The East Siberian Sea (ESS) is a marginal sea of the Arctic Ocean, lying off the northeastern coast of Asia between the New Siberian and Wrangel islands. It occupies a central position in the system of epicontinental Siberian seas; in the west it borders the Laptev Sea, and in the east, the Chukchi Sea. The ESS lies almost entirely on the continental shelf; its average depth is 54 m, the greatest is 915 m, while half the basin has depths less than 30 m. The ESS shelf is the widest in the Arctic. The 200-m isobath lies at a distance of 700–750 km from the coastline, and the 100-m isobath, at a distance of 660–680 km [13].

The ESS is the harshest and iciest of the Arctic epicontinental seas. From October-November to June-July it is completely covered with ice; the largest loss of seasonal ice is observed in August–the very beginning of September [15]. Current climate changes in the basin are clearly visible, but less pronounced than in other Arctic seas. According to satellite data, the positive trend in air temperature in the region in summer averages 1.01°C/10 years, at sea surface temperature ‒0.69°C/10 years [54]. A decrease in sea ice cover period and increase in the duration of open water in summer have been observed since the 1980s, and these trends have especially intensified in the last 15 years [26, 33, 34, 48]. Years are frequently observed when almost the entire ESS free of ice in the summer.

The ice conditions of the ESS are characterized, as in other Arctic seas, by significant interannual and seasonal variations in ice cover observed in the last decade, which is also important for the functioning of regional ecosystems [32, 34]. This trend is well illustrated by Arctic ice maps of the last 15 years (http://www.aari.ru/clgmi/index.html; http://siows. solab.rshu.ru; https://nsidc.org). The strongly pronounced interannual variability of the ESS ice cover in summer is the background on which weakly pronounced modern climate trends manifest.

Continental runoff into the East Siberian Sea is low, about 250 km3/year, which is slightly less than 10% of the total volume of continental runoff into all Arctic seas. Freshwater runoff is mainly determined by the influx of Kolyma (132 km3/year) and Indigirka waters (60 km3/year) [13, 40]. Approximately 90% of the runoff occurs, as in other Arctic seas, in the summer months. The large area of the ESS and small volumes of continental runoff determine that its influence affects the hydrological regime only over the inner and southern part of the middle shelf. Low desalination of the upper layer of the sea and weak stratification allows autumn–winter convection in part of the basin, in contrast to other Siberian Arctic seas, to penetrate to the bottom at depths of 40–50 m, which occupy more than 70% of its entire area, and up to 70–80 m in deeper water areas. In general, it can be said that, in contrast to the Kara and Laptev seas, river runoff does not determine to such a strong extent the hydrological appearance of the basin and the features of its ecosystem.

Constant currents on the surface of the ESS are weakly expressed and form a cyclonic circulation, which can change under different synoptic situations [13, 51]. The influence of Atlantic waters hardly reaches the ESS, while in the eastern part of the basin, there is a weak influence of relatively warm Pacific waters, which determines the meridional nonuniformity in the distribution of many oceanological characteristics [44, 55, 56]. The boundary between the modified shelf waters of the ESS and Pacific waters is distinctly visible on the shelf and, on average, lies at 162° E. Depending on the local circulation characteristics and direction of desalinated water transport, the change in its meridional position can reach 10° [55], and it shifts to the east [28]. The presence of this boundary results in the separation of the ESS shelf into two areas, western and eastern, which differ significantly in oceanographic conditions important for the development of pelagic biota. The advection of Pacific waters onto the ESS shelf is an important phenomenon, which is part of the interaction between the Arctic and the Pacific Basin, which exhibits the spread of climate signals coming from southern to high latitudes.

The ESS is the least studied epicontinental Arctic basin, due to its inaccessibility, low biological productivity, and scarcity of biological resources [7, 8, 13, 21]. Low estimates of its hydrocarbon potential have also played an important role in the “passivity” of research into its ecosystem [14, 20].

Information about the pelagic ecosystem of the ESS is incommensurable with what we have today for other Siberian seas. Estimates of primary production in the basin are rare [8, 36, 37], while only [8] is based on mass measurements of primary production over a large area of the basin, taking into account all components of phytoplankton. Comprehensive data on ESS phytoplankton are contained only in publications [28, 53]. The most complete studies of the composition and quantitative characteristics of phytocenoses and their connection with environmental conditions in the western and eastern parts of the sea are presented in [28].

To date, studies of zooplankton in the ESS are sparse and cover only limited areas of the basin [4, 5, 21, 22, 25, 41, 42]. The most complete study [42] summarizes original data from 2009 and 2015 and published and archival data obtained in 1946 and 1948. The observations during these years spanned different areas of the ESS shelf, but together they covered a significant part of it. Data obtained in 1946 and 1948, as well as in 1973 [21], represent a cold and icy period in the Arctic until a radical change in climatic conditions in the region, which, according to some studies, occurred in 1990 [33], and according to other data, in the early 2000s [26]. During this period, in summer, the ESS was clear of ice only in a narrow coastal strip, as well as in the southeastern part near the De Long Strait and Chaun Bay, and in the west near the New Siberian Islands. Observations were concentrated in the latter two areas. Materials from 2009 and 2015 represent a period of modern warming, accompanied by a significant loss of ice cover in the ESS in summer. Moreover, in 2009 and 2015, the ice cover of the ESS during the period of maximum ice retreat, when the material was collected, varied significantly (http://old. aari.ru/odata/_d0015.php?mod=1). Thus, [42] combines data representing fundamentally different climatic periods and years with different climatic and ice conditions. In [4, 5], based on material collected in August 2015, estimates of zooplankton biomass and the contribution of dominant species and taxonomic groups to it are given, averaged for a small area of the inner ESS shelf adjacent to the mouth of the Kolyma River.

