The Role of Phytoplankton in the Assessment of the Ecological State of the Floodplain Lakes of the Irtysh River, Kazakhstan

Abstract

Floodplain lakes play a significant role in maintaining biological diversity and providing a food base for aquatic organisms. In 2023–2024, for the first time, we studied phytoplankton of five floodplain lakes of the transboundary Irtysh River in Kazakhstan. A total of 149 species and forms of planktonic algae were recorded, with a low level of similarity between the lakes. The ratio of indicator species (predominance of eutraphents and meso-eutraphents), abundance (3301.6–168,961.1 thou. cells L⁻¹), biomass (2.41–83.67 mg L⁻¹) of phytoplankton communities, and composition of dominant phyla and species (Cyanobacteria: Microcystis pulverea, M. aeruginosa, Aphanizomenon flos-aquae; Chlorophyta: Volvox globator; Dinoflagellata: Ceratium hirundinella and others) testified to a high level of organic pollution of floodplain lakes. Chemical variables (nitrogen compound content, PI) supported this conclusion. Analysis of the RDA revealed that the biomass of Cyanobacteria was controlled by nitrate nitrogen, while phosphates controlled that of Chlorophyta. The applied integrated approach showed an improvement in the trophic status of lakes in a high-water year and can be useful in assessing the ecological state of aquatic ecosystems in other regions.

1. Introduction

Floodplain lakes are widely distributed in various regions of the world. They are a special type of aquatic ecosystem that periodically connects with the river during the spring flood [1]. With a decrease in the water level during the low-water period, this relationship is interrupted. Floodplain lakes play an important role in maintaining biological diversity [2,3], providing food for waterfowl and other organisms, and regulating pollution levels [3]. Some of them are of fishery or recreational importance [4].

Shallow depths and, as a rule, good warming of the water column contribute to the rapid reproduction of phytoplankton, resulting in a high rate of primary production [5] and eutrophication of floodplain lakes [6].

The species composition, structure, and productivity of phytoplankton communities depend on various environmental factors, including the total dissolved solids (TDS) content, water chemistry, temperature, and nutrient availability [7]. The rapid response of phytoplankton communities to changes in external conditions makes them effective indicators of the ecological state of aquatic ecosystems [8,9].

In the Kazakh part of the Irtysh basin, floodplain lakes are confined to the lower reaches of the river, on its section from the city of Kurchatov to the border with Russia. Along the riverbed are the industrial cities of Kurchatov, Aksu and Pavlodar, as well as numerous villages. The main anthropogenic load is experienced by the Irtysh River, where municipal and industrial wastewater is discharged [10,11]. Floodplain lakes located at different distances from the channel and settlements are subject to anthropogenic pollution to a lesser extent. With annual changes in water levels, suspended sediments, nutrients, pollutants, and organisms are exchanged between lakes, catchments, and the river.

The phytoplankton of water bodies in the Irtysh basin has a long history of study [12,13,14,15,16]. The available data relate to the Irtysh River [14,17,18,19,20], reservoirs [21,22,23,24,25,26], Lake Zaisan [27], and some salt lakes [16].

The study of phytoplankton communities and the assessment of the ecological state of floodplain lakes on the Irtysh River appear to be an urgent task that has not been carried out to date. This work partially fills this gap. Its purpose is to analyse the diversity, ecological preferences, and structure of phytoplankton in response to external factors and to assess the ecological state of five lakes located in the floodplain of the Irtysh, within the territory of the Pavlodar region.

2. Description of the Region

2.1. Site Description

The transboundary Irtysh River originates on the slopes of the Mongolian Altai in China and flows into the Ob in Russia. Its total length is 4280 km, of which 1964 km are in Russia and 1698 km in Kazakhstan.

In Kazakhstan, the Irtysh can be divided into three unequal sections.

The upper section, from the border with China to the confluence with Lake Zaisan, is known as the Black Irtysh. Its length is about 80 km. The width of the floodplain ranges from 1.2 to 3.6 km. Before flowing into the lake, the river forms a vast delta that is up to 25 km wide.

