# Environmental factors controlling seasonal and spatial variability of zooplankton in thermokarst lakes along a permafrost gradient of Western Siberia

Yury A. Noskov, Rinat M. Manasypov, Nadezhda I. Ermolaeva, Denis V.

Antonets, Liudmila S. Shirokova, Oleg S. Pokrovsky

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Yury A. Noskov, Rinat M. Manasypov, Nadezhda I. Ermolaeva, Denis V. Antonets, Liudmila S. Shirokova, et al.. Environmental factors controlling seasonal and spatial variability of zooplankton in thermokarst lakes along a permafrost gradient of Western Siberia. Science of the Total Environment, 2024, 922, pp.171284. 10.1016/j.scitotenv.2024.171284. insu-04831472

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# Environmental factors controlling seasonal and spatial variability of zooplankton in thermokarst lakes along a permafrost gradient of Western Siberia

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Yury A. Noskov<sup>a,b,*</sup>, Rinat M. Manasypov<sup>a</sup>, Nadezhda I. Ermolaeva<sup>c</sup>, Denis V. Antonets<sup>d</sup>, Liudmila S. Shirokova<sup>e</sup>, Oleg S. Pokrovsky<sup>f,**</sup>

## HIGHLIGHTS

* Seasonal and spatial variability of zooplankton composition in thermokarst lakes

* Water temperature, lake size, pH, DOC, and pCO₂ controlled zooplankton development.

* Copepods were positively related to lake depth and pH and increased northward.

* Cladocerans decreased in abundance from sporadic to continuous permafrost zone.

* Partial disappearance of copepods and increase of cladocerans in the north upon climate warming

## GRAPHICAL ABSTRACT

<figure><img src="image_4.png" /></figure>

## ARTICLE INFO

Editor: Ashantha Goonetilleke

**Keywords:**

Plankton

Humic lakes

Peatland

Cladocera

Copepoda

Rotifera

Organic carbon

## ABSTRACT

Humic thermokarst lakes of permafrost peatlands in Western Siberia Lowland (WSL) are major environmental controllers of carbon and nutrient storage in inland waters and greenhouse gases emissions to the atmosphere in the subarctic. In contrast to sizable former research devoted to hydrochemical and hydrobiological (phyto- plankton) composition, zooplankton communities of these thermokarst lakes and thaw ponds remain poorly understood, especially along the latitudinal gradient, which is a perfect predictor of permafrost zones. To fill this gap, 69 thermokarst lakes of the WSL were sampled using unprecedented spatial coverage, from continuous to sporadic permafrost zone, in order to assess zooplankton (Cladocera, Copepoda, Rotifera) diversity and abun- dance across three main open water physiological seasons (spring, summer and autumn). We aimed at assessing the relationship of environmental factors (water column hydrochemistry, nutrients, and phytoplankton

<footnote><sup>a</sup> Biological Institute, BIO-GEO-CLIM Laboratory, Tomsk State University, 36 Lenin av., 634050 Tomsk, Russia</footnote>

<footnote><sup>b</sup> Institute of Systematics and Ecology of Animals SB RAS, 11 Frunze str., 630091 Novosibirsk, Russia</footnote>

<footnote><sup>c</sup> Institute for Water and Environmental Problems SB RAS, 1 Molodezhnaya str., 656038 Barnaul, Russia</footnote>

<footnote><sup>d</sup> MSU Institute for Artificial Intelligence, Lomonosov Moscow State University, 119192 Moscow, Russia</footnote>

<footnote><sup>e</sup> Federal Center for Integrated Arctic Research, Institute of Ecological Problem of the North, 23 Nab. Severnoi Dviny, 163000 Arkhangelsk, Russia</footnote>

<footnote><sup>f</sup> GET UMR 5563 CNRS University of Toulouse (France), 14 Avenue Edouard Belin, 31400 Toulouse, France</footnote>

<footnote>* Corresponding author at: Biological Institute, BIO-GEO-CLIM Laboratory, Tomsk State University, 36 Lenin av., 634050 Tomsk, Russia.</footnote>

<footnote>** Corresponding author.</footnote>

<footnote>E-mail addresses: yunoskov@gmail.com (Y.A. Noskov), oleg.pokrovsky@get.omp.eu (O.S. Pokrovsky).</footnote>
Nutrients
Arctic ponds

parameters) with the abundance and diversity of zooplankton. A total of 74 zooplankton species and taxa were detected, with an average eight taxa per lake/pond. Species richness increased towards the north and reached the maximum in the continuous permafrost zone with 13 species found in this zone only. In contrast, the number of species per waterbody decreased towards the north, which was mainly associated with a decrease in the number of cladocerans. Abundance and diversity of specific zooplankton groups strongly varied across the seasons and permafrost zones. Among the main environmental controllers, Redundancy Analysis revealed that water temperature, lake area, depth, pH, Dissolved Inorganic and Organic Carbon and CO₂ concentrations were closely related to zooplankton abundance. Cladocerans were positively related to water temperature during all seasons. Copepods were positively related to depth and lake water pH in all seasons. Rotifers were related to different factors in each season, but were most strongly associated with DOC, depth, CH₄, phytoplankton and cladoceran abundance. Under climate warming scenario, considering water temperature increase and permafrost boundary shift northward, one can expect an increase in the diversity and abundance of cladocerans towards the north which can lead to partial disappearance of copepods, especially rare calanoid species.

# 1. Introduction

High-latitude limnetic systems are important elements in the carbon and greenhouse gases cycles (Serikova et al., 2019; Karlsson et al., 2021). At the same time, due to their high vulnerability to the climate warming and anthropogenic pressure (Vonk et al., 2015; Walvoord and Kurylyk, 2016; Wik et al., 2016; Turetsky et al., 2019), these systems can act as important sentinels of ongoing environmental changes. The maximal number and areal coverage of lakes and ponds occur in continental plains (wetlands) that present high storage of both surface and soil water and organic carbon, in the form of peat deposits, lake sediments and dissolved organic carbon (DOC) pool. Among these regions, thermokarst lakes of permafrost peatlands in Western Siberia Lowland (WSL, 2 million km²) are especially interesting due to their location within the climate and permafrost gradient on similar relief and lithology (Manasypov et al., 2015, 2021). Over the past decade, sizable research efforts demonstrated that WSL lakes act as major environmental controllers of carbon and metal storage in inland waters (Polishchuk et al., 2017, 2018) together with greenhouse gases emissions to the atmosphere (Serikova et al., 2019; Shirokova et al., 2021). Together with bacterioplankton and phytoplankton, zooplankton exerts the major impact on C and nutrient cycle, C storage in the biomass and CO₂ and CH₄ emissions to the atmosphere. Given that thermokarst lakes of Western Siberia are typically devoid of higher organisms such as fish (Pokrovsky et al., 2014a, 2014b), zooplankton represents a terminal part of the food web, capable of accumulating toxicants and controlling the fluxes of C and nutrients from the water column to the sediments.

In contrast to sizable former research devoted to WSL lake hydrochemistry (Pokrovsky et al., 2011, 2013; Shirokova et al., 2013; Pokrovsky et al., 2016; Manasypov et al., 2015, 2020), phytoplankton (Pavlova et al., 2016; Pokrovsky et al., 2022), sedimentary carbon (Audry et al., 2011; Manasypov et al., 2022), and processes controlling their total dissolved solute composition (Manasypov et al., 2017; Kuzmina et al., 2022), zooplankton characteristics of WSL lakes remained virtually unknown, especially across the permafrost gradient in this environmentally-important region. Furthermore, whereas zooplankton communities of Arctic and Subarctic lakes outside of Western Siberia were studied in sufficient details (Rautio, 2001; O'Brien et al., 2004; Samchyshyna et al., 2008; Carter and Schindler, 2012; Paterson et al., 2014; Bégin and Vincent, 2017; Hayden et al., 2017; Novichkova and Azovsky, 2017; MacLeod et al., 2018; Beaver et al., 2019; Lau et al., 2020), the majority of works on zooplankton in Siberian inland waters concern southern, permafrost-free zone (i.e., 80 references Web of Science by 2024), and only a few works addressed zooplankton features in permafrost-affected Siberian waters, exclusively in northeastern Siberia such as the Lena River basin (Nigamatzyanova et al., 2016; Abramova et al., 2017; Frolova and Nigamatzyanova, 2019; Chertoprud and Novichkova, 2021; Chertoprud et al., 2023) and the Anadyr region (Streletskaya, 2010). Recently, limited data became available on zooplankton in permafrost peatlands of NE European Arctic (Nazarova et al., 2021; Sobko et al., 2023) and high altitude permafrost-affected

lakes of Tibet (Lin et al., 2017; Ren et al., 2022; Su et al., 2018, 2022; Zhao et al., 2022). Overall, the information on zooplankton in peatland-hosted (organic-rich) thermokarst waters is highly limited.

