INTRODUCTION

The Yamal Peninsula, located in the permafrost zone, is characterized by a great number of thermokarst water bodies at different stages of succession. It is a convenient object for comparative limnological analysis. The development cycle of these water bodies includes several main stages: flat–hilly swamp, mochaga (prelake), thermokarst lake, and khasyrei (the remains of a thermokarst lake after its water has been drained into an adjacent reservoir or river) (Manasypov et al., 2012). It is of practical importance to elucidate the patterns of thermokarst processes, including the classifications of water bodies, to assess the environmental status and predict possible economic damage to oil and gas fields.

In 2015–2019, research using multispectral satellite images was performed as part of the Program for the Remote Sensing of Yamal Water Bodies in the area of the BOGCF. At the same time, 52 water bodies were examined by in situ methods and the data necessary for interpreting and detailing the results of satellite imagery analysis were obtained. The most detailed hydrobiological surveys were carried out at 25 water bodies of different sizes (Ermolaeva, 2016; Zarubina, 2016a, 2016b; Koveshnikov, 2018). These studies included the sampling of zoobenthos, one of the key objects of environmental monitoring listed in the standards for a comprehensive assessment of the environmental status. At the next stage of the work, it is planned to compare the results of the zoobenthos study with the data obtained by an analysis of parallel in situ sampling and remote sensing. The goal of the comprehensive classification of thermokarst lakes is to determine the patterns of natural and anthropogenic succession. This step is necessary to predict the direction and rate of changes of the hydrological, hydrochemical, and hydrobiological parameters of the water bodies, as well as predict the changes in water quality. In the future, it is planned to develop a method for the remote classification of water bodies and assessment of hydrobiological and hydrochemical parameters of the environment from satellite remote sensing in the areas of development of continental deposits in the Arctic.

This study aims to track the dynamics of the development of benthic macroinvertebrate communities during the natural succession of thermokarst water bodies in the central Yamal and classify water bodies according to zoobenthos parameters.

MATERIALS AND METHODS

According to the geographic zoning of the continental tundra, the study area is located in the moss–lichen subzone of the typical tundra zone of the Yamalo-Tazovsky District of the West Siberian Province of the Kola-Gydan region (Gorbatskii, 1967). Complex studies were performed in 2015, 2018, and 2019 near the BOGCF, located in the central Yamal Peninsula (basin of the Baidaratskaya Bay of the Kara Sea). The water bodies were studied in the interfluves of Seyakha and Mordyyakha rivers, the main watercourses of this area. Floodplain water bodies located in the valleys, the upper floodplain water bodies of watershed elevations, and the upper floodplain water bodies at the border of these areas were considered. Hydrobiological samples, including zoobenthos, were collected at 25 water bodies to classify thermokarst water bodies according to a set of hydrophysical and hydrobiological indicators based on the characteristics of the benthic community of invertebrates (Table 1).

Table 1. Codes of the water bodies, sampling sites, and sampling dates in 2015–2019
Full size table

Table 1 may be used in the Google Earth program as an interactive illustration to discuss the results of the study. Zoobenthos was collected in late August–early September at the depths of up to 1 m with a scraper (mesh size of 350 µm). Two or three combined cuts, depending on the bottom sediments, were performed at each station. Collection and laboratory processing were carried out by conventional methods (Rukovodstvo …, 1992). Invertebrates were identified mainly according to the latest reviews (Opredelitel …, 1997–2006). Taxonomic classification and valid names are given in accordance with the international zoological nomenclature.Footnote 1

RESULTS

In total, 161 taxa of species rank of zoobenthos belonging to 100 genera, 36 families, 20 orders, 12 classes, and 5 types (Cnidaria, Nemathelmintes, Annelida, Molluska, and Arthropoda), were found in the BOGCF area. Insects (112 species) with a predominance of chironomids (88 species) were the most diverse group. The abundance of zoobenthos varied within a wide range of 0.02–23.4 thousand ind./m2, biomass from 0.05 to 73.10 g/m2, and number of species per sample from 1 to 33. The minimum abundance and biomass were observed on peat beaches and the maximum on detritus among coastal water sedge thickets. According to the zoobenthos biomass, the trophic status of water bodies (Kitaev, 2007) varied over the widest range, from ultraoligotrophic to hypereutrophic.