The middle course, from Lake Zaisan to the city of Semey, is regulated by the Upper Irtysh cascade of three reservoirs. Lake Zaisan is currently part of the Bukhtarma reservoir. Downstream, there are two more reservoirs, Shulbinskoye and Ust-Kamenogorskoye. In this section, the Irtysh receives its main tributaries, including the Kurchum, Ulba, and Oba, among others.

The third, lowest section, is located in the Pavlodar and Abay regions. There are no tributaries and reservoirs. The width of the floodplain varies from 8.3 to 15.0 km. The main part of the floodplain lakes is located here. The lakes were formed as a result of changes in the riverbed and, by their type and location, are classified as floodplain–valley lakes [28]. The hydrological and hydrochemical regimes of floodplain lakes depend on the volume of floodwater, the frequency, and intensity of water releases from the upstream Upper Irtysh cascade of reservoirs [29].

2.2. Climate

The Kazakh part of the Irtysh basin is located in three natural zones: steppe, semi-desert and mountain [30]. The climate is sharply continental, characterised by cold, relatively snowy winters and hot, dry summers. The annual range of air temperatures in the steppe and foothill regions is about 50.0 °C and can reach 70.0–80.0 °C in the north and mountainous areas. The amount of precipitation varies from 100–200 mm/year in the plains to 600–2000 mm/year in the mountains. Most of the precipitation falls from April to October.

According to agroclimatic zoning [31], in Northern Kazakhstan, the transition of average daily air temperatures through the 10 °C mark occurs in early May. The climatic features of the region cause a sharp increase in air temperature at the end of the calendar spring and its rapid decrease in late August–early September. A feature of the climate of a significant part of the territory of Kazakhstan is the return of cold weather in the spring and at the beginning of the calendar summer.

2.3. Geomorphological Zoning and Geological Substrate

According to the geomorphological zoning scheme of Kazakhstan [32], the floodplain of the Irtysh River falls within two distinct geomorphological regions. The upper reaches are located in the area of the accumulative and denudation plains of the Zaisan depression of the orogenic belt; the lower reaches belong to the accumulative plain of the Irtysh region. The main component of the geological substrate in floodplain territories is alluvial deposits of various genesis. They are sedimentary rocks that are transported and deposited by the river during floods and high waters. The composition and thickness of these deposits vary depending on the distance from the riverbed. The near-river zone accumulates the largest amount of gravel and sand particles, while the central and near-terrace parts accumulate silt and dust particles.

2.4. Description of the Surveyed Floodplain Lakes

The Baskol, Shoptykol, Kurkol, Orlovskoye and Stary Irtysh Lakes are located in the left-bank part of the Irtysh floodplain (Figure 1) at altitudes from 82–117 m above sea level. Lakes Baskol, Shoptykol, and Kurkol are within the zone of influence of industrial enterprises in the city of Aksu. Orlovskoye and Stary Irtysh are located at a considerable distance from anthropogenic sources of pollution.

All lakes are shallow, have different shapes (Figure 1) and a small area (

Table 1. Coordinates and hydrophysical characteristics of the surveyed floodplain lakes of the Irtysh River.

Variable	Baskol	Shoptykol	Kurkol	Orlvoskoye	Stary Irtysh
Abbreviation	BA	SH	KU	OR	SI
Coordinates	N 51.726238 E 77.353618	N 51.470083 E 77.144481	N 51.826111 E 77.177222	N 53.154167 E 75.733611	N 53.641389 E 75.115556
Area, km2	1.03	1.68	1.13	0.82	0.42
Altitude above the sea level, m	117	114	113	85	82
Depth, m	1.5–1.7	1.5–2.2	1.5–2.0	2.0	1.5
Transparency, m	1.0–1.3	0.3–0.5	2.0	2.0	0.3
Bottom sediments	black and grey silt	grey silt	black silt	black sludge with a smell of hydrogen sulphide	black silt
Aquatic and coastal vegetation	reed	pondweed, cattails, reed	cattails, reed	reed, elodea, pondweed, hornwort	reed, water lily

). The distribution of depths is uniform, a characteristic generally observed in steppe lakes of Kazakhstan [33]. Water transparency varies from 0.3 m in Lakes Stary Irtysh and Shoptykol to 1.0–2.0 m in the remaining lakes. Bottom sediments are represented by black and grey silt, sometimes with the smell of hydrogen sulphide, and detritus.