The present work is devoted to characterizing environmental factors controlling zooplankton distribution in thermokarst lakes of Western Siberia. We aimed to i) quantify zooplankton abundance and diversity across seasons and permafrost zones, and ii) assess the relationships between specific zooplankton groups and major potential external factors controlling zooplankton distribution, such as seasons, permafrost coverage, phytoplankton, and water column chemistry (pH, [DOC], greenhouse gases, nutrients). Based on the knowledge of freshwater plankton diversity in high latitude regions, which stems essentially from studies on European and Canadian Arctic (e.g., Pinel-Alloul et al., 2013; Anas et al., 2015; Bégin and Vincent, 2017; Lau et al., 2020; Vucic et al., 2020; Sobko et al., 2023), as well as on northern parts of Eastern Siberia (Dubovskaya et al., 2010; Abramova et al., 2017; Novichkova and Chertoprud, 2020; Chertoprud and Novichkova, 2021; Chertoprud et al., 2023) and two large-scale circumpolar investigations (Novichkova and Azovsky, 2017; Schartau et al., 2022) we hypothesized that i) along a permafrost and climate gradient in Western Siberia, the species richness and zooplankton abundance will decrease northward, from sporadic to continuous permafrost zone; ii) the abundance and composition of zooplankton in thermokarst lakes of the WSL will be related to phytoplankton and nutrients concentration, given that these isolated waterbodies lack higher-level consumers such as fish; iii) there will be a shift in the composition of zooplankton communities towards the north, from the predominance of cladocerans to copepods. We tested these hypotheses along a sizable (~1500 km) latitudinal gradient of sporadic, discontinuous and continuous permafrost conditions by selecting several lakes and ponds of various sizes in each zone. We anticipate that assessing environmental factors that control both zooplankton abundance and diversity across permafrost and climate gradients over different seasons should allow integrating the acquired knowledge into pan-Artic hydrobiological models incorporating external factor control on the biota of limnetic systems.

# 2. Materials and methods

## 2.1. Thermokarst lakes and ponds of Western Siberia

Western Siberia Lowland comprises highly sensitive and potentially unstable limnic ecosystems that could play a pivotal role in the modification of the carbon (C) cycle under future warmer climate conditions (Smith et al., 2007; Anisimov et al., 2013). The latitudinal gradient in the WSL is known to be a perfect predictor of permafrost zones (Manasypov et al., 2020; Pokrovsky et al., 2022). The thermokarst (thaw) lakes of the WSL are located essentially within the sporadic, discontinuous and continuous permafrost zones and belong to the northern taiga, forest-tundra and tundra biomes (Fig. 1). Here, the permafrost zones are defined according to the Global Terrestrial Network for Permafrost (Biskaborn et al., 2015). Despite the fact that the boundaries between
permafrost zones are not strictly defined and the type of soil permafrost is directly related to latitude, we used the division into permafrost zones in describing the results, since hydrochemical indicators of the lakes vary between different zones (Fig. S1 of the Supplement; Manasypov et al., 2020).

All thermokarst lakes studied in this work were formed due to thawing of frozen peat (histosols). At the lake watershed, the peat is in average 1 to 2 m thick and underlayed by sand and clay substrates. The physio-geographical and climate parameters of three main studied sites are provided in Table S1. The same set of thermokarst lakes and thaw ponds sampled for a previous study of phytoplankton (i.e., Pokrovsky et al., 2022) and hydrochemistry (Manasypov et al., 2020) was used in this work about zooplankton communities. The lakes (> 10,000 m²) and ponds (10–10,000 m²) are fishless, shallow (0.5 to 1.5 m depth) and ranged in size from 12 m² to 1.19 km² (see Manasypov et al., 2020 for details), which is typical for most lentic systems of Western Siberia (Polishchuk et al., 2017, 2018). All the lakes are non-connected closed basins (no inlet/outlet) located within permafrost peatlands, developed over similar peat (histosols) and sand/silt mineral substrate. The lakes are rounded, flat/concave shallow basins with very small catchments so that the watershed area typically does not exceed the water surface of the lake. The distance between lakes in each selected permafrost zone strongly (1 to 2 order of magnitude) exceeded the size of the lake catchment. Because of extremely flat relief of the WSL, the lakes and ponds are not nested hydrologically, i.e., the small ponds are not within the catchments of larger lakes. Considering sizable variation in lake water area, lack of lake connection to the groundwater and extremely small catchment size, and given that our lakes are non-interconnected, multiple lakes/ponds sampled in each site (permafrost zone) in this study can be considered as nearly independent (see Section 2.5 for details of statistical analyses).

Physical and chemical parameters of these water bodies have been extensively studied in the past across three main hydrological seasons, i.

e., May/June, August, and September/October, corresponding to biological spring, summer and autumn, respectively (Manasypov et al., 2015, 2020). Based on their nutritional status (Serikova et al., 2019), the lakes can be considered as oligotrophic to weakly mesotrophic. One particular feature of the WSL thaw ponds is that small water bodies (<1000 m²) exhibit low pH (between 3 and 4) and high DOC concentration (>20–30 mg L⁻¹). Due to their shallow depth and open surrounding area (tundra and forest-tundra), there is no significant thermal and redox stratification during open water seasons. The evolution of thermokarst lakes in the WSL starts from permafrost depressions and very small fens (<10 m²), which are transformed into small thaw ponds (10–10,000 m²) that actively grow via peat abrasion at the borders. Finally, in large thermokarst lakes (>10,000 m²), the shoreline is stabilized (Audry et al., 2011; Pokrovsky et al., 2014a, 2014b) and these lakes terminate their existence via drainage to another lake or a river. In this sequence of events due to permafrost thaw (“depression → thaw pond → thermokarst lake”) as determined in former studies (Pokrovsky et al., 2011; Loiko et al., 2020), there is a progressive increase in pH and [DIC] and a decrease in pCO₂ and nutrients with an increase in the lake size and age (Shirokova et al., 2013; Manasypov et al., 2020; Zabelina et al., 2021).

## 2.2. Lake sampling strategy

Similar to the design of a previous phytoplankton study (Pokrovsky et al., 2022), 69 lakes and ponds have been sampled in 2016 during open water period (no seasonal freeze/thaw event). This included biological Spring (20 May–13 June) after the ice melt, Summer (8–24 August) and Autumn (25 September–8 October), just before the ice-on. There were 15 to 19 lakes in each of four sites as detailed below. In the sporadic permafrost zone (Kogalym, Fig. 1), we sampled 19, 18 and 1 lakes in May, August and October, accordingly. Note that all except one lake in this zone in October were either dried out or frozen solid. In the

<figure><img src="image_5.png" /><figcaption>**Fig. 1.** Map of the study area in the Western Siberia Lowland (WSL). Four sampling sites – Kogalym, Khanymey + Urengoy, and Tazovsky – correspond to sporadic, discontinuous and continuous permafrost zones, respectively. A, Kogalym; B, Khanymey; C, Urengoy; D, Tazovsky. Satellite images (A–C) show all studied lakes in each area. Thick blue line demarks the boundary between the sporadic and the discontinuous permafrost zone.</figcaption></figure>
discontinuous permafrost zone (Khanymey and Urengoy), we sampled 32, 19 and 13 lakes in May, August and September, accordingly. Finally, in the continuous permafrost zone (Tazovsky), we sampled 16, 16, and 13 lakes in May, August and September, accordingly. The lakes were sampled in the direction from the south to the north in spring and from the north to the south in autumn, following the available road infrastructure of Western Siberia. This allowed collecting water column and zooplankton three times during open water period at approximately the same hydrological stage and biological season.

## 2.3. Lake water sampling and analyses

Sampling and analyses of pH, Specific Conductivity (S.C.), Specific UV absorbance at 254 nm (SUVA), DOC, CO₂, CH₄, DIC, major anions (Cl and SO₄), major cations (Na, Mg, Ca, K), macro-nutrients (P<sub>tot</sub>, P-PO₄, N-NO₃⁻, N-NH₄⁺, Si) concentrations were performed as described in details elsewhere (Manasypov et al., 2020; Serikova et al., 2019). The water was filtered in situ, immediately after sampling, through disposable Sartorius filter (0.45 µm) and the filtrates were collected into pre-cleaned polypropylene vials. For cation analysis by inductively coupled plasma mass spectrometry (ICP MS), the filtrate was acidified with bidistilled HNO₃. Major cations (Ca, Mg, Na, K), Si, and some trace elements were analyzed using an ICP-MS Agilent ce 7500 with In and Re as an internal standard and three various external standards, placed every 10 samples in a series of samples. The SLRS-5 (Riverine Water Reference Material for Trace Metals certified by the National Research Council of Canada) was used to check the accuracy of analyses. No acidification was used for preserving the filtrates in the second vial, collected for dissolved inorganic carbon (DIC), DOC, anions, and SUVA measurements. The samples were stored in a refrigerator at 4 °C during 1–2 weeks before analyses. Temperature, pH, and specific electrical conductivity (S.C.) were measured in the field (Multi 3430 m, WTW).