In general, the materials listed above, with rare exceptions, were obtained in areas of the ESS that are fundamentally different in environmental conditions. This made it possible to compile a detailed regional list of species and larger taxonomic groups of zooplankton and to obtain estimates of abundance and biomass for these areas, but they do not allow us to analyze the patterns of the spatial structure of the community, its temporal dynamics, and the relationship of these characteristics with environmental parameters on a basinwide scale. This requires data obtained within one season over most of the sea. In addition, there are virtually no quantitative assessments for the ESS of the role of zooplankton in consuming phytoplankton biomass and production. The materials that form the basis of this article, although limited to the ice-free water area in summer 2017, to some extent fill this gap for situation with certain climatic and ice conditions.

This study is based on material collected over 5 days in the western and eastern parts of the ESS shelf during the maximum ice-free period of the basin. This makes it possible to comparatively characterize the composition, biomass and role of dominant groups and species in it, the features of the cross-shelf distribution of zooplankton communities in connection with the most important features of the pelagic environment, and to assess the level of differences in the structure and trophic characteristics of communities in the western and eastern parts of the basin and their relationship with the oceanographic boundary separating these areas. These data constitute the basis for comparative estimates of the parameters of zooplankton communities in the ESS and other Siberian seas based on materials obtained by the authors over the past 20 years using similar methods [2, 10, 23, 29, 30, 42, 45, 47, 52].

EXPLORED AREA, MATERIAL, AND METHODS

The material was obtained on cruise 69 of the R/V Akademik Mstislav Keldysh as part of the program Marine Ecosystems of the Siberian Arctic. From September 5 to 9, 2017, two quasi-meridional transects were made on the ESS shelf (Fig. 1). The southern stations of the transects were in areas most strongly influenced by Indigirka and Kolyma river runoff, and the northern stations were at the edge of the ice field. The distance between transects was about 290 km. The Indigirka transect of 11 stations was located between 71°28′ and 76°09′ N and had a length of 610 km. Zooplankton in the transect was sampled at eight stations, CTD probing was done at 11; repeat observations were made at two stations, 5602 and 5606, on September 11. The length of the Kolyma transect was 550 km. On this transect, between 69°56′ and 74°23′ N, nine stations with CTD probing were made and zooplankton was sampled at six stations.

Fig. 1.
figure 1

Sketch map of transects and stations of cruise 69 of Akademik Mstislav Keldysh in East Siberian Sea in September 2017. Dotted line shows position of seasonal ice boundary during research period.

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Plankton samples were obtained by vertical catch with a Juday net (mouth square 0.1 m2, 180 μm filter cone mesh), raised at a rate of 0.6 m/s. The entire water column was sampled in layers: the upper mixed layer from the pycnocline to the surface and the layer from the bottom to the pycnocline. The volume of water filtered by the nets was determined by the distance covered. The net was carefully washed with seawater on deck to maximize the preservation of collected organisms. Mesoplankton samples were fixed with a 4% formaldehyde solution and processed in a standard manner under a binocular microscope in a Bogorov chamber, identifying animals to species, genus, and, sometimes, larger taxon, as well as measuring body length. When taking into account common forms, an aliquot of the sample was analyzed to obtain statistically significant estimates; rare forms were counted in the entire sample. For subsequent calculations of the wet biomass of species populations and total biomass of the community, the individual wet weight of animals was determined using the relationship between body length and weight for different species and groups [6], as well as Chislenko nomograms [31]. For conversion to dry weight, coefficients [3] were used.

The intensity of consumption of autotrophic phytoplankton by mass species of zooplankton was assessed using the fluorescent method based on the content of phytopigments (chlorophyll a and pheopigments) in the gut (G) and time of food digestion (T) [50]. Zooplankton for analysis was collected with a Juday net. At shallow-water stations (5598, 5615, 5619, and 5620), the layer from the bottom to the surface was sampled; at the remaining stations, samples were collected from the upper mixed layer and from the layer under the pycnocline. The methodology for selecting zooplankters and measuring G is described in detail in [9]. For common species, we used literature data on the time of food digestion [52], reduced to the average temperature for the layer where animals were caught, taking into account Q10 = 2.2 [43].

