layer waters, occurred in June when the flood of the Yenisei was maximal, while the further drift of the desalinated waters to the west of the Ob-Yenisei estuary was forced by the prevailing northern wind. The additional desalination (by 2–3 psu and even more) of the upper layer waters near the eastern coast of Novaya Zemlya might be related to the melting of the Novozemelskiy ice massif. The regularities of the temporal evolution of the upper desalinated layer, as well as the influence of this layer on the hydrological structure and dynamics of the southwestern Kara Sea, are discussed.     

Phytoplankton of the south-western part of the Kara Sea

The research was performed along a transect from the Yamal Peninsula coast towards the outer shelf of the southwestern part of the Kara Sea in September 2007. 130 phytoplankton species have been identified, among which 63 were found in the area for the first time. The total phytoplankton numbers varied within the range of 0.2 to 11.3 × 10⁹ cells/m², while biomass within the range of 43 to 1057 mgC/m². A well pronounced cross-shelf zoning in the phytoplankton communities was ascertained. The inner shelf zone about 30 km wide with depths down to 30 meters was characterized by the predominance of diatoms (up to 80% of the total algae numbers and biomass). The second group by value was dinoflagellates. Seaward in the area of the depth increase from 30 to 140 m, the zone of the Yamal Current was located, which was 40 km wide and notable for its active water dynamics. The total cell numbers in the zone reached a maximum for the entire investigated area: up to 11.3 × 10⁹ cells/m². The leading group in the phytoplankton was autotrophic flagellates, whose share in the total numbers reached 56–82%. Further than 70 km from the shore, the outer shelf zone was found with the water column rigidly stratified. The highest for the whole area phytoplankton biomass was identified here (up to 1.06 gC/m⁹), 80% of which was concentrated above the halocline. Diatoms dominated in the phytoplankton numbers (up to 92%) and biomass (up to 90%), which was related to the mass development of two species: Chaetoceros diadema and Leptocylindrus danicus.     

Phytoplankton production characteristics in the southern Atlantic and the Atlantic sector of the Southern Ocean in the austral summer of 2009–2010

In December–January of 2010 the spatial distribution of the phytoplankton production characteristics was studied along transects in the vicinity of the Greenwich meridian (I) and in the Drake Passage (II). On transect I, the surface chlorophyll a concentration and primary production varied from 0.11 up to 3.57 mg/m³ and from 4.38 up to 37.47 mgC/m³ per day, respectively. The chlorophyll a in the photosynthetic layer and the integrated primary production varied from 10.7 up to 66.1 mg/m² and from 83 to 646 mgC/m² per day, respectively. On transect II in the surface layer, the chlorophyll a concentration changed within the range of 0.09–1.02 mg/m³ and the primary production ranged from 2.08 to 9.49 mgC/m³. The integrated values ranged from 6.32 to 38.29 mg/m² and from 41 to 221 mgC/m² per day, respectively. The moderate means of themaximum quantum yield (F _v/F _m) on transects I and II (0.41 and 0.35, respectively) testify to the low activity of the phytoplankton’s photosynthetic apparatus. The studied water areas in the Southern Ocean differed both in the phytoplankton biomass expressed in the chlorophyll a concentration values and in the conditions of the primary production formation.     

Bacterial and primary production in the pelagic zone of the Kara Sea

Data on the bacterial and primary production, which were obtained simultaneously for the same water samples, are presented for three regions of the Kara Sea. The samples were collected for the transect westwards of the Yamal Peninsula, along the St. Anna Trough, and the transect in Ob Bay. Direct counts of the DAPI-stained bacterial cells were performed. The bacterial production and grazing rates were determined using a direct method when metabolic inhibitors vancomycin and penicillin were added. The primary production rates were estimated using the ¹⁴C method.     

Bloom of Euglena sp. (Euglenophyceae,Euglenales) in the Outer Part of the Ob River Estuary

Oceanology - The material was obtained on July 18–24, 2016 in the area comprising the Ob River estuary and the adjacent shelf of the Kara Sea (71°28.3′–74°38.8′...     

Circulation in the southwestern part of the Kara Sea in September 2007

During cruise 54 of the R/V Akademik Mstislav Keldysh to the southwestern Kara Sea (September 6 to October 7, 2007), a large amount of hydrophysical data with unique spatial resolution was obtained on the basis of measurements using different instruments. The analysis of the data gave us the possibility to study the dynamics and hydrological structure of the southwestern Kara Sea basin. The main elements of the general circulation are the following: the Yamal Current, the Eastern Novaya Zemlya Current, and the St. Anna Trough Current. All these currents are topographically controlled; they flow over the bottom slopes along the isobaths. The Yamal Current begins at the Kara Gates Strait and turns to the east as part of the cyclonic circulation. Then, it turns to the north and propagates along the Yamal coast over the 100-m isobath. The Eastern Novaya Zemlya Current (its core is located over the eastern slope of the Novaya Zemlya Trough) flows to the northeast. Near the northern edge of Novaya Zemlya, it encounters the St. Anna Trough Current, separates from the coast, and flows practically to the east merging with the continuation of the Yamal Current. A strong frontal zone is formed in the region where the two currents merge above the threshold that separates the St. Anna Trough from the Novaya Zemlya Trough and divides the warm and saline Arctic waters from the cooler and fresher waters of the southwestern part of the Kara Sea. This threshold, whose depth does not exceed 100–150 m, is a barrier that prevents the spreading of the Barents Sea and Arctic waters to the southwestern part of the Kara Sea basin through the St. Anna Trough.     

Phytocenoses of the Ob Estuary and Kara Sea Shelf in the Late Spring Season

Oceanology - Based on the material obtained in the spring–summer season of 2016, the composition and quantitative distribution of phytoplankton in the Ob estuary and over the Kara Sea shelf...     

The role of zooplankton in the transformation of the organic matter in the Ob estuary,on the shelf,and in the deep regions of the Kara Sea

The data for the present study were collected at 20 sampling stations in the Kara Sea along the transect from the Ob estuary to the deep sea St. Anna Trough in September 2007. Based on the hydrophysical features, the distribution of the Chl a, and the primary production, we distinguished six habitats: the river, estuary, inner and outer shelf, continental slope, and trough. The impact of the small-size (<0.5 mm) and large-size (>0.5 mm) fractions of the zooplankton on the phytoplankton’s organic carbon in the different regions of the Kara Sea was estimated. The ingestion rate was assessed using the analysis of the gut fluorescence content and the gut evacuation rate. The zooplankton grazed 1–2% of the phytoplankton biomass in the river and estuary; 3.5% over the shelf; and 6 and 10% in the regions of the trough and slope, respectively. The grazing impact of the small-sized zooplankton increased from the river zone to the deep regions (from 1 to 90%) along with their share in the total zooplankton abundance (from 18 to 95%). From 72 to 86% of the primary production was grazed over the shelf and slope. The primary production did not cover the feeding requirements of the zooplankton in the estuarine regions and St. Anna Trough in the autumn. In the estuarine regions, the major portion of the organic matter settles on the bottom due to the strong inflow of the allochthonous matter and the relatively low zooplankton grazing.     

Structural and Functional Characteristics of Zooplankton in the Ob Estuary and Adjacent Shelf Areas of the Kara Sea in Summer

Oceanology - The study was carried out in the Ob estuary and the adjacent shelf of the Kara Sea in July 2016. For the first time data on the species composition, abundance and distribution as well...