## 2.4. Zooplankton sampling and identification

A total of 147 zooplankton samples were collected from spring to autumn 2016. For the samples, 50 L of lake water was collected using a 10 L plastic bucket, filtered through 64 µm Apstein's net and immediately fixed with 4 % formaldehyde solution in 50 mL Falcon tubes. Samples were taken in the central part of the lake approximately the same time of the day (2–5 p.m.). Given specific features of thermokarst lakes in Western Siberia, such as shallow depth, the absence of a pronounced littoral zone and lack of large accumulations of macrophytes, as well as the long duration of daylight during the ice-free period, we assume that sampling period could not have a significant effect on the spatial distribution of zooplankton at different time of the day.

The total abundance of individuals was measured in Bogorov's chamber using Stemi 2000C stereomicroscope (Zeiss). Species identification has been carried out using Primo Star microscope (Zeiss). For zooplankton species identification both standard keys and recent taxonomic revisions have been used (Rylov, 1948; Borutsky, 1960; Kutikova, 1970; Alekseev and Tsalolikhin, 2010). Nauplial and copepodite stages of copepods have not been identified to species level.

## 2.5. Data treatment

The zooplankton diversity was evaluated using the Shannon (H), Pielou (P) and Simpson (S) indices based on the abundance. The species composition similarity of thermokarst lakes across different permafrost zones and seasons was measured by Sørensen-Dice index (Qs) (Wolda, 1981). Comparisons of zooplankton abundance between different permafrost zones and between seasons for normally distributed data (Shapiro-Wilk test, *p* > 0.05) were performed using one-way ANOVA followed by Tukey's or Fisher's least significance differences (LSD) post-hoc tests; for abnormally distributed data (Shapiro-Wilk test, *p* < 0.05), Kruskal-Wallis ANOVA followed by Dunn's post-hoc test was used.

Relationships between zooplankton abundance and hydrochemical parameters were determined by Spearman correlation analysis. Redundancy analysis (RDA) was applied to determine the relationships between zooplankton characteristics and environmental factors. Rare zooplankton taxa were removed if they did not occur in at least two lakes. Although there is significant correlation between the various factors, there is no clear redundancy in the environmental data. The absolute values of mutual correlation coefficients did not exceed 0.6–0.73. Thus, none of the analyzed variables can be excluded a priory (Fig. S2 of the Supplement). Environmental and biological data were log (x + 1) transformed prior performing RDA to reduce the importance of dominant taxa. The software STATISTICA 12 (StatSoft Inc., USA), PRISM 5 (GraphPad Software, USA) and PAST v. 4.12 (Hammer and Harper, 2001) were used for the analyses.

We have also used the GLMM approach to take into account the nested nature of our sampling design (both in space and time) and also to assess if addition of either latitude (Lat) or permafrost zone, or both as fixed effects can improve the model. Several random effects were assessed: particular lake and season combination; permafrost zone, lake and season combination; we also tested separate random effects for permafrost zone, lake and season and different combinations of those. The GLMM regression analysis was performed in R (v. 4.2.1) (R Core Team, 2013) using lme4 (v. 1.1-35.1) and DHARMa (v. 0.4.6) (Hartig, 2022) packages. Since the variables in our data had different scales, they were transformed and scaled. The pH column was taken unchanged; from Lat column we subtracted 60; column T (water temperature) values were divided by 10; columns Depth, PO₄, NO₃, NH₄ and TP were transformed with log1p, i.e. log(x + 1); S (lake area), O₂, S.C. (specific conductivity), CH₄, CO₂, Cl, SO₄, SUVA₂₄₅, [DOC], [DIC], phytoplankton abundance and phytoplankton biomass columns were log-transformed. We also created an additional dataset where variable values were standardized after the transformation, but did not see any significant difference when building GLMM models as compared to the unstandardized data and thus did not use it further. Model comparison and selection was based on ANOVA R function likelihood ratio test results and Akaike information criterion (AIC). The main results of the GLMM analysis are shown in Supplementary Material (Text Section 1, GLMM regression models).

Although the analyzed lakes are not interconnected, they cannot be treated as completely independent, since the dispersion of phyto- and zooplankton from neighboring lakes with wind and birds might also play a role. Therefore, Moran's test for autocorrelation was performed to check if there is a significant spatial autocorrelation in crustacean counts and environmental factors (Supplementary Material, Text Section 2, Spatial autocorrelation test).

# 3. Results

## 3.1. The diversity of zooplankton communities

With our GLMM analysis the only random effect that was found to be significant was the particular lake-season combination and the obtained results did not contradict to conclusions drawn from our correspondence analysis and ANOVA analysis. The GLMM results are presented the Supplementary Material Text Section 1.

In total, 74 zooplankton species and taxa were found in thermokarst lakes across the seasons and permafrost zones. Thirty of them belonged to Cladocera (25 Anomopoda, 3 Ctenopoda, 2 Onychopoda), 19 to Copepoda (13 Cyclopoida, 6 Calanoida), and 25 to Rotifera (see Table S2 of the Supplement for the full list of zooplankton). Average zooplankton species richness in the thermokarst lakes across seasons was low (8.4 ± 0.28), and differed significantly (Kruskal-Wallis test, chi² = 42.6, *p* < 0.0001), ranging from 1 to 17 species per lake. The highest (11.5 ± 0.5) and lowest (5.5 ± 0.6) average number of species per lake was recorded in spring in sporadic and continuous permafrost zones, respectively (Dunn's test, *p* < 0.001). The total species richness varied in different
permafrost zones and increased from south to north, reaching the maximum of 53 species in the continuous permafrost zone, 13 of which were found only in this zone, although sporadically (Table S2). On the contrary, the species richness per lake declined from the sporadic (9.6 ± 0.5) to continuous (6.6 ± 0.8) permafrost zone when averaged across seasons. It resulted essentially from a change in the number of cladoceran species, which decreased northward in all seasons (Table 1). Opposite to cladocerans, the species richness of copepods increased with latitude (northward) in every season. There were no regularities in the dynamic of the number of rotifer species in different permafrost zones and seasons. Their species richness was more stable in the continuous permafrost zone, compared to the other two zones (Table 1).

The most common species were small cladocerans *Bosmina* (Eubosmina coregoni Baird, 1857 and *Chydorus sphaericus* (O.F. Müller, 1785), which were found in 102 of 147 samples (69 %). The cladocerans *Acroperus elongatus* (Sars, 1862), *Alona* sp., *Alonella nana* (Baird, 1843), *Ceriodaphnia quadrangula* (O.F. Müller, 1785), *Polyphemus pediculus* (Linnaeus, 1761), the copepod *Cyclops vicinus vicinus* Uljanin, 1875 and the rotifers *Testudinella patina* (Hermann, 1783), *Conochilus unicornis* Rousselet, 1892, *Trichotria* sp. were also quite common and occurred in 23–52 % of the samples. The occurrence of other species did not exceed 20 %, most of which (50 taxa) were found in <10 % of the samples. The calanoid copepod *Arctodiaptomus acutilobatus acutilobatus* (Sars G. O., 1903) was found in high abundance only in the continuous permafrost zone.

The Shannon diversity index (H) based on abundance was low and ranged from 0.99 (spring, continuous zone) to 1.36 (spring, sporadic zone). In all seasons, there was a downward trend of the index from south to north (Fig. S3 A), which was most pronounced in spring (1-way ANOVA, F<sub>2.64</sub> = 2.98, *p* = 0.057). The Simpson index (S) of the abundance demonstrated a trend towards a decrease from the south to the north, which was pronounced only in spring (Fig. S3 B, Table 1). Values of both indices reflected a decrease in the level of the diversity and evenness with increasing latitude. It should be noted that, in the continuous permafrost zone, we observed a slight increase in the H and S indices from spring to autumn, which was caused by an increase in the evenness of the zooplankton, given that the species richness did not increase (Table 1).

When comparing the zooplankton species composition of different permafrost zones across seasons, the Sørensen-Dice index ($Q_S$) showed a

high degree of similarity – from 0.69 to 0.77, with the highest similarity being observed between sporadic and discontinuous permafrost zones and the lowest similarity between discontinuous and continuous zones (Table 2). Comparison of the $Q_S$ between permafrost zones in different seasons showed that the communities were most similar in summer and least similar in autumn (Table 2). Note that the similarity between sporadic and continuous permafrost zones in summer was higher than between sporadic and discontinuous zones (0.64 and 0.59, respectively).

## 3.2. Abundance and diversity of zooplankton in different seasons along the latitude

For crustacean abundance, we observed only a moderate to high spatial autocorrelation for Copepoda abundance in summer (which is directly associated with water temperature) and a moderate spatial autocorrelation for Cladocera abundance in autumn. However, for such factors as water temperature, pH, DIC, and SUVA<sub>245</sub>, we observed a statistically significant spatial autocorrelation ranging from moderate to very high in each season. Thus, given the fact that spatial autocorrelation of crustacean abundance was found to be much weaker than that of

<caption>**Table 2**</caption>

The Sørensen-Dice index of zooplankton species composition similarity in thermokarst lakes of Western Siberia Lowland between different permafrost zones in different seasons.