Daily consumption of autotrophic phytoplankton in chlorophyll units (IChl, ng Chl/ind/day) was calculated as IChl = (G1t1 + G2t2)/T, where G1 and G2 are the amount of phytopigments in the guts of zooplankters during the day and night (ng Chl ind.–1), t1 and t2 are the durations of the light and dark time of the day, 14 and 10 h, respectively. Total consumption of autotrophic phytoplankton biomass by populations of the studied mesozooplankton species (EСhl, mg Chl/ind./day) was calculated using the formula

$${{E}_{{Chl}}} = \mathop \sum \limits_{i{\kern 1pt} {\kern 1pt} = {\kern 1pt} {\kern 1pt} 1}^n {{I}_{i}}{{N}_{i}},$$

where Ii is the daily consumption of Chl for a given species, Ni is the species abundance in the layer (ind./m2), and n is the number of layers.

To convert the total daily consumption of phytoplankton into carbon units (Ec, µg S/m2/day), data on the biomass of autotrophic algae in units of organic carbon (Cph) [28] and chlorophyll a concentration were used [8]. Based on these data, the ratio Cph/Chl was calculated for each station (Table 1).

Table 1.   Ratio of content of organic carbon of phytoplankton and chlorophyll a (Cph/Chl) at stations
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Data on the temperature and salinity distribution on the transect were obtained by vertical profiling with a SBE911plus CTD system (SeaBirdTM Electronics Inc.), carried out synchronously with zooplankton collection.

RESULTS

Ice conditions. In the ESS in the last two decades, in general, two aspects should be emphasized about the ice conditions. After “breakup” in the ice cover of the basin at the end of the 20th–beginning of the 21st century [26, 33], a significant part of the ESS is free of ice in summer. At the same time, there is a pronounced interannual variability in the ice conditions, which is well illustrated by a comparison of 2018 and 2019. (Fig. 2). The ESS in 2017, when our studies were carried out, can be characterized as a year with average ice conditions against the background of climate trends observed in the last decade and interannual variability of ice cover. The ice edge in mid-September, during the period of its maximum retreat to the north, was located over the middle shelf at the 55–60 m isobath. On the Indigirka transect, this corresponded to 76°09′.9 N; on the Kolyma transect, to 74°23′.0 N (Fig. 1, 2). Compared to 2012, the year with the least ice in the Arctic since the beginning of the 21st century (Fig. 2), the ice edge in summer 2017 was more than 5° farther south.

Fig. 2.
figure 2

Ice cover maps of East Siberian Sea for mid-September by year (http://old.aari.ru/odata/_d0015.php?mod=1).

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Oceanographic conditions. The temperature and salinity distributions on the transects are shown in Fig. 3.

Fig. 3.
figure 3

Temperature (t, °C) and salinity (S, PSU) distributions on Indigirka (a) and Kolyma (b) transects.

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On the Indigirka transect (Fig. 3a), the temperature of the upper ~7 m mixed layer (UML) varied from 6.2°C in the coastal part, exposed to the maximum impact of river runoff, to –1.3°C at northern stations at the edge of the ice field. Corresponding surface salinity estimates ranged from 15.2 to just over 30.0 PSU. The boundary of surface waters desalinated (<24 PSU) by river runoff were at a distance of 335 km from the coast (73°40′ N, station 5603). The water column throughout almost the entire transect, with the exception of the area adjacent to the ice edge (station 5607, Fig. 3a), was stratified. Below the thermohalocline, the water temperature was positive (0.6–3.0°C) only in the 200 km coastal zone. North along the transect from 72°40′ N at a distance of more than 220 km from the coast, it decreased to –1.2 to –1.6°C. Salinity in the lower layers of the water column varied from 25.5 PSU in the coastal part of the transect to 33.4 PSU in the northern part (Fig. 3b). Shelf Arctic cold (≤0°C) and saline (≥30 PSU) water was observed in the surface 15 m layer at ~75°30′ N at a distance of 585 km from the coast; deeper than 20 m, it spread to the south up to 72°20′ N (station 5600, ~165 km from the coast).

The waters crossed by the Kolyma transect ~290 km to the east differed significantly in hydrological characteristics (Fig. 3b). In the coastal zone, at a distance of up to 110 km from the coast, in a thin (2–7 m) surface layer, the water temperature was ≥6.0°C and its salinity was 17–18 PSU. At a depth of 12.5–15.0 m, the temperature dropped to 1.0°C and salinity increased to 27 PSU. Surface desalination associated with river runoff (<24 PSU) was traced at a distance of up to 210–230 km from the mouth of the Kolyma River (station 5617, ~71°30′ N). The area of the middle shelf between stations 5614 and 5616 at a distance of 250–380 km from the coast was characterized by very weakly pronounced vertical stratification (Fig. 3b). At depths of 22–27 m, there was no UML and underlying thermohalocline, characteristic of the Arctic shelf with the exception of some narrow coastal shallow areas. The temperature of the entire water column here was positive and gradually decreased in a northerly direction from 5.0 to 2.5°C. Salinity varied slightly from the surface (28 PSU) to the bottom (28.5 PSU). This shelf zone with quasi-uniform temperature and salinity characteristics in the water column represented a kind of “well” in which waters are intensively mixed from the surface to the bottom. This nature of vertical stratification fundamentally distinguished the hydrological structure of this shelf area from a similar area lying to the west, which crossed the Indigirka transect. North of 71°30′ N (stations 5612–5613) at a distance of more than 400 km from the coast in the area adjacent to the ice edge, a well-defined UML and an underlying thermohalocline, characteristic of Arctic shelf waters, were observed in the temperature and salinity distributions. Cold (≤0°C) and saline (≥30 PSU) water enters this zone from the north at depths greater than 15–20 m.