All the lakes are overgrown with varying degrees of higher aquatic and coastal vegetation. In the coastal zone, the common reed (Phragmítes austrális (Cav.) Trin. ex Steud) grows (Figure 2). Commonly found are cattails (Typha spp.) and reeds (Scírpus sp.). The bottom of Orlovskoye Lake is overgrown with elodea (Elodea sp.), to a lesser extent with shiny pondweed (Potamogeton lucens L.), and hornwort (Ceratophyllum sp.). Among the aquatic plants, in the Stary Irtysh, the water lily Núphar sp. is found.

3. Materials and Methods

3.1. Sampling

Studies of floodplain lakes were conducted in July 2023 and June 2024. Baskol and Shoptykol lakes were surveyed in 2024, while Orlovskoye and Kurkol were surveyed in both years, and Stary Irtysh was surveyed in 2023. Considering the small areas and depths, material was collected at three designated stations on each lake (Figure 1), following the recommendations [34]. A total of 24 samples were collected for each type of analysis. The coordinate reference of the stations was carried out using a Garmin eTrex 22x GPS navigator (Garmin Ltd., Taoyuan, Taiwan). Temperature and pH measurements were performed using a HORIBA device (HORIBA Ltd., Kyoto, Japan). The transparency of the water was determined using the Secchi disc.

At each station, water samples were taken to determine the total dissolved solids (TDS) content, oxygen content, nutrient levels, and the content of easily oxidizable organic substances, as indicated by the permanganate index (PI). Samples for the determination of nutrients were collected in glass bottles with a volume of 0.5 L and preserved with chloroform (1 mL) [35]. Water samples for the determination of easily oxidizable organic substances (PI) were collected in glass containers with a volume of 0.2 L and fixed with chemically pure sulfuric acid in a 1:3 dilution. Phytoplankton samples were collected by scooping up 1 L of water from surface horizons and fixed in a 4% neutral formaldehyde solution. Oxygen determination was performed in the field immediately after sampling using the Winkler iodometric method [35].

3.2. Sample Analysis

Hydrochemical analysis was conducted according to Semenov [35] and Fomin [36]. The complexometric method was used to determine the following calcium ions with the addition of murexide, magnesium ions with the addition of eriochrome-black T, and total hardness with the addition of a special chromogen (black ET–00). The gravimetric method was used to determine the concentration of sulphates in the form of a water-insoluble barium sulfate precipitate formed during the interaction of sulphate ions with barium salts. If the content of chloride ions in water exceeded 20 mg L⁻¹, it was determined by the volumetric argentometric method. The total content of sodium and potassium ions was calculated by subtracting the sum of cations from the sum of anions. The total dissolved solids (TDS) content was calculated. The concentration of nitrites and nitrates in the water was measured using a spectrophotometer (PE5400, Ekros, Saint Petersburg, Russia). Depending on the type of analysis, Griss or Nessler reagents, ammonium molybdate with ascorbic or sulfosalicylic acid were used. Determination of the content of easily oxidizable organic substances was carried out according to the Kubel method under acidic conditions. The samples were analysed in three and four repeats.

Phytoplankton samples were analysed according to the recommendations [37] with some modifications. Each sample was sequentially concentrated to a specific volume (15, 10, or 5 mL) and three drops were examined at each dilution. Species identification of algae was carried out using a Soptop EX30 microscope (Ningbo Sunny Optical Co, Ltd., Ningbo, China) and identification keys [38,39,40,41,42,43,44,45,46,47,48,49,50,51,52]. The taxonomic names of algae species and phyla were provided by AlgaeBase [53].

The number of cells was counted under a microscope in a Goryaev chamber. The phytoplankton abundance was calculated according to the equation:

$$N={\displaystyle \frac{n{v}_1}{v_{2}w}}$$

(1)

where N is the abundance, thou. cells L⁻¹; n is the arithmetic mean of the number of cells in the viewed grids of the Goryaev chamber; v₁—sample concentrate volume (5, 10, 15 mL); v₂ is the volume of the Goryaev chamber mesh; and w is the volume of the sample taken.