<table><caption>**Table 1**</caption><thead><tr><th>Total</th><th>Discontinuous</th><th>Continuous</th></tr></thead><tbody><tr><td><strong>Sporadic</strong></td><td>0.77</td><td>0.71</td></tr><tr><td><strong>Discontinuous</strong></td><td>1</td><td>0.69</td></tr><tr><td><strong>Spring</strong></td><td><strong>Discontinuous</strong></td><td><strong>Continuous</strong></td></tr><tr><td><strong>Sporadic</strong></td><td>0.76</td><td>0.38</td></tr><tr><td><strong>Discontinuous</strong></td><td>1</td><td>0.41</td></tr><tr><td><strong>Summer</strong></td><td><strong>Discontinuous</strong></td><td><strong>Continuous</strong></td></tr><tr><td><strong>Sporadic</strong></td><td>0.59</td><td>0.64</td></tr><tr><td><strong>Discontinuous</strong></td><td>1</td><td>0.68</td></tr><tr><td><strong>Autumn</strong></td><td><strong>Discontinuous</strong></td><td><strong>Continuous</strong></td></tr><tr><td><strong>Sporadic</strong></td><td>0.4</td><td>0.33</td></tr><tr><td><strong>Discontinuous</strong></td><td>1</td><td>0.55</td></tr></tbody></table>

Quantitative parameters of zooplankton in thermokarst lakes of Western Siberia Lowland, for each season and three main permafrost zones.

<table><thead><tr><th rowspan="2"></th><th colspan="3">Spring</th><th colspan="3">Summer</th><th colspan="3">Autumn</th><th rowspan="2">Cumulative data</th></tr><tr><th>Sporadic</th><th>Discontinuous</th><th>Continuous</th><th>Sporadic</th><th>Discontinuous</th><th>Continuous</th><th>Sporadic</th><th>Discontinuous</th><th>Continuous</th></tr></thead><tbody><tr><td>Number of species (n)</td><td>11.5 ± 0.5</td><td>7.5 ± 0.6</td><td>5.5 ± 0.6</td><td>8.5 ± 0.6</td><td>9.9 ± 0.5</td><td>7.9 ± 1</td><td>9</td><td>6.7 ± 0.7</td><td>6.5 ± 0.7</td><td>8.1 ± 0.6</td></tr><tr><td>n Cladocera</td><td>7.5 ± 0.3</td><td>4.6 ± 0.5</td><td>1.6 ± 0.4</td><td>7.2 ± 0.6</td><td>5.4 ± 0.5</td><td>3.8 ± 0.7</td><td>8</td><td>3.1 ± 0.5</td><td>1.7 ± 0.3</td><td>5.4 ± 0.5</td></tr><tr><td>n Copepoda</td><td>0.9 ± 0.2</td><td>1.2 ± 0.2</td><td>1.8 ± 0.2</td><td>1 ± 0.2</td><td>1.2 ± 0.2</td><td>2.1 ± 0.4</td><td>0</td><td>0.9 ± 0.4</td><td>2.2 ± 0.4</td><td>1.25 ± 0.3</td></tr><tr><td>n Rotifera</td><td>3.2 ± 0.4</td><td>2.1 ± 0.2</td><td>2.5 ± 0.3</td><td>0.5 ± 0.2</td><td>3.5 ± 0.5</td><td>2.4 ± 0.5</td><td>1</td><td>2.8 ± 0.6</td><td>2.8 ± 0.5</td><td>2.3 ± 0.4</td></tr><tr><td>Average abundance, thousand ind./m<sup>3</sup></td><td>7556 ± 1257</td><td>5994 ± 1834</td><td>11,300 ± 2971</td><td>8551 ± 2190</td><td>49,611 ± 35,210</td><td>6954 ± 1780</td><td>10,220</td><td>5845 ± 2920</td><td>4067.7 ± 1242</td><td>12,670 ± 4650</td></tr><tr><td>N Cladocera</td><td>2008 ± 489</td><td>1864 ± 577</td><td>133 ± 49</td><td>7642 ± 2150</td><td>41,545 ± 35,274</td><td>1440 ± 600</td><td>780</td><td>4592 ± 2782</td><td>165 ± 68</td><td>4570 ± 4600</td></tr><tr><td>N Copepoda</td><td>3381 ± 818</td><td>3534 ± 1764</td><td>9525 ± 2952</td><td>848 ± 248</td><td>2382 ± 751</td><td>5318 ± 1376</td><td>160</td><td>499 ± 248</td><td>1775 ± 564</td><td>3454 ± 573</td></tr><tr><td>N Rotifera</td><td>2166 ± 1155</td><td>597 ± 146</td><td>1644 ± 722</td><td>61 ± 36</td><td>5683 ± 1950</td><td>196 ± 76</td><td>9280</td><td>754 ± 316</td><td>2128 ± 1168</td><td>1670 ± 350</td></tr><tr><td>Shannon index (Hs) by abundance</td><td>1.36 ± 0.11</td><td>1.35 ± 0.09</td><td>0.99 ± 0.13</td><td>1.28 ± 0.1</td><td>1.23 ± 0.13</td><td>1.14 ± 0.14</td><td>1.4</td><td>1.25 ± 0.17</td><td>1.23 ± 0.15</td><td>1.24 ± 0.04</td></tr><tr><td>Simpson index (1-D) by abundance</td><td>0.57 ± 0.05</td><td>0.61 ± 0.04</td><td>0.47 ± 0.06</td><td>0.59 ± 0.04</td><td>0.55 ± 0.06</td><td>0.52 ± 0.06</td><td>0.68</td><td>0.58 ± 0.07</td><td>0.56 ± 0.06</td><td>0.56 ± 0.06</td></tr><tr><td>Equitability</td><td>0.53 ± 0.04</td><td>0.66 ± 0.04</td><td>0.5 ± 0.06</td><td>0.58 ± 0.04</td><td>0.52 ± 0.05</td><td>0.53 ± 0.05</td><td>0.61</td><td>0.66 ± 0.08</td><td>0.63 ± 0.05</td><td>0.58 ± 0.05</td></tr><tr><td>Evenness_eH/S</td><td>0.34 ± 0.04</td><td>0.55 ± 0.04</td><td>0.43 ± 0.04</td><td>0.44 ± 0.03</td><td>0.37 ± 0.04</td><td>0.42 ± 0.05</td><td>0.4</td><td>0.56 ± 0.07</td><td>0.54 ± 0.07</td><td>0.45 ± 0.04</td></tr></tbody></table>
environmental factors, we concluded that, with inclusion of these environmental factors into our models, we also accounted for spatial patterns in our data at a sufficient degree and that the inclusion of distances between the analyzed lakes was not necessary (Supplementary Material Text Section 2, Spatial autocorrelation test).

The total zooplankton abundance, when averaged across seasons, did not differ significantly between permafrost zones (1-way ANOVA, $F_{2,146} = 1.01, p = 0.36$). However, when averaged across permafrost zones, it significantly differed between seasons (1-way ANOVA, $F_{2,144} = 6.53, p = 0.002$), with lowest abundance observed in autumn, compared to spring and summer (Fisher's LSD, $p < 0.05$). There were significant differences in the abundance of zooplankton between adjacent permafrost zones in spring (1-way ANOVA, $F_{2,64} = 4.61, p = 0.013$) and summer (Kruskal-Wallis test, $\chi^2 = 6.4, p = 0.04$). In particular, the lowest and highest average abundances were observed in the discontinuous zone in spring (Tukey's test, $p < 0.05$) and in summer (Dunn's test, $p < 0.05$), respectively.

When comparing zooplankton specific groups, Cladocera significantly differed in abundance between permafrost zones (Kruskal-Wallis test, $\chi^2 = 40.4, p < 0.0001$; Fig. 2) and decreased significantly from south to north (Dunn's test, $p < 0.001$). Opposite to cladocerans, copepods increased in the abundance towards the north and also significantly differed between permafrost zones (Kruskal-Wallis test, $\chi^2 = 18.71, p < 0.0001$). Abundance of this group was significantly higher in the continuous zone, compared to the other zones (Dunn's test, $p < 0.001$). Abundance of rotifers varied between the zones (Kruskal-Wallis test, $\chi^2 = 6.53, p = 0.038$), with the lowest values observed in the sporadic and highest values in the discontinuous permafrost zones (Dunn's test, $p = 0.01$).