Species and group composition of zooplankton is characterized by Table 2, which lists the main components of the community and their occurrence at transect stations.

Table 2.   Occurrence of species and taxonomic groups of mesozooplankton at stations of Indigirka and Kolyma transects
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The materials presented in Table 2 allow us to identify the following specific features of the species composition of mesoplankton on the transects. Species of the deep Arctic basin Calanus hyperboreus, Atlantic C. fnmarchicus, and Metridia longa were noted in very small numbers only at the northernmost stations of the transects, which indicates lack of significant influence of water transgression from deep-sea areas on the composition of communities of the middle shelf of the ESS. Mass species of Arctic mesoplankton associated with desalination—Limnocalanus macrurus, and Drepanopus bungei—are widely represented on the Indigirka transect, while on the Kolyma transect, they are recorded as single individuals or not found at all: (D. bungei). On the Indigirka transect, the following species are significantly more widely represented: Microcalanus pygmaeus, Pseudocalanus acuspes, Oncaea borealis, jellyfish Euphysa fammea and Obelia longissima. Gammaridea spp. were recorded only in the western part of the ESS shelf. On both transects, the following species are found almost universally: Acartia longiremis, Calanus glacialis, Jaschnovia tolli, Pseudocalanus minutus, Pseudocalanus spp., Oithona similis, Parasagitta elegans.

The obtained estimates of the total mesoplankton biomass for the Indigirka and Kolyma transects demonstrate significantly lower values for the eastern part of the sea. The corresponding numbers are, respectively, 771.00 (n = 10; SE = 125.44) and 287.84 (n = 6; SE = 56.27) mg/m3 in wet weight and 39.96 (n = 10; SE = 5.33) and 16.19 (n = 6; SE = 5.24) mg/m3 in dry weight units. For both parameters, the differences are larger than twofold. The estimates of the average mezooplankton biomass for each transect are shown, for three large taxonomic categories: zooplankton excluding chaetognaths and jellyfish (MZ), chaetognaths (CT), and jellyfish (JF) (Fig. 4, Table 3). All three groups in the western part of the basin were characterized by higher average biomass and, without exception, higher maximum and minimum values in wet and dry weight units (Fig. 4, Table 3). The differences in average biomass values in dry weight units for MZ were 2.4 times, for JF, 3.3 times. The biomass of the quantitatively poorest group, CT, on the Indigirka transect was also 1.5 times higher, although the differences were not statistically significant.

Fig. 4.
figure 4

Average biomass values for Indigirka and Kolyma transects (mg/m3, dry weight) jellyfish, chaetognaths, mesozooplankton without jellyfish and chaetognaths, and total mesozooplankton biomass (Total). (1) Indigirka transect, (2) Kolyma transect; vertical lines, standard deviation.

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Table 3.   Average (Bav), minimum (Bmin), and maximum (Bmax) biomass values (mg/m3) by taxonomic categories for Indigirka and Kolyma transects. SE—standard error of mean; n—number of determinations
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When comparing the average indicators of the share of identified large taxonomic groups in the total mesoplankton biomass in the western and eastern parts of the sea, a generally similar pattern was observed. The maximum contribution to the biomass in units of dry weight on both transects came from MZ, the contribution of JF was 2.7–3.6 times lower, and the least significant group was CT; their biomass was 5.8–9.0 times lower than that of MZ (Fig. 4, Table 4).

Table 4.   Comparison of biomass of large taxonomic groups of mesozooplankton on Indigirka and Kolyma transects using Mann–Whitney U-test
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The distribution of large taxonomic groups of mesozooplankton on transects is shown in Fig. 5. The western and eastern regions of the sea differed significantly in character of the latitudinal distribution of groups and their role in the formation of the total biomass. On the Indigirka transect (Fig. 5a), the composition of mesoplankton in the 200 km coastal zone (stations 5598–5600), exposed to the effects of the most pronounced desalination, and in the northern regions with similar values of the total biomass of MZ and CT, differed significantly. The coastal region was characterized by a relatively high MZ biomass and low CT biomass. The wet biomass of MZ exceeded the biomass of CT by 8–1000 times; the corresponding differences in dry weight were by 40 to more than 5000 times. Against this background, the JF biomass was the highest for the transect, reaching 23.9 mg/m3 dry weight, and JF:MZ biomass ratio was as 1 : 2. The ratio of biomass of large taxonomic groups at the northern stations of the transect (5605–5608, 5606_2) was fundamentally different. JF biomass in the northern regions decreased to values of 2.0–6.2 mg/m3 dry weight, and the ratio of JF and MZ biomass averaged 1 : 6. The role of chaetognaths in the biomass of mesoplankton increased significantly. With the values of the total biomass of these two groups close to those observed in the coastal desalinated area, the wet biomass of CT differed slightly from the biomass of MZ or exceeded it by 1.2–1.7 times. In dry weight units, the MZ biomass was only 2.5–5.1 times higher than the CT biomass. The maximum values of the total biomass of MZ and CT for the transect were noted in the area close to the ice edge at station 5608 above depths of 50 m–430.2 mg/m3 in wet weight and 33.7 mg/m3 in dry weight.