When analyzing the characteristics of zoobenthos, the difference in the conditions of its formation in water bodies occupying different positions in the basin was considered. These were the upper ones (on the watershed elevations), middle ones (at the edge of the valleys), and lower ones (in floodplain part of the valley). The species of the dominant complex contributing ≥10% to the biomass of individual samples of this water body and B dominants for each water body, indicating that the species brings the maximum contribution in one of its sections, are listed separately (Tables 2–4).

Table 2. Characteristics of water bodies in the upper part of the basins
Full size table

On the watersheds, there are wetlands with the smallest thermokarst water bodies (prelakes, or mochagas). Larger lakes, occupying the upper position in the basins, are relatively isolated from each other and have a rounded shape. The largest lakes (above the floodplain) are characterized by high scree banks made of redeposited peat and dry silt, which are eroded due to wind-wave processes. They are characterized by a sandy–silty bottom, and there are sandy and silty beaches as well. The tundra surrounding the floodplain water bodies is hilly, easier to pass, and predominantly brown in color due to the outcrop of the rock, and it is less bright due to low vegetation when compared to that at the floodplain valleys. Background coastal vegetation is presented by lichens, moss, cloudberry, dwarf willow, and dwarf birch. There are more terraces than in floodplain water bodies due to the steeper slopes and the presence of an upper (moss–birch) area above the willow forest.

The lakes occupying the upper and middle positions in the basin form series; their areas increase towards the floodplain valleys lying below. The middle and, especially, the lower water bodies are characterized by smoother banks; they are located in the wide relief depressions and tend to unite at high water level (Table 3).

Table 3. Characteristics of water bodies in the middle part of the basins
Full size table

Along floodplain lakes of medium size, the coastal sphagnum quagmire is typical; along large shapeless lakes, there are peat beaches. The tundra surrounding the lower water bodies has a rich green color due to dense vegetation and is impassable due to high humidity, tall grass, and shrubs. Background coastal vegetation is presented by moss, sedge, cinquefoil, cotton grass, and willow. There are fewer lake terraces than near floodplain lakes due to the flatter relief; the uppermost terraces are overgrown by dwarf willow. In such areas, in addition to round and largest shapeless lakes, there are arcuate water bodies, which are shallowed thermokarst lakes. The valleys of similar drying lakes in Yakutia are called alas. In Yamal, both the valley and the arcuate reservoir are called khasyrei. Relief depressions containing floodplain backwaters and old riverbed lakes, outwardly similar to khasyreis due to the same shape and similarly overgrown banks, are called similarly khasyrei or khasre in the local dialect (Valgmanova et al., 2012). However, the latter are adventitious water bodies of a different genesis. According to our data, they have the biological features of an old lake or a young khasyrei, characterized by conditions favorable for the formation of zoobenthos of the same chironomid–mollusk type, but an even greater proportion of gastropods is noted in the benthic community structure (Table 4).

Table 4. Characteristics of water bodies in the lower part of the basins
Full size table

DISCUSSION

Hydrological and glaciological peculiarities of the cycle of thermokarst lakes in the northern tundra have been described, including the stages of a flat–hilly swamp, wetlands, a thermokarst lake, and khasyrei (Manasypov et al., 2012). However, it is practically difficult to distinguish between numerous tundra water bodies by the stage of their development during route surveys. There was an attempt to classify these water bodies by visual observations based on a description of near-water vegetation (Loiko et al., 2018). Our study continues this new approach, but considers the zoobenthos dynamics and the conditions for its formation during the natural succession of thermokarst water bodies. In order to classify the water bodies of the central Yamal by the example of the BOGCF area, the accepted terms are applied, but a larger number of stages are proposed. The surveyed water bodies are ranked by eight types, representing all subsequent stages of the succession cycle, based on the results of original studies of zoobenthos, vegetation, and hydrological parameters of water bodies in 2015–2019 (Zarubina, 2016a, 2016b; Koveshnikov, 2018) and published data (Manasypov et al., 2012; Loiko et al., 2018). The stage of succession may be determined by a set of features; the length of the reservoir along the midline, water body shape, position in the drainage basin, the nature of bottom sediments, and the structure of animal and plant communities are the most significant. The scheme of approximate classification of water bodies is presented in the final table (Table 5).

Table 5. Types of thermokarst water bodies in the central Yama
Full size table

There is a relationship between the succession stage of the reservoir and its position in the basin. The stage of development from the raised bog and wetland to the mid-stage lake occurs only on hills; the isolation of such water bodies from each other is a characteristic feature. During the formation of a group of closely located lakes, this section of the relief decreases and a common valley for these lakes is formed. As a result, most mature lakes are located in the lower part of the valley; a lack of isolation due to the common hydrographic network and the flooded plain are characteristic features. Water bodies of earlier stages are located on the edge of the valley that formed. All khasyreis are lower floodplain water bodies with clear brown water and a large amount of detritus (where a Gammarus-type benthic community is formed).