Zooplankton can be roughly divided by feeding type into “predatory” (active grabbers) and “peaceful” (filter feeders and gatherers). Most rotifers, cladocerans and copepod nauplii are peaceful forms that consume algae, bacteria and detritus. Exceptions in our collections are the predatory *P. pediculus*, *Bithotrephes crassicadus* (Sars, 1890), and *Asplanchna* sp. Adult cyclops are mostly active grabbers. Along with this, some species (for example, *C. vicinus*) are capable of consuming various groups of phytoplankton with sufficient efficiency, whereas the benthic forms, such as *Paracyclops affinis* (Sars, 1863), *Paracyclops fimbriatus* (Fischer, 1853), and *Eucyclops* sp. are gatherers. Most calanoid copepods are also filter feeders, consuming mostly diatoms and detritus, with the exception of *Heterocope*, which is active grabber but has a mixed diet (phytoplankton/detritus/zooplankton) (Lampert, 1988; Lampert and Sommer, 2007).

In spring, copepods dominated in all permafrost zones, with ratio ranging from 45% (sporadic zone) to 84% (continuous zone) of the total abundance (Fig. 3A). Nauplii were the most abundant among copepods and the other groups (37–71% of total abundance). In the sporadic zone, in addition to nauplii (37% of the total abundance), *Kellicottia longispina*

<figure><img src="image_6.png" /><figcaption>**Fig. 3.** Partial contribution of Cladocera (white), Copepoda (light grey) and Rotifera (dark grey) in thermokarst lakes of Western Siberia Lowland in spring (A), summer (B) and autumn (C) across sporadic (left vertical panel), discontinuous (middle vertical panel) and continuous (right vertical panel) permafrost zones. In total, 147 samples were used for the analysis. Note that only one lake was examined in the sporadic permafrost zone.</figcaption></figure>

(Kellicott, 1879) (14 %), *Keratella* sp. (8 %), *Ch. sphaericus* (7 %) and *B. (E.) coregoni* (5 %) were also abundant. In the discontinuous zone, nauplii predominated in abundance (54 %), whereas *B. (E.) coregoni* (12.6 %) and *T. patina* (6 %) were subdominants. In the continuous zone, nauplii were the most abundant (71 %), followed by *T. patina* (11 %) and *Cyclops scutifer* G. O. Sars, 1863 (8 %). The proportion of filter feeders in the sporadic zone was 84 %, including species with mixed nutrition that consumed detritus and phytoplankton (28 %); the proportion of predators was 16 %. In the discontinuous zone, the proportion of filter feeders decreased to 74 % (31 % of species with mixed nutrition), whereas the proportion of predators was 26 %. In the continuous permafrost zone, most of the community were represented by active predators (79 %), and the proportion of filter feeders decreased to 21 % (5 % of them were verticators, filter feeders and sedimentators, and remaining 16 % were species with mixed nutrition, including consuming detritus).

<figure><img src="image_7.png" /><figcaption>**Fig. 2.** Average abundance (mean ± SEM) of Cladocera, Copepoda and Rotifera in thermokarst lakes of Western Siberia Lowland in sporadic (Kogalym, 38 samples), discontinuous (Khanymey and Urengoy, 64 samples) and continuous (Tazovsky, 45 samples) permafrost zones (shown in the legend). For convenience, the data from three seasons are pooled. Different letters above whiskers corresponds to significance of the difference between permafrost zones.</figcaption></figure>
In summer, cladocerans predominated in the sporadic and discontinuous zones (89 % and 84 % of the total abundance, respectively; Fig. 3B); copepods were more abundant in the continuous zone (76 %). In the sporadic zone *B. (E.) coregoni* (45 %) and *P. pediculus* (28 %) were the most abundant species. One species predominated in the discontinuous zone – *B. (E.) coregoni* (79 %). Nauplii (52 %) were most abundant in the continuous zone, *A. acutilobatus acutilobatus* (15 %), *Ch. sphaericus* (10 %), *B. (E.) coregoni* (8 %) and copepodites (6 %) were subdominants. The main part of the zooplankton community in lakes of the sporadic permafrost zone was represented by filter feeders (51 %). Organisms with a mixed diet, such as scrapers and gatherers, accounted for 16 % of the total abundance, and the proportion of predators was 33 %. In the discontinuous zone, the proportion of filter feeders was significantly higher (92 %) than in the sporadic zone (of which only 9 % were species with mixed feeding) and the proportion of predators, respectively, was 8 %. In the continuous zone, the zooplankton community was predominated by detritivorous gatherers (67 %), followed by phytophilic filter feeders (26 %). The proportion of predatory

organisms decreased to 7 %.

In autumn, zooplankton communities of the thermokarst lakes differed more pronouncedly in terms of dominants' composition (Fig. 3C). Sporadic permafrost zone was characterized by the predominance of rotifers, specifically *T. patina* (46 %), *Asplanchna* sp. (28 %), *S. pectinata* (17 %). The most abundant species in the discontinuous zone was *B. (E.) coregoni* (74 %), copepodites were subdominants (7 %). In the continuous permafrost zone, *S. pectinata* (33 %), *A. acutilobatus acutilobatus* (24 %), copepodits (14 %) and *Keratella* sp. (10 %) predominated in the community. In the sporadic zone, predatory forms with active food capture predominated (92 %). The proportion of filter feeders was 4.9 %, and species with mixed nutrition constituted only 2.7 %. In the discontinuous and continuous permafrost zones, the most abundant were filter feeders and detritivores, accounting for 88 and 8.7 %; 57 and 34 % of the total abundance, respectively. The proportion of predators in the discontinuous zone was 3.1 %, and in the continuous zone it reached 8.6 %.

To summarize, zooplankton specific groups' abundance significantly

<figure><img src="image_8.png" /><figcaption>**Fig. 4.** RDA of zooplankton communities and environmental factors of Western Siberia Lowland thermokarst lakes in latitudinal gradient across all seasons and permafrost zones. In total, 147 samples were used for the analysis. Sampling sites symbolled as follows: red dot – Sporadic zone; yellow square – Discontinuous zone; blue triangle – Continuous zone. Abbreviations: N Clad, N Cop and N Rot – total abundance of Cladocera, Copepoda and Rotifera, respectively; nClad, nCop, nRot – number of species of Cladocera, Copepoda and Rotifera, respectively; N – total zooplankton abundance; n – total number of species; Phyt.(N), Phyt(B) – total number of cells and biomass of phytoplankton, respectively; log Area – log transformed lake sizes; Depth – lake depths; S.C. – specific conductivity.</figcaption></figure>
varied between seasons and permafrost zones. Cladocerans decreased and copepods increased in abundance towards the north. The highest abundance of Cladocera, Rotifera and Copepoda were registered in the sporadic, discontinuous and continuous permafrost zones, respectively. Most abundant species, except juvenile copepods, in different seasons and permafrost zones were *K. longispina*, *Keratella* sp., *Ch. sphaericus*, *B. (E.) coregoni*, *T. patina*, *C. scutifer*, *P. pediculus*, *A. acutilobatus acutilobatus*, *Ch. sphaericus*, *S. pectinata*, *Asplanchna* sp.

### 3.3. Multifactor analyses of possible environmental controllers

The phytoplankton cell number and biomass of the same lakes sampled during the same seasons (Pokrovsky et al., 2022) did not exhibit any correlation with total zooplankton abundance (not shown). Considering only filter feeders, we found a weak positive correlation with phytoplankton biomass (Spearman $r = 0.24, p = 0.04$). No correlation between filter feeders' abundance and phytoplankton biomass/abundance in specific season and/or permafrost zone was registered.

The RDA of full dataset demonstrated that the first two axes are capable explaining 27.5 % of the zooplankton variation and was statistically significant (permutation test: $F = 3.627, adjusted R^2 = 0.255, p = 0.001$). Cladoceran abundance was positively associated with water temperature, and negatively associated with latitude, lake area, depth, pH, DIC and $CO_2$ concentration (Fig. 4). Copepods' abundance (mainly nauplii) was positively related to pH, [DIC], depth and lake area and negatively associated with [DOC] and phytoplankton biomass (Fig. 4). Among environmental factors positively related with abundance of rotifers were $[CH_4]$, phytoplankton abundance, [TP] and $CO_2$; negative relationships were observed with water temperature and $[SO_4]$.

The RDA analysis of the zooplankton specific group and environmental factors in spring demonstrated significant control (permutation test: $F = 2.248, adjusted R^2 = 0.264, p = 0.001$). The first (22.7 %) and second (15.6 %) axes explained 38.3 % of the cumulative variation in the zooplankton specific group data. Cladoceran abundance and diversity positively related to $T_{water}$, [DOC] and $[SO_4]$ (Fig. 5 A). These factors were positively correlated with mainly small cladocerans *Alona* sp., *A. nana*, *B. (E.) coregoni*, *C. quadrangula*, *Ch. sphaericus*, *P. pediculus* (Fig. S4 A). However, abundance and diversity of cladocerans were inversely related to depth, lake area, pH, phytoplankton abundance, SUVA, concentrations of $CH_4$, $CO_2$ and DIC (Tables S3-S4 of the Supplement). These factors were positively associated with copepod *C. scutifer* and rotifers *T. patina*, *Pompholyx sulcata* Hudson, 1885 (Fig. S4 A). Copepods also positively related to deep lakes with large area, higher pH values and poor with [DOC]. Copepods were mainly represented by juveniles (nauplii), whereas rotifers were dominated by *K. longispina* (Fig. S4 A). In addition, two cladoceran species – *Sida crystallina* (O.F. Müller, 1776) and *Acroperus elongates* (G.O. Sars, 1862) – were positively associated with second axis.