To calculate the phytoplankton biomass, the dimensional parameters of each algae species (width, length, height, or diameter) were initially determined. Then, the individual cell mass (volume) was calculated using the volume formulas for geometric shapes [54,55]. The biomass of each species in the sample was then determined by multiplying the average cell volume (mass) by the number of cells.

The species occurrence (%) was found as the ratio of the number of samples in which the species was detected to the total number of samples. To describe the structure of phytoplankton communities, the average number of species per sample, Shannon-Ab and Shannon-Bi indices [56], and the average mass (volume) of the algal cells in the sample were calculated.

The Shannon Diversity Index was calculated using the equation:

$$H=-\sum _{i=1}^n{P}_i\mathit{log}2p_i$$

(2)

where H is the Shannon index, P_i is the share of the i-th species in the total abundance or biomass, log2 is the logarithm of base 2, n is the number of species in the sample, and ∑ is the sum of values.

The analysis of species similarity in phytoplankton communities was conducted using the Bray–Curtis index in Primer 7 software [57] and JASP [58]. To calculate the similarity index, the values of species abundance were transformed by the square root. For all variables, we calculated the mean values and standard error in Excel. To assess the variability of phytoplankton (response variables) to environmental factors (independent variables), a multivariate statistical analysis, RDA, was performed using the CANOCO programme [59].

The trophic state classification assessment was conducted based on phosphorus concentration [60], which is typically used to calculate the Trophic State Index (TSI) [61]. Assessment of Water Quality Class based on the nutrient variables (nitrogen and phosphorus) concentration was performed according to the classification [62].

3.3. Bioindication

The assessment of the ecological state of floodplain lakes was conducted based on the species composition and structure of phytoplankton communities. In the first case, the ratio of the number of indicator species of algae for which their ecological preferences are known was analysed: the ratio to substrate, temperature, oxygen conditions, trophic state, and nitrogen content [8]. For bioindication of the structure of phytoplankton communities, the values of abundance, biomass, composition of dominant phyla and species, values of the Shannon diversity index and the value of the average individual cell mass were used [63,64].

4. Results

4.1. Hydrophysical and Hydrochemical Characteristics

The water temperature of floodplain lakes varied within relatively small limits (

Table 2. Hydrochemical parameters of floodplain lakes of the Irtysh River (mean values and standard error).