Fig. 5.
figure 5

Distribution of mesozooplankton biomass (mg wet weight/m3) on Indigirka (a) and Kolyma (b) transects. (1) biomass without chaetognaths and jellyfish (B1), (2) chaetognaths biomass (B2), (3) jellyfish biomass (B3).

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As a characteristic feature of the mesoplankton composition, we noted the large role of small-sized copepods of the genus Pseudocalanus and appendicularia Oikopleura vanhoeffeni in the biomass (Fig. 6). The biomass of Pseudocalanus spp. on the transect varies from 25.6 to 145 mg wet weight/m3, and the contribution of the genus to the total biomass (excluding jellyfish) was from 5 to 40%. The biomass of O. vanhoeffeni at most stations is estimated as 11–70 mg wet weight/m3, and the share of appendicularia in the mesoplankton biomass ranges from 2 to 19%.

Fig. 6.
figure 6

Distribution of mesozooplankton species on Indigirka (solid line) and Kolyma (dashed line) transects. B, wet biomass, mg/m3.

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On the Kolyma transect, the latitudinal distribution of the identified large taxonomic groups of mesoplankton and their ratio were fundamentally different (Fig. 5 b). At stations within the 200 km coastal desalinated area, the community structure varied significantly. Station 5620, located 35 km from the coast, stood out from the entire studied area due to the dominance of chaetognaths in the biomass. In units of wet biomass, the values for CT were 7.5 times higher than for MZ; similar differences in dry weight were two times higher. As well, at more seaward station 5619, 100 km from the coast, there were no CTs in the mesoplankton. The characteristics of mesoplankton at station 5617, located at the boundary of the coastal desalinated area (surface salinity 23.5 PSU, depth 22 m) 210 km from the coast, were unique for the entire study area. With an extremely low total biomass of 15.3 mg/m3 in wet weight and 0.5 mg/m3 (!) in dry weight, the dominant group was JF, which accounted for 0.3 mg/m2 dry biomass. The northern part of the transect, at a distance of 320–570 km from the coast, was characterized by values of the total biomass of MZ and CT, close to those observed in this area at the Indigirka transect: 147.3–457.6 mg/m3 in wet weight and 3.4–33.8 mg/m3 in dry weight. At the same time, the contribution of CT was significantly less than in the western part of the basin–the biomass of MZ exceeded the biomass of CT from 2 to 28 times in wet weight units and from 8 to 15 times in dry weight. The maximum values of the total MZ and CT biomass, as on the Indigirka transect, were noted in the area close to the ice edge (station 5612, depth 52 m) and amounted to 402.6 mg/m3 wet weight and 33.8 mg/m3 dry weight. At the same time, the ratio of taxonomic groups differed significantly: MZ demonstrated quantitative predominance—it exceeded CT in wet biomass by 2.6 times, and in dry weight, by 14.4. The biomass of jellyfish at stations in the outer part of the transect ranged from 2.6 to 5.5 mg/m3 dry weight at stations 5612 and 5613 and was 4–12 times lower than the total MZ and CT biomass. In the central part of the shelf, in an area with no vertical stratification (station 5615, Fig. 3b), a unique for the entire studied area ratio of taxonomic groups in the mesoplankton community was observed: in dry weight, the JF biomass exceeded the total MZ and CT biomass by 1.8 times.

On the Kolyma transect, as well as in the western part of the sea, a high proportion of Pseudocalanus spp. and Oikopleura vanhoeffeni was noted in the biomass (Fig. 6). The maximum values of 27.1–32.9 mg wet weight/m3 the biomass of Pseudocalanus spp. reached at the southernmost and northern stations of the transect (Fig. 6), where the contribution of the genus to the total mesoplankton biomass was 9–17%. The high biomass of Oikopleura vanhoeffeni—19–24 mg wet weight/m3—was noted in the area adjacent to the ice field boundary at stations 5612 and 5613.

Figure 6 shows the distribution on transects of the main mesozooplankton species that formed the community in the studied area of the ESS. By the nature of distribution, several groups were distinguished among these species. The first includes the copepods Calanus glacialis, Oithona similis, Oncaea borealis, and Microcalanus pygmeus. These species have a similar cross-shelf distribution on the Indigirka and Kolyma transects and were characterized by very similar biomass values in shelf areas of the same latitude. The second group of species demonstrates, conditionally, a similar latitudinal distribution trend, but at the same time, their biomass in the eastern part of the basin was significantly lower: two to four times lower than in the western part. This group includes the copepods Acartia longiremis, appendicularia Oikopleura vanhoeffeni and Fritillaria borealis, jellyfish Aglantha digitale, Halitholus joldia-arcticae. The distribution of species of this group does not demonstrate a single latitudinal trend; it can be characterized by an increase in abundance both in the northern and coastal or central parts of the transects. The next group, the most abundant, included the copepods Drepanopus bungii, Jaschnovia tolli, Limnocalanus macrurus, genus Pseudocalanus (without taking into account P. minutus and P. major), P. minutus, P. major, the pteropods Limacina helicina chaetognaths Parasagitta elegants. The latitudinal distribution of these species on the western and eastern transects has no common features. Most of them were characterized by a significantly higher biomass in the western part of the sea, while some were found sporadically in the eastern part of the basin. Preferring desalinated waters, L. macrurus was not recorded on the eastern transect. Exceptions to this group of species were L. helicina, the biomass of which on the eastern transect was an order of magnitude higher than on the western, and P. elegans. The distribution of this species on the transects has a “mirror” nature: in the west of the basin, the highest biomass was recorded at the northern stations of the transect, and in the east, similar values were recorded in the coastal zone. Groups with similar and different types of distribution in the western and eastern parts of the basin include species with both relatively high and low biomasses.

Grazing of mass species of zooplankton and their role in consuming phytoplankton. Table 5 summarizes the data on the content of plant pigments in the guts of the dominant species of herbivorous mesoplankton in the day- and nighttime. The nighttime G value of C. glacialis and Limacina helicina was significantly higher than during the day (Mann–Whitney U-test, p < 0.001), and in copepods of the genus Pseudocalanus and appendicularia Oikopleura vanhoeffeni, it did not differ significantly.

Table 5.   Content of plant pigments in guts (G, ng/ind.) of common zooplankton species at different times of day. Mean values ±SD and number of replicates are given in parentheses
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Since the size of appendicularia varied significantly at different stations, we obtained the addiction of G on body length, L, mm, for them (G = 1.09L1.83, r 2 = 0.75, n = 64), from which we calculated the daily consumption of autotrophic algae by individuals of different sizes (Table 6).

Table 6.   Daily consumption of autotrophic phytoplankton (IChl, ngСhl ind.–1 days–1). IChl was reduced to average temperature of 2°C for all stations
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The total consumption of phytoplankton of the studied zooplankton populations in organic carbon units on the Indigirka transect varied from 0.5 to 15 mgC/m2/day, averaging 5 mgC/m2/day. Low values of 0.5–1.5 mgС/m2/day are typical of the 300 km area adjacent to the coast (stations 5598–5602) and of the northernmost station 5607 at the ice edge (Fig. 7a). The consumption of biomass and phytoplankton production in these areas did not exceed 3%. At stations 5604–5606 and 5608, phytoplankton consumption was significantly higher, on average, 9 mgC/m2/day. The consumption of phytoplankton biomass and production at these stations was 2–8 and 20–64%, respectively.

Fig. 7.
figure 7

Total consumption (E, mgC/m2/day) and consumption of daily biomass (E/Bd, %) and production (E/PP, %) of phytoplankton by mass species of zooplankton on Indigirka (a) and Kolyma (b) transects. O.v, Oikopleura vanhoeffeni; Ps, Pseudocalanus spp.; L. h., Limacina helicina; C. g., Calanus glacialis.

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On the Kolyma transect, the total consumption of phytoplankton at coastal stations (5617–5620) did not exceed 0.1 mgC/m2/day, and the consumption of phytoplankton biomass and production–0.5% (Fig. 7b). At northern stations, phytoplankton consumption increased to 1–5 mgC/m2/day, and grazing, up to 3–10% of biomass and up to 5–27% of phytoplankton production. The main contribution to the total consumption in the coastal areas of two transects (70–100%) came from small copepods of the genus Pseudocalanus. At most of the northern stations, grazing was determined by the population of Oikopleura vanhoeffeni, the contribution of which to the total consumption of phytoplankton was 50–90%. The exception was station 5615 of the Kolyma transect, in which, with a low number of appendiculars, the leading role in grazing was played by the population of Limacina helicina.

DISCUSSION

Comparison of the data on the species and group composition of mesozooplankton obtained in this material with the estimates presented in summary [42] shows almost complete identity of the lists for the areas of the inner and middle shelves of the ESS. The absence of a number of species in our materials is determined solely by the fact that the 2017 collections did not cover the outer shelf, the continental slope zone, and adjacent deep-sea areas.

The following minor differences should be noted. In our material, Eucalanus bungii was not detected, whereas in the 1946 collections [16], it was found in the area of the inner shelf adjacent to the mouth of the Kolyma River. This species is a Pacific expatriate, an indicator of Pacific waters on the ESS shelf, and our data, in comparison with [16], provide biological confirmation that this phenomenon is “impulsive.” Noteworthy is the absence of a mass species of jellyfish Aglantha digitale in the 1946 and 1948 collections [16, 22], which may likely be due to failure to take jellyfish into account when processing samples after long-term storage. In the 2009 materials obtained on the inner shelf [42], A. digitale was noted with a biomass of 0.2 mg/m3 dry weight, which is close to the values we obtained: 0.1–0.3 mg/m3.

The values of mesozooplankton biomass we obtained—40.0 ± 16.8 mg/m3 dry weight in the western ESS—are above the average of 16.8 mg/m3 dry weight given in [42] for stations on the middle shelf carried out within the area of our research in September. As well, for the eastern part of the basin, our material gives very similar figures: 16.2 ± 12.8 mg/m3 dry weight (Table 2). The above comparative estimates of mesoplankton biomass for the ESS shelf in 2015 and 2017 were obtained against the background of similar ice conditions in the basin in these years (Fig. 2).

A significant difference between our data and the materials of previous observations in the ESS in which jellyfish were taken into account in detail [41, 42] is a higher biomass of Cnidaria. Research data from 2009 and 2015 give an average figure of 0.91 (SE ± 0.43) mg dry weight/m3 for maximum values of 1.2 mg dry weight/m3. The values we obtained exceed these by three to ten times: 10.07 (SE ± 2.32; maximum 23.9) for the Indigirka transect and 3.07 (SE ± 0.83; maximum 5.51) mg dry weight/m3 for the Kolyma transect (Table 2). In order to talk about a pronounced trend, we require confidence in the preservability of jellyfish in the 2009 and 2015 samples at the time of their processing and correct quantitative assessment of the group, which we, unfortunately, do not know. However, the found differences are of undoubted interest.

The values of mesoplankton biomass we obtained for the western and eastern shelves of the ESS allow, with some adaptation, a comparison with data for other Siberian Arctic seas. For shelf areas of the Laptev Sea, in [2, 46, 49, 52], the following range of figures is mainly given: 8–50 mg/m3 dry weight. The highest, up to 104–270 mg/m3 dry weight, just like the lowest, <10 mg/m3 dry weight, were noted in narrow coastal zones under the influence of continental runoff [49, 52].

The shelf of the westernmost and “warmest” of the Siberian Arctic seas, the Kara Sea, in previous studies is characterized by values of the total wet weight of mesoplankton in summer–fall of 70–295 mg/m3 [10, 19, 39]. According to our data [29], the total wet biomass of mesoplankton on the inner shelf of the Kara Sea ranged from 74 to 236 (average 162.1 ± 84.0) mg/m3 wet weight. In [47], biomass values of 15–36 mg/m3 dry weight are given, which exclude jellyfish. In dry weight units for the mesoplankton biomass on the Kara Sea shelf, the following figures are given in [45]: from 1.8 to 5.0 g/m2; in [39], from 0.1 to 41.4 g/m2 with an average value of 6.2 ± 1.3 g/m2. In autumn 2013 and 2015, for the eastern (160 E) most Arctic part of the Kara Sea, we obtained estimates of mesoplankton biomass in dry weight (excluding jellyfish) of, respectively, 17–112 (average 48 ± 35) mg/m3 and 27–80 (average 51 ± 17) mg/m3 [12].

Our estimates, adapted for comparison with the above data, for the western ESS in wet weight units range from 337.3 to 1537.0 (771 ± 472.6) mg/m3, and for the eastern, from 15.3 to 436.6 (287.8 ± 229.5) mg/m3. If from the estimates we exclude jellyfish, which have a high biomass on the ESS shelf (Figs. 4, 5; Table 2), we obtain the following corresponding averages: 267.7 ± 170.6 mg/m3 and 134.4 ± 67.6 mg/m3. In dry weight units, these values range from 10.6 to 49.1 (average 29.9 ± 5.5) mg/m3.

Clearly, our estimates for the mesoplankton biomass in the ESS for summer–fall are within the range of values characteristic of other Siberian Arctic seas. Moreover, this is true even for the eastern, more depleted part of the ESS shelf. Our figures for the western part of the basin are closer to the upper limit of the values given above for the Laptev and Kara seas. All this allows us to say that the common thesis about the extreme poverty of the ESS ecosystem due to its long ice cover period is not true, at least for such a key component of the pelagic ecosystem as mesoplankton.

The zonal distance between the Indigirka and Kolyma transects is only 290 km, the southern parts of both transects are adjacent to the estuaries of large Arctic rivers and are influenced by desalination from river runoff. Both transects intersect areas of similar depths of the inner and middle shelves of the ESS, but at the same time, the water structure and mesoplankton communities on the transects differ significantly. The differences in hydrophysical conditions are illustrated in Fig. 3 and Table 7. The central part of the Kolyma transect crossed a specific latitudinal zone with a length of ~250 km (this is ~1/3 the width of the ESS shelf), in which vertical stratification, characteristic of the shelves of the Siberian Arctic seas, was almost completely absent (Fig. 3; 25 m isobath in Table 7). The bottom temperature in this area reached 4.04°C vs. –1.10°C at the same depths in the western part of the sea and was almost equal to the temperature of the surface layer. The differences in surface and bottom salinity were only 0.14 PSU; on the Indigirka transect, they exceeded 6 PSU. All this indicates intense vertical mixing of the entire water column, which creates unique conditions for the existence of planktonic communities on the Arctic shelf, the most important properties of which in other areas are determined by rigid vertical stratification [2, 10, 23, 27–29]. The reasons for the formation of such conditions are probably related to the influence of Pacific waters coming from the east on the eastern part of the ESS shelf [36, 38, 51, 55] and the relatively weak comparative to other Siberian shelf influence of river runoff.