The RDA analysis of the zooplankton specific groups and environmental factors in summer was also statistically significant (permutation test: $F = 1.89, adjusted R^2 = 0.245, p = 0.001$). The first (22.5 %) and second (16.6 %) axes explained 39.2 % of the cumulative variation in the zooplankton specific group data. Cladoceran abundance was directly related to $T_{water}$ (Fig. 5 B), among which *P. pediculus* exhibited the strongest relationship with this factor (Fig. S4 B). Cladoceran abundance was inversely related to latitude, pH and $CO_2$ (Table S5). Copepods had strong associations with lake area, [DIC], $CO_2$, $PO_4$, [TP], and pH. These factors are most strongly associated with *A. acutilobatus acutilobatus*, *Heterocope borealis* (Fischer, 1851) and nauplii (Fig. S4 B). Abundance of copepods was inversely related to water temperature and $CH_4$ concentration.

Rotifers' abundance was directly related to [DOC], [TP], abundance and biomass of phytoplankton, SUVA and S.C., but inversely related to $SO_4$, pH and Depth (Fig. 5B; Table S5). Five rotifer species were strongly related to these factors: *Ploesoma hudsoni* (Imhof, 1891), *C. unicornis*, *Keratella* sp., *T. patina*, *Ploesoma triacantha* (Bergendal, 1892) (Fig. S4 B).

Both copepods and rotifers abundance were directly related to latitude in summer, unlike cladocerans.

In autumn, the RDA analysis of the zooplankton specific group and environmental factors was significant only at $p = 0.07$. The first (30.5 %) and second (23.4 %) axes explained most of the cumulative variation (54 %) in the zooplankton specific group data. Cladoceran abundance was directly related to $NO_3$, $NH_4$, $SO_4$, $O_2$ and SUVA, but had strong negative correlation with depth and pH (Fig. 5C; Table S6). Three cladoceran species *B. (E.) coregoni*, *A. nana*, *Alona* sp. most strongly related to the mentioned factors (Fig. S4). Copepods' abundance was directly related to [DIC], depth and area of the lakes, pH and $CO_2$. The most abundant copepods *A. acutilobatus acutilobatus*, *H. borealis* and nauplii were strongly related to these parameters. Rotifers were associated with shallow acidic lakes rich with phytoplankton and with low phosphorus concentrations (Fig. 5C). Only three species dependent on these factors: *T. patina*, *Keratella* sp. and *Trichotria* sp. (Fig. S4 C).

Summarizing the RDA results, the abundance of cladocerans was directly related to temperature during all seasons. Copepods' abundance was positively associated with the lake depth and pH in all seasons, but inversely related to $CH_4$ (in spring and summer), $T_{water}$, [DOC], phytoplankton abundance (in summer), [DOC] and $CO_2$ (in autumn). The abundance of rotifers depended on various factors across seasons, and these relationships could be opposite in different seasons. For example, this group was directly related to depth of the lakes in spring and inversely related to depth in summer and autumn.

## 4. Discussion

### 4.1. Diversity, abundance and controlling factors of zooplankton

#### 4.1.1. Spatial variations of zooplankton

We found 74 zooplankton species and taxa in total (Table S2). In contrast to the first hypothesis of this study, on the decrease of zooplankton richness and abundance northward, the highest diversity was observed in the lakes of the continuous permafrost zone. Lakes from this zone harbored up to 13 unique species, most of which belonged to Rotifera (6 species) and Copepoda (5 species). Two species of Cladocera – *Biapertura affinis* (Leydig, 1860) and *Daphnia middendorfiana* (Fisher, 1851) were detected solely in the continuous zone. Only one of these species, *D. middendorfiana*, occurred in several lakes in spring and summer (5 and 6 lakes, respectively), indicating its non-occasional presence in lakes in this zone. Indeed, *D. middendorfiana* inhabits the far north of the Holarctic (Alekseev and Tsalolikhin, 2010) and is often registered in high latitudes (Hobæk and Weider, 1999; Novichkova and Chertoprud, 2020; Sobko et al., 2023) or in highlands (Tiberti, 2011), which explains its presence exclusively in the continuous permafrost zone. Among these 13 species, in addition to *D. middendorfiana*, another inhabitant of cold waters living mainly in northern latitudes was noted – the rotifer *Notholca labis* (Gosse, 1887). However, this species was recorded in only one lake in autumn. The remaining 11 species are not recognized as inhabiting exclusively high latitude environments and were represented in very low numbers in single lakes across the seasons. Given that these species are not confined exclusively to high latitudes, their absence in other permafrost zones may therefore reflect competitive exclusion (Shurin et al., 2000), or, most likely, limitations of the sampling procedure caused by their low abundance.

The dominance of Cladocera in the sporadic and discontinuous permafrost zone in summer indicates adequate water heating given that the temperature directly influences their metabolism and growth rate (de Eyto and Irvine, 2001). The cladocerans *Bosmina* (*Eubosmina*) *coregoni*, *Chydorus sphaericus* and rotifer *Testudinella patina* were the most common species, occurring in >50 % of the sampled lakes, and the vast majority of species (71 out of 74) were found in less than half of the lakes. Moreover, 70 % of the species were found in <10 % of the sampled lakes, which is consistent with data from Norwegian high latitude lakes (Hessen and Walseng, 2008). Considering that the distribution of species
<figure><img src="image_9.png" /></figure>

<figure><img src="image_10.png" /></figure>

<figure><img src="image_11.png" /></figure>

(caption on next column)

**Fig. 5.** RDA of zooplankton specific groups' abundance and environmental factors of thermokarst lakes of Western Siberia Lowland along latitudinal gradient in Spring (A), Summer (B) and Autumn (C). Sixty-seven, 53 and 27 samples were used for the analysis in each season, respectively. Sampling sites symbolled as follows: red dot – Sporadic zone; yellow square – Discontinuous zone; blue triangle – Continuous zone. Abbreviations: N Clad, N Cop and N Rot – total abundance of Cladocera, Copepoda and Rotifera, respectively; nClad, nCop, nRot – number of species of Cladocera, Copepoda and Rotifera, respectively; N – total zooplankton abundance; n – total number of species; Phyt.(N), Phyt(B) – total number of cells and biomass of pyutoplankton, respectively; log Area – log transformed lake sizes; Depth – lake depths; S.C. – specific conductivity.

was not limited geographically and most of them were found in all permafrost zones, it can be assumed that ecological plasticity and, possibly, competitive abilities of zooplankton species, as well as specific features of lakes determine the frequency of species occurrence (Shurin, 2000; Hessen et al., 2006). It should be noted that the species richness in a single lake in our study was low, which confirms possible competitive exclusion, especially under conditions of poor food supply (Shurin et al., 2000).

The highest zooplankton abundance was observed in the discontinuous permafrost zone in summer. However, high average abundance of zooplankton in this zone was due to a summer outbreak of *B. (E.) coragoni* in one lake (up to 667,000 ind. m⁻³). A possible reason for this particular case could be abnormally high abundance of heterotrophic bacteria fueled by terrestrial OM as carbon source, as it is known for alpine lakes (Zhang et al., 2016). In the Khanymey site, this could be due to peat abrasion at the border of this particular lake or input of surface water leachates of terrestrial vegetation (i.e., Shirokova et al., 2013, 2021). If this sample is excluded from the analysis, the average abundance of zooplankton becomes comparable to that in lakes of the sporadic permafrost zone. Higher development of zooplankton in these two zones is certainly linked to generally warmer conditions in the southern part of the WSL. Interestingly, the lowest average abundance was also registered in the discontinuous zone (in spring), which may indicate certain instability of environmental conditions in this zone, such as temperature fluctuations. In general, the discontinuous permafrost zone included species found in both sporadic and continuous permafrost zones, thus, this zone may be considered as an ecotone. For example, relatively thermophilic copepods (Rylov, 1948; Borutsky et al., 1991) *Mesocyclops leuckarti* (Claus, 1857) and *Heterocope appendiculata* (Sars, 1863) were recorded in higher and lower abundances in lakes of the sporadic and discontinuous permafrost zones, respectively, and were absent in lakes of the continuous permafrost zone. Conversely, psychrophilic copepods *Megacyclops gigas* (Claus, 1857) and *Heterocope borealis* (Fisher, 1851) (Rylov, 1948; Borutsky et al., 1991) were found mainly in lakes of the continuous permafrost zone and less often in lakes of the discontinuous permafrost zone, but were not recorded in the sporadic permafrost zone. In the lakes of sporadic zone, the number of species was higher in spring, although the highest average temperature was registered in summer. The decrease in species diversity in this zone in summer was due to a decrease in the number of rotifer species, which could be caused by an increase in the abundance of predatory cladocerans *P. pediculus*, which selectively feed on small rotifers and protozoa (Packard, 2001). In addition, we observed an increase in the total number of cladocerans. Large species of this group are known to contribute to a decrease in the abundance and species diversity of rotifers (MacIsaak, Gilbert, 1989). Besides, this may also be linked with an increase in the specificity of environmental conditions during the open water period leading to preferential survival of most adapted species. In the discontinuous and continuous permafrost zones, the highest species richness was observed during summer, which was certainly linked to maximal water heating and sufficient food supply.