Some late-stage lakes remain isolated from the valley due to the absence of a breakthrough in the high banks, so they do not turn into arcuate khasyrei. The characteristic features of such lakes are steep eroded banks and sandy–silty shallow muddy water and a small amount and low diversity of zoobenthos with a dominance of small bivalves. The relationship between the stage of succession and the position of the reservoir in the basin is clearly manifested when the water bodies are aligned along the gradient of their length along the median line within each identified type (Fig. 1).

Fig. 1.
figure 1

Relationship between the size (average length, m) of thermokarst water bodies, the stage of the succession cycle (I–VIII), and the position of the reservoir in the basin (up, upper; mid, middle; low, lower).

Full size image

According to a number of parameters (Table 5), some water bodies are characterized as intermediate between the selected types. When calculating the average values for each type, these water bodies have been combined with the earlier stage (VI+ and VII+) based on the size and shape of the water body.

At the beginning of the cycle (swamp and wetland), when silt is nearly absent but phytoperiphyton abundance is high, the highest abundance and biomass of zoobenthos is observed in the chironomid-type community, where phytophagous animals dominate. As the lake grows and soft sediments accumulate, the zoobenthos abundance decreases (Fig. 2), but its taxonomic diversity increases (Fig. 3) together with the increase in microbiotope diversity along the coastline.

Fig. 2.
figure 2

Average abundance (N, thousand ind./m2) and biomass (B, g/m2) of zoobenthos at different stages of succession of thermokarst water bodies in the central Yamal (I–VIII) and the average length of water bodies of this type (L, 10 m).

Full size image
Fig. 3.
figure 3

Total number of zoobenthos species (S) in thermokarst water bodies of the central Yamal at different stages of succession (I–VIII).

Full size image

At the lake stages, the most complex community develops, where bivalves, oligochaetes, and chironomids dominate by biomass. At the same time, zoobenthos is nearly absent at large peat beaches of late-stage lakes, and only single mollusks may be found. However, the diversity of zoobenthos is much higher on the silty bottom and in the thickets of late-stage lakes (Fig. 3).

After the lake discharge and the formation of a khasyrei rich in detritus, the taxonomic diversity of zoobenthos gradually decreases down to the level of accessory water bodies. First the proportion of gammarids increases significantly, and then that of gastropods, so they form together the basis of the zoobenthos until the reservoir disappears and the cycle starts over. Consequently, the predominant feeding type of zoobenthos gradually changes from phytophagous through a wide range (including filtration) to phytophagous and detritus feeders. During succession, small forms give way to larger ones, and the zoobenthos abundance decreases gradually. At the lake stages, the overall taxonomic diversity increases, but the biomass decreases. At the khasyrei stages, after the dominant complex has changed, the diversity and biomass of zoobenthos return to the prelake level.

The above description is preliminary and phenomenological; it is based on the search for internally consistent trends. There is still not enough data for a deep statistical analysis, so the method of primary reconnaissance analysis has been applied. In particular, a dendrogram of the Euclidean distance has been constructed by the method of single links for generalized species lists of different stages of the lake cycle. Despite the changes in the dominant complex, the benthic fauna of the extreme shallow water stages (swamps, wetlands, and khasyreis) unites together with the oxbow lake into a common cluster, which is successively nested in clusters of lakes of the early, middle, and late stages of development. This indicates similar conditions for bottom communities in shallow water bodies of the tundra, i.e., a low taxonomic diversity of zoobenthos with a high biomass. On the other hand, all shallow-water objects and lakes are characterized by high taxonomic diversity, but low abundance and low biomass. As expected, the Jaccard similarity index for the generalized species lists of invertebrates in water bodies of different stages of succession testifies to the greatest change in the composition of zoobenthos during the transition from the old khasyrei to the bog (VIII–I). The greatest similarity (almost the same level) is noted within different lakes (III–IV–V) and within different khasyreis (VI–VII–VIII). This indicates similar habitat conditions within these categories, as well as a significant change in the living conditions and species composition during the transformation of the lake into khasyrei (V–VI), which can be explained by a sharp change in the reservoir depth. The number of unique species in the taxonomic lists of water bodies of different stages starts from a maximum of 41.2% in the swampy tundra (I) and decreases to zero in late-stage khasyreis (VIII).