Variable	Baskol	Shoptykol	Kurkol		Orlovskoye		Stary Irtysh
	2024	2024	2023	2024	2023	2024	2023
Temperature, °C	23.5 ± 0.15	19.7 ± 0.35	21.5 ± 0.23	20.5 ± 0.11	22.2 ± 0.30	13.3 ± 0.10	23.4 ± 0.56
O2, mg L−1	8.54 ± 0.79	7.98 ± 0.15	5.49 ± 1.14	10.94 ± 0.38	12.61 ± 0.86	9.62 ± 0.58	12.56 ± 0.40
pH	6.74 ± 1.68	8.53 ± 0.08	8.30 ± 0.08	8.46 ± 0.008	8.59 ± 0.45	7.43 ± 0.08	7.99 ± 0.02
Na+ + K+, mg L−1	67.50 ± 1.32	75.33 ± 7.29	28.83 ± 5.71	24.17 ± 1.01	43.00 ± 2.25	22.17 ± 1.64	23.3 ± 1.83
Ca, mg L−1	40.08 ± 1.15	33.73 ± 1.46	30.06 ± 0.58	27.72 ± 0.84	22.71 ± 3.54	18.71 ± 0.67	26.05 ± 1.16
Mg, mg L−1	22.29 ± 0.40	15.00 ± 1.07	11.15 ± 0.54	7.09 ± 0.54	13.98 ± 0.61	6.69 ± 0.35	12.56 ± 1.07
HCO3, mg L−1	207.47 ± 1.76	256.29 ± 18.64	138.31 ± 5.38	136.28 ± 4.07	91.46 ± 25.39	101.70 ± 2.03	138.31 ± 4.07
SO4, mg L−1	99.97 ± 0.83	29.30 ± 1.12	27.00 ± 4.79	23.87 ± 0.19	10.00 ± 7.21	23.76 ± 0.45	29.63 ± 0.76
Cl, mg L−1	39.72 ± 0.61	39.73 ± 0.61	18.40 ± 0.00	6.62 ± 0.47	41.48 ± 1.74	7.57 ± 0.59	13.48 ± 0.00
Hardness, mg L−1	3.80 ± 0.00	2.93 ± 0.03	2.38 ± 0.09	1.93 ± 0.03	2.25 ± 0.21	1.47 ± 0.03	2.40 ± 0.003
TDS, mg L−1	479.03 ± 3.30	449.37 ± 26.07	253.70 ± 10.56	225.75 ± 4.60	268.93 ± 18.45	180.58 ± 4.29	243.36 ± 4.39
PI, mgO L−1	22.62 ± 0.63	15.21 ± 0.41	18.58 ± 0.40	11.09 ± 0.48	13.33 ± 0.23	9.99 ± 0.27	8.62 ± 0.82
N-NO3, mg L−1	0.250 ± 0.159	0.200 ± 0.135	0.615 ± 0.276	0.201 ± 0.142	0.010 ± 0.010	0.201 ± 0.111	0.0003 ± 0.0003
N-NO2, mg L−1	0.038 ± 0.003	0.040 ± 0.009	0.009 ± 0.004	0.018 ± 0.009	0.065 ± 0.011	0.032 ± 0.010	0.060 ± 0.018
N-NH4, mg L−1	0.957 ± 0.338	0.469 ± 0.084	0.088 ± 0.021	0.212 ± 0.021	1.084 ± 0.077	0.160 ± 0.005	0.898 ± 0.180
Nitrogen Sum, mg L−1	1.245 ± 0.347	0.708 ± 0.221	0.711 ± 0.300	0.431 ± 0.156	1.159 ± 0.071	0.394 ± 0.099	0.959 ± 0.189
PO4, mg L−1	0.210 ± 0.006	0.025 ± 0.006	0.088 ± 0.023	0.019 ± 0.002	0.027 ± 0.003	0.01 ± 0.00	0.038 ± 0.006
P-PO4, mg L−1	0.068 ± 0.002	0.008 ± 0.002	0.029 ± 0.008	0.006 ± 0.001	0.009 ± 0.001	0.003 ± 0.000	0.012 ± 0.002

). The exception was Lake Orlovskoye, where a very low water temperature was recorded in 2024. An exception was Lake Orlovskoye, where the average water temperature in June 2024 was 8.94 °C lower than in July 2023. This is due to the fact that in 2024, phytoplankton samples were collected after a sharp cold snap. The invasion of cold air masses caused a decrease in daytime air temperature from 25 to 13–15 °C, and nighttime temperatures from 10 to 0 °C [65]. Warm weather returned within a few days, and the water temperature in the nearby Kurkol Lake was typical for the region at the beginning of summer. Thus, the described differences did not reflect the actual temperature regime of Lake Orlovskoye, but were associated with weather anomalies. The pH values characterised the alkaline reaction of the water, with the exception of Lake Baskol, which had a neutral reaction. Daytime oxygen levels were high everywhere, except in Lake Kurkol in 2023.

According to the classification [62,66], the water in the lakes is fresh and soft. Based on the ion ratio, the water belonged to the carbonate class of the calcium group, a second type.

The highest PI values were recorded in Baskol in 2024 and Kurkol in 2023. The total content of mineral nitrogen varied by an order of magnitude, with maximum values in lakes Baskol (2024) and Orlovskoye (2023). The phosphate content also changed by an order of magnitude, with a maximum in Baskol. From 2023 to 2024, in Kurkol and Orlovskoye lakes, TDS, PI values, total nitrogen and phosphate content decreased.

4.2. Phytoplankton

4.2.1. Species Composition

149 species and forms of planktonic algae were recorded in the composition of lacustrine phytoplankton, including Heterokontophyta-36, Chlorophyta-59, Cyanobacteria-38, Euglenophyta-9, Dinoflagellata-3, Charophyta-4 (

Table A1. Species composition and occurrence of planktonic algae in floodplain lakes of the Pavlodar Irtysh region, 2023–2024.

Species Name	Occurrence, %
	Lakes *
	BA	SH	KU		OR		SI
	2024	2024	2023	2024	2023	2024	2023
Charophyta