Contrary to our second hypothesis, zooplankton in thermokarst lakes of the WSL globally did not depend on phytoplankton. Only a slight
correlation was found between the season-averaged abundance of filter feeders and phytoplankton biomass. Moreover, a comparison of the zooplankton abundance and phytoplankton biomass and cell number in specific seasons and permafrost zones did not reveal any pairwise correlation. RDA demonstrated weak positive relationships between rotifers and phytoplankton in summer and autumn; in addition, a weak positive relationship was found between the abundance of cladocerans and phytoplankton biomass in autumn. On the contrary, RDA revealed a negative relationship between cladocerans, copepods and phytoplankton in spring and summer, respectively. Considering high abundance and diversity of phytoplankton in thermokarst lakes of the WSL (Pokrovsky et al., 2022), we can conclude that food supply is not a limiting factor in the development of zooplankton in general and these groups in particular. Every specific zooplankton group was associated with different factors in each season and in each permafrost zone, which is discussed below. A comparison of trophic structure of zooplankton communities from different permafrost zones demonstrated an increase in relative amount of detritivorous gatherers northward (Fig. S5). This may indicate an increased role of detritus as food source for zooplankton in the continuous permafrost zone where the phytoplankton abundance decreased.

Our third hypothesis was confirmed and there was a decrease in the abundance and species diversity of cladocerans and an increase of copepods with increasing latitude, which was more pronounced in autumn season. Although cladocerans are able to form ephippiums and survive harsh conditions, they can actively develop and reproduce parthenogenetically (indicating optimal conditions) only at temperatures above 10 °C (Nevalainen and Luoto, 2010) and the duration of egg development sharply decreases when temperatures falls below 10 °C (Bottrell, 1975). A stronger dependence of cladocerans on temperature was also noted in other studies of northern lakes (Novichkova and Azovsky, 2017; Chertoprud and Novichkova, 2021). As latitude increases, the number of days with optimal temperatures for cladocerans' development decreases. Thus, the low species richness and abundance of cladocerans in the continuous permafrost zone is in accordance with their greater dependence on water temperature. Copepods are more thermotolerant than cladocerans and therefore predominate in high latitudes (Novichkova and Azovsky, 2017). Rotifers have a faster development rate compare to Cladocera and Copepoda. Rotifers hatch from resting eggs in small portions during the summer season and develop in high abundance only under optimal conditions (Walsh et al., 2014; Mnatsakanova and Polishchuk, 1996). Among the reasons for relatively low abundance of rotifers in high latitudes may be the pressure from predatory forms (copepods), low concentration of bacterioplankton, and the proportion of green and golden algae in phytoplankton (Pokrovsky et al., 2022).

### 4.1.2. Seasonal variations of zooplankton

In spring, copepods dominated in abundance the WSL thermokarst lakes in all permafrost zones. This was likely linked to low water temperatures, at which other groups, especially cladocerans, develop slowly (Novichkova and Azovsky, 2017). Note the dominance of nauplii, which constituted between 67 and 91 % of the total number of copepods. This finding is in agreement with their life cycle in northern waterbodies, when copepods emerge from resting eggs in early spring after the ice melts (Abramova et al., 2017). The RDA and Spearman correlations revealed positive relationships between the abundance of copepods, lake depth and pH of the water column, and a negative relationship with [DOC] concentration. Presumably, deep lakes warm up more slowly (i. e., Manasypov et al., 2015), which contributes to longer predomination in the abundance of copepods. A positive correlation with pH cannot be explained by copepods' preferences for certain pH values, as the physiological response of different copepods species to pH may vary significantly (Nilssen and Wærvågen, 2003). Most likely, it is linked to elevated pH in lakes of the continuous permafrost zone (i.e., Manasypov et al., 2020), where the copepods' abundance was highest. The [DOC] concentration decreased in lakes towards the north, which explains a

negative correlation of [DOC] with copepods' abundance. The abundance of cladocerans was negatively related to latitude and a number of hydrochemical parameters of the water column (DIC, CO₂, pH, CH₄), which, in turn, were strongly related to the latitude. This could be linked to unfavorable oxygen regime for zooplankton, when the generation of CO₂ and CH₄ in thermokarst lake sediments becomes more pronounced under partially anoxic conditions (Serikova et al., 2019; Manasypov et al., 2024). Interestingly, the abundance of Cladocera in spring negatively correlated with [TP], abundance and biomass of phytoplankton, while there was a positive relationship with [DOC] and T<sub>water</sub>. Note that the biomass and cell number of phytoplankton in the WSL lakes correlate with [DOC] and phosphate (Pokrovsky et al., 2022). We therefore suggest the grazing of phytoplankton by zooplankton, since the abundance of phytoplankton in the WSL thermokarst lakes in spring is low (Pokrovsky et al., 2022). Indeed, phytoplankton develops poorly at low water temperatures, even with an excess of nutrients (Bergström et al., 2013). However, we did not take into account benthic algae, which, despite their low quality in terms of some polyunsaturated fatty acids (Mariash et al., 2011), can serve as a food source for zooplankton in shallow subarctic and Arctic water ponds (Rautio and Vincent, 2007). It is also possible that the importance of allochthonous organic matter as a food source for filter feeders and suspension (predominantly calanoid copepods; i.e., Barnett et al., 2007) in thermokarst lakes of the WSL is more important in spring than in summer, as it was shown for a calanoid copepod *Eudiaptomus gracillis* G. O. Sars, 1863 from oligotrophic Loch Ness lake (Grey et al., 2001). Overall, the control of external factors such as terrestrial carbon input on the zooplankton abundance and diversity is not always straightforward and often can be obscured by other in-lake factors that are more important for organism development such as light attenuation (including UV and photosynthetically-active radiation), heat retention, oxygen availability and bioavailability of nutrients (reviewed by Brett et al., 2017).

In summer, cladocerans dominated in the lakes of sporadic and discontinuous permafrost zones. This group of zooplankton correlated positively with T<sub>water</sub> and negatively with latitude, as well as with pH and CO₂. Note however, that both pH and pCO₂ of the lake water increased northward. Thus, in summer, one of the most important factors in the development of cladocerans in WSL lakes is water temperature. Copepods dominated in abundance only in the continuous permafrost zone and their abundance was negatively related to water temperature, which was the lowest in the lakes of this zone and did not exceed 17 °C. The dominance of Copepoda in the continuous permafrost zone is consistent with their adaptation to cold water via specific metabolism and nutrient storage (Frisch, 2002; Martynova et al., 2009; Hobbs et al., 2020). Rotifers reached the highest relative abundance in the discontinuous permafrost zone, where the lakes were richest in [DOC], compared to the lakes of the other two zones. At the same time, rotifers positively correlated with [DOC] concentration, latitude, as well as the abundance and biomass of phytoplankton. High concentrations of [DOC] can promote the development of heterotrophic bacteria (Bergström et al., 2013), that serve as a food source for rotifers. On the other hand, the presence of diverse and abundant phytoplankton in WSL lakes in summer (Pokrovsky et al., 2022) enriches the food supply and reduces competition with cladocerans. Moreover, during this period, cladocerans were represented predominantly by small-sized species that did not exclude rotifers due to exploitative competition and their ability for long-term coexistence (MacIsaac and Gilbert, 1989).

In autumn, rotifers dominated in the sporadic and continuous permafrost zones whereas cladocerans dominated in the discontinuous permafrost zone. The dominance of rotifers in the continuous permafrost zone indicates certain instability of environmental conditions in this zone, such as sizable water temperature variations, full freezing of the water column and poor nutrient status (i.e., Manasypov et al., 2015, 2020). In addition, in lakes of the discontinuous permafrost zone in autumn, the average abundance of cladocerans was comparable to summer values (with an exception of one lake), while the abundance
and biomass of phytoplankton was low (Pokrovsky et al., 2022). Most likely, in lakes of this zone, rotifers were excluded by abundant cladocerans under conditions of low food supply.

The food chains of the communities in the studied lakes exhibited a complex and dynamic trophic structure. In summer, along with an increase in primary production, the proportion of filter feeders in the zooplankton community naturally increased. However, since the total abundance of zooplankton in thermokarst lakes was low, in summer it occurred under conditions of excess food (high abundance and biomass of phytoplankton). Therefore, it was statistically impossible to trace the relationship between the food and its consumer in an equilibrium system. Such a relationship can be revealed when there is a significant pressure from the consumer, especially in the presence of factors regulating the numerical indicators of zooplankton (Kovalenko et al., 2023). It is known that in northern water bodies, a significant proportion of zooplankton nutrition is provided by detritus and microbial mats (Rautio and Vincent, 2006). In the WSL lakes, such a pattern was valid only for the summer and autumn seasons. In spring, in higher latitudes, predatory forms predominated in zooplankton.