Table 7.   Characteristics of conditions at 15, 25, 50 m isobaths of Indigirka (152°54′–163°03′ E) and Kolyma (162°32′– 168°12′ E) transects
Full size table

The total biomass of mesoplankton and biomass of the main large taxonomic groups in the western part of the ESS is higher than in the eastern part (Fig. 4, Table 3). Only for CT, although the trend is obvious, are the differences not significant. Similar quantitative characteristics of the community in the range of 270–500 mg wet weight/m3 (excluding jellyfish) were observed only in the northern parts of the transects at a distance of more than 550 km from the coast in the area adjacent to the ice boundary (Fig. 1, 5).

In the western and eastern parts of the basin, the cross-shelf distribution of the biomass of both total mesoplankton and large taxonomic groups and many of the dominant species and their share in the biomass showed significant differences (Figs. 5, 6). Particularly indicative in this regard is the distribution of CT, species of the genus Pseudocalanus, and JF. Differences in the cross-shelf distribution of mesoplankton across transects were observed against the background of not only pronounced differences in the vertical structure of the water column, but also the latitudinal distribution of surface salinity, reflecting the level of influence of the river runoff over the area. The surface 23.5 PSU isohaline in the western part of the basin was at a distance of 350 km from the coast, and in the eastern part, 200 km (Figs. 3, 5). Variations in surface salinity in other Arctic seas are the main environmental property with which cross-shelf features of the structure of mesoplankton communities are associated [2, 10, 29, 52].

The distribution features of mesoplankton were fully reflected in the role of zooplankton in the consumption of primary organic matter in the ESS. The estimates, which were obtained for the first time, showed significant differences in the level of grazing of phytoplankton by mass species of zooplankton in the western and eastern parts of the shelf. These differences are especially pronounced in the coastal zone influenced by river runoff: the consumption of phytoplankton biomass and production in the eastern part (0.2%) is an order of magnitude lower than in the western part (2.5–3.3%). These differences persist in the northern regions, although they are less pronounced: in the eastern part, zooplankton consumed 1–5% of phytoplankton biomass and 5–27% of the production; in the western part, 5–8 and 20–64%, respectively.

In general, with the exception of minimal values in the eastern part of the shelf, our estimates of consumption of phytoplankton biomass and production by herbivorous zooplankton are within the range of values given for other Siberian Arctic seas in autumn. On the Kara Sea shelf, the consumption of phytoplankton biomass was 1.6–12.5% [1, 23, 45]; on the Laptev Sea shelf, 1–8% of biomass and 1–72% of production [2, 52]. The main consumer of planktonic algae on the shelves of these seas were abundant species of copepods.

In the ESS at shelf stations located outside the zone of distribution of desalinated waters, the grazing of phytoplankton was determined by appendicularia Oikopleura vanhoeffeni. According to our calculations, the population of this species consumed 1–8% of biomass and 3–55% of phytoplankton production. However, our method allows us to estimate the amount of phytoplankton ingested, while from 10 to 60% of phytoplankton can remain on the mucus filters of the “houses” of appendicularia (mucus formations that form during its life and are regularly discharged when the filter apparatus is clogged) [35]. Therefore, the contribution of O. vanhoeffeni population to grazing of phytoplankton may be significantly higher. Estimates of grazing, obtained using the marker bead method, which allows for the fraction attached to houses to be taken into account, reached 13% of phytoplankton biomass in the coastal waters of Newfoundland. With high levels of phytoplankton consumption, appendicularia play a prominent role in the vertical organic carbon flux in the northern regions of the ESS shelf. According to sediment trap data, the contribution of fecal pellets from appendicularia at station 5606 accounted for about a third of the total organic carbon flux [11].

Our data show that the mesoplankton communities of the western and eastern parts of the ESS differ significantly in quantitative characteristics, features of cross-shelf distribution, and the role of dominant groups and species in phytoplankton grazing. The boundary between communities with different structural and trophic characteristics runs approximately along 160°–163° E. The differences are probably associated with the influence of Pacific waters on the hydrophysical and hydrochemical environmental conditions in the eastern EES, the existence of a well-defined boundary of this influence in the central part of the basin [38, 44, 51, 55], as well as significantly smaller impact of river desalination on the eastern shelf of the ESS. The existence of this boundary is confirmed by differences in the processes of biogeochemical transformation of carbon [36] and structure of phytoplankton communities [28].

On the ESS shelf, also at a latitude of ~160°–165° E, there is a well-defined boundary (habitat boundary) between Atlantic and Pacific benthic biotic assemblages [17, 24]. And although there is the opinion that this boundary is not directly related to advective propagation of the Pacific signal [18], obviously, there are powerful natural factors supporting it.

Pelagic and benthic fauna, primarily mesoplankton communities, due to their relatively rapid response to environmental changes, are a representative object for identifying climate change occurring in Arctic ecosystems over the past few decades. The ESS is of particular interest in this sense, due to the significant increase in the area of ice-free water in summer and the duration of the ice-free period in the last two decades [26, 34, 48], which can lead to changes in biodiversity and increase in productivity of the base trophic levels of the ecosystem. Clearly, in the ESS, comparisons of materials on mesoplankton in search of ongoing changes should be carried out taking into account the fundamental differences between pelagic biotopes and communities of the western and eastern parts of the basin and the well-defined boundary between them, localized, at least in the last 20 years, in the area of 160°–165° E.