A preliminary classification of thermokarst water bodies is presented for a typical moss–lichen tundra of the central Yamal by the example of the Seyakha-Mordyyakha interfluve. Eight types of water bodies have been identified, representing the stages of the succession cycle. The succession of several closely located thermokarst water bodies is interconnected, resulting in the formation of a common valley and hydrographic network. Water bodies of the initial stages of the cycle are located on hills, lakes of the middle and late stages may be located at the edge of the valleys, and the most mature lakes and all khasyreis are obligatorily located in the lower part of the valleys. Some separately located old lakes remain isolated on the watersheds; they are characterized by muddy water bodies with steep eroded banks.

The cycle begins with a flat–hilly swamp (stage I). These are temporary water bodies and channels among sedge tussocks and shrubs. Such water bodies are hypereutrophic and rich in phytoperiphyton. The zoobenthos is represented by 17 species; the abundance reaches 23 400 ind./m2 and biomass 73.1 g/m2. These are water bodies of the “chironomid” type, in which scrapers feeding on phytoperiphyton predominate. Larvae of the mosquito Chironomus (Camptochironomus) macani form most of biomass (up to 86%; B dominant).

The lowering of the swamp area leads to the formation of the prelake, or mochaga (II). These are small, oval, eutrophic water bodies up to 50 m in diameter. Their bottom is represented by the flooded vegetation of the surrounding tundra, rich in phytoperiphyton and detritus. Zoobenthos is represented by 22 species; abundance of 11.1–12.6 thousand ind./m2 and biomass of 25.2–39.8 g/m2. These are still water bodies of the chironomid type, where scrapers predominate. Chironomus (Camptochironomus) tentans (≤84%) and Chironomus nigrifrons (≤42%) are B dominants.

A further increase in the wetland due to the melting of the permafrost beneath it leads to the formation of an early-stage lake (III). Deep round water bodies up to 1000 m in diameter appear, characterized by a silty bottom overgrown with hydrophytes (mainly Potamogeton). Apparently, the presence of pondweeds may be considered a distinctive feature of the true lakes, but this requires additional research. Compared to the previous stages, the trophic status of the reservoir significantly decreases (from β-mesotrophic to ultraoligotrophic), as does zoobenthos abundance, but the species diversity increases along with an increase in the diversity of microbiotopes. The variability of the trophic behavior of zoobenthos significantly increases; zoobenthos is represented by 54 species (0.5–2.9 thousand ind./m2 and 0.7–10.1 g/m2). B dominants are Micruropus sp. (≤57%), Henslowiana sp. (≤52%), Gammarus lacustris (≤48%), Limnodrilus hoffmeisteri (≤46%), and Lumbriculus variegatus (≤32%).

The shores of young lakes are overgrown with helophytes; sometimes there are quagmires covered with coastal vegetation form. There are deformed lakes of the middle stage (IV) with a diameter of up to 1500 m and with a curved coastline and humic-colored water. According to our observations, the sphagnum quagmire of tundra lakes in the central Yamal is significantly smaller in size than the quagmire of forest tundra (Nadym) and even more forest lakes (Nizhnevartovsk) of the permafrost zone. It is likely that thick deposits of peat were formed in Yamal during a warmer epoch, while its erosion and redeposition is taking place in modern lakes. The trophic status of midstage lakes varies from α-mesotrophic to α-eutrophic. Zoobenthos is represented by 42 species (3.0–10.8 thousand ind./m2 and 4.1–19.8 g/m2). These are definitely water bodies of the mollusk type, where small filtering bivalve mollusks predominate. In the bottom communities, Euglesa (Casertiana) sp. (≤28%) is B-dominant; in helophyte thickets, it is Cincinna (Sibirovalvata) confusa (≤19%).

The further growth of lakes often leads to the uniting of neighboring water bodies, which gives them the sunglass shape. The largest late-stage lakes (V) are formed; they are several kilometers long. Due to the large area, wind-wave processes are most pronounced here; sandy beaches and deposits of old peat appear along the water’s edge. The trophic status is ultraoligotrophic; it was α-eutrophic only in one case. Zoobenthos is represented by 87 species (0.02–4.9 thousand ind./m2 and 0.05–17.8 g/m2). These are still water bodies of the mollusk type. B dominants are Henslowiana (Arcteuglesa) sp. (≤100%), Chironomus fl. plumosus (≤84%), Sphaerium (Asyociclas) asiaticum (≤65%), and Chanomphalus (Pseudogyraulus) sp. (≤40%).

A growing lake comes into contact with a river or another lower reservoir, discharges water into it, and becomes shallow, dividing into peat deposits and an arcuate reservoir (khasyrei) at an early stage of development (VI). These residual water bodies, up to several kilometers long and with a silty–detrital bottom, are slowly overgrown with semisubmerged vegetation. As the depth decreases, the trophic status of water bodies begins to increase again (from α-eutrophic to hypereutrophic), and the diversity of zoobenthos decreases. Zoobenthos is represented by 49 species (3300–7500 ind./m2 and 10.9–49.3 g/m2). In bottom communities, B dominants are Sphaerium (Sibirisphaerium) levinodis (≤39%) and Pisidium amnicum (≤38%); in coastal thickets of helophytes, they are Chanomphalus (Pseudogyraulus) sp. (≤30%). Mollusks still predominate, but the proportion of detritus feeders is increasing.

As a result of secondary freezing of the talik and permafrost heaving, the khasyrei continues to shallow; its banks are overgrown with tundra vegetation. These are remaining water bodies of a distinctly crescent form, i.e., khasyreis of the middle stage (VII) with a length of less than 1500 m. The trophic status is high again, from α-eutrophic to β-eutrophic. Collecting species play the main role in food chains. Now they are water bodies of a definitely Gammarus type. Zoobenthos is represented by 45 species (3.3–7.5 thousand ind./m2 and 10.1–49.3 g/m2). B dominants are Micruropus sp. (≤39%) and Gammarus lacustris (≤37%).

At the end of the cycle, the mid-stage khasyrei breaks up into a crescent-shaped row of separate small water bodies less than 500 m in length, or khasyreis of the late stage (VIII). The latter completely overgrow later with tundra vegetation. The trophic status of such water bodies returns to a maximum (from α-eutrophic to hypereutrophic), but these are water bodies of the Gammarus type, but not of the chironomid type, as at the beginning of the cycle. Zoobenthos is represented by 28 species (1.9–5.3 thousand ind./m2 and 10.3–66.5 g/m2). B dominants are Gammarus lacustris (≤59%) and Chanomphalus (Pseudogyraulus) sp. (≤29%).

A separate type of frequently encountered water bodies are river oxbow lakes, similar to crescent midstage khasyrei. Howeve, in contrast to khasyrei, the role of amphipods is small here, and gastropods and chironomids predominate. The trophic status of the examined oxbow lake is α-eutrophic. Zoobenthos is represented by 32 species (6600 ind./m2 and 18.2 g/m2). B dominants are Chironomus fl. plumosus (≤71%) and Cincinna (Sibirovalvata) confusa (≤37%).

CONCLUSIONS

In 2015, 2018, and 2019, 161 species of zoobenthos were found in 25 water bodies in the area of the Bovanenkovo oil and gas condensate field. Zoobenthos diversity and abundance varied over a wide range: the number of species per sample was 1–33, abundance was 0.02–23.40 thousand ind./m2, and biomass was 0.05–73.09 g/m2. Eight types of thermokarst water bodies have been identified for a typical moss–lichen tundra of the central Yamal. These types corresponded to different stages of successional cycle: (I) temporary reservoir of a flat–hilly swamp, (II) mochaga (prelake), (III) early-stage lake, (IV) midstage lake, (V) late-stage lake, (VI) early-stage khasyrei, (VII) mid-stage khasyrei, and (VIII) late-stage khasyrei. The water bodies of the initial stages of succession (I–IV) were located on the uplands, the lakes of the subsequent stages (IV–V) at the edge of the valleys, and most of the lakes of the late stage of development and all khasyreis (V–VIII) were in the lower part of the valleys. The trophic status of water bodies changed from hypereutrophic at the beginning of the cycle to ultraoligotrophic at intermediate stages of lakes and again to hypereutrophic at the end of the cycle. The predominant type of nutrition of zoobenthos changed accordingly from phytophagy, through a wide spectrum (including filtration), to phytophagy and detritus-feeding. The biomass-dominating complex changed as well, from chironomid-type at the beginning of the cycle, through the predominance of bivalves and oligochaetes at the lake stages, to the predominance of gastropods and gammarids at the end of the cycle. The decrease in zoobenthos abundance during succession was due to the replacement of small forms by larger ones.