## 4.2. Comparison with available data on thermokarst lakes in high latitudes

The highest diversity of zooplankton in WSL thermokarst lakes was due to cladocerans, similar to what was reported for the NE European peatlands (Sobko et al., 2023). However, this result was in contrast to water pools of the Lena River delta, where the species richness of cladocerans was the lowest and rotifers were the most diverse group (Abramova, 2017). The least diverse specific group in our study was Copepoda, opposite to what was reported in the recent studies of thermokarst lakes of the Lena River delta (Chertoprud and Novichkova, 2021; Chertoprud et al., 2023) and in a large-scale study of zooplankton from 1665 Canadian lakes, where Calanoida and Cyclopoida together accounted for 50 species (33 and 17, respectively), although species richness of Cladocera (33 species) was comparable to our data (30 species) (Pinel-Alloul et al., 2013). Zooplankton composition of six thermokarst lakes from the Lena River delta was represented by 60 species (Abramova et al., 2017), which is lower than that in the WSL (74 species). Of the 9 and 11 (respectively) most common species in both regions, only one, Ch. sphaericus, matched. It should be noted that our study was carried out during one year open-water period, while the thermokarst lakes of the Lena River delta were studied for 11 years. In addition, in our study, some zooplankton individuals were identified to the genus level and could include more than one species. Accordingly, the species composition of zooplankton inhabiting WSL thermokarst lakes can be expanded. In another work of thermokarst lakes in the Lena River delta (Chertoprud and Novichkova, 2021), 55 species of Cladocera and Copepoda were registered with 13 species per sample in average, which is higher than that in the WSL (8.4). One of the three most common species (Ch. sphaericus) in the Lena River delta coincided with WSL lakes. Cladocerans were mainly associated with annual water temperature dynamic and copepods (non-harpacticoid) variations were explained by hydrochemical factors, which is similar to results obtained in the present study.

Results of the present study are also comparable to those on 50 Quebec (Canada) lakes located within sporadic to discontinuous permafrost zones, where Rotifera predominated in abundance in most of the thaw ponds in August (Begin and Vincent, 2017). Species diversity of Rotifera in the WSL is similar to Quebec (25 and 24 species, respectively) while the number of species in a single lake was lower in the WSL than in Quebec (0 to 8 and 2 to 14, respectively). The authors suggested four most important environmental variables, which control the diversity of Rotifera in Canadian lakes, namely [DOC], O₂, Chla concentrations and Chaoborus abundance. Note that we have not identified larvae of Chaoborous in the WSL thermokarst lakes. At the same time, in the WSL, the environmental factors controlled essentially the diversity of rotifers

rather than their abundance. Specifically, we observed mostly biotic factors (cladocerans diversity and abundance) affecting rotifers abundance and there was only weak influence of [DOC], phytoplankton and CH₄ in summer.

The composition and diversity of Cladocera and Copepoda in the WSL as revealed in this study is drastically different from another study of Quebec lakes (MacLeod et al., 2018). The zooplankton species richness is higher in the WSL than in Quebec (49 and 34, respectively); however, average number of species in one sample was comparable – 8.4 (1–17) and 9.1 (6–16), respectively. A common dominant species for both territories is Ch. sphaericus. While in Quebec, a positive relationship between species richness and lake depth, area, concentration of Ca, [DIC], Mg and Si was shown (MacLeod et al., 2018), in the WSL, the total number of species had only a weak correlation with water temperature and sulfate concentration and varied across permafrost zones and seasons. Note however, that thermokarst lakes of Canada are characterized by significantly greater depths and higher pH values than the WSL lakes, which undoubtedly affects the diversity and development of zooplankton.

## 4.3. Possible future changes of WSL zooplankton communities

Summarizing the obtained results, the maximal similarity of species composition was observed between sporadic and discontinuous permafrost zones, likely due to similar environmental conditions across seasons, especially water temperature. The dominance of Cladocera in the southern regions likely reflected the warmest environment and generally more stable conditions. Less stable environment was observed in the continuous permafrost zone, where Rotifera predominated in autumn. Based on these findings, it is possible to predict a positive, bottom-up relationship between rotifers and phytoplankton under climate change. Even a slight extension of the period of open water, combined with an increase in water temperature and, as a consequence, an additional supply of organic matter from thawed soils, will stimulate more active development of phytoplankton, including green, golden and blue-green algae, which are the optimal food source for rotifers. In this case, the species diversity and abundance of rotifers at high latitudes may increase (Belle et al., 2021).

Taking advantage of sizable permafrost gradient of thermokarst lakes sampled in the present study, we can apply a substituting space for time approach (i.e., Blois et al., 2013a, 2013b) in order to foresee possible changes in zooplankton abundance and dominant groups upon climate warming and permafrost boundary shift northwards. The suitability of Western Siberia for space-for-time substituting scenario is justified by highly homogeneous relief, landscape, bedrock, soils and vegetation distribution in this vast and relatively pristine territory. Such an approach has been successfully used for predicting possible evolution of hydrochemical composition of different environmental components, such as lake water (Manasypov et al., 2014, 2020), river (Vorobeyv et al., 2017; Krickov et al., 2019), soil porewaters (Raudina et al., 2021) and the GHG regime (Serikova et al., 2019) in this region. When applying this scenario for zooplankton distribution pattern in the WSL lakes, there will be a sizable increase in total abundance in each permafrost zone during summer, which might not be pronounced during other seasons. The dominance of some Copepoda, especially psychrophilic species, in the continuous permafrost zone might be replaced by small as well as large-sized cladocerans or other thermophilic species (see Graphical Abstract). Under climate warming scenario, there is a possibility of changes in the ratio of zooplankton ecological groups – an increase in the proportion of phytophagous filter feeders and a decrease in detritivorous gatherers in northern territories. Note the presence of calanoid copepod A. acutilobatus acutilobatus in high abundance in the northernmost part of the studied area, continuous permafrost zone. Previous studies showed that the distribution of this species was limited to the southern subarctic, that is, the forest-tundra zone (Dubovskaya et al., 2010). Thus, the species probably moved northward quite
successfully, given its high abundance in these lakes.

## 5. Conclusions

Unprecedented spatial coverage of thermokarst lakes sampled in Western Siberia Lowland across a permafrost gradient (sporadic, discontinuous and continuous) over three main seasons allowed quantifying the main pattern of zooplankton distribution and testing the environmental control on zooplankton abundance, biomass and diversity. The total zooplankton abundance significantly differed between seasons (the lowest abundance was observed in autumn) and permafrost zones (the lowest and the highest average abundances were observed in the discontinuous zone in spring and in summer, respectively). Lakes from continuous permafrost zone exhibited the highest abundance of Copepoda and the lowest abundance of Cladocera. The maximal similarity of species composition was observed between sporadic and discontinuous permafrost zones, likely due to similar environmental conditions across seasons, especially water temperature.

Regardless of season, there was significant change in the abundance of specific zooplankton groups along the permafrost gradient, between the southern and northern lakes: a decrease of cladocerans and an increase of copepods. We did not evidence a straightforward control of phytoplankton and nutrients on zooplankton development in general. The dominance of Copepoda (predominantly Calanoids) in the continuous permafrost zone is consistent with literature data and associated with the presence of species confined to the northern Holarctic. Upon climate warming and permafrost thaw, we expect an increase in total zooplankton abundance in each zone during summer, which might not be pronounced during other seasons. The dominance of Copepoda in the continuous permafrost zone might be replaced by Cladocera.

The strong spatial and seasonal variability of zooplankton populations in thermokarst lakes in response to water pH and temperature, as well as the lack of control by phytoplankton identified in this study, require testing of the relationship between bacterioplankton and zooplankton in these highly dynamic ecosystems.

### CRediT authorship contribution statement

Yury A. Noskov: Writing – original draft, Methodology, Conceptualization. Rinat M. Manasypov: Investigation, Formal analysis. Nadezhda I. Ermolaeva: Methodology, Investigation. Denis V. Anto-nets: Visualization, Formal analysis. Liudmila S. Shirokova: Investi-gation, Formal analysis. Oleg S. Pokrovsky: Writing – original draft, Conceptualization.

### Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

### Data availability

All the data are presented in the paper and supplement

### Acknowledgements

The study was funded by the TSU Development Program "Priority-2030". RM thanks partial support from the RSF grant No. 21-77-10067 (15%, lake water sampling, analysis and interpretation). OP and LS are grateful for partial support from project PEACE of PEPR FairCarboN ANR-22-PEXF-0011.

### Appendix A. Supplementary data

Supplementary data to this article can be found online at https://doi.

org/10.1016/j.scitotenv.2024.171284.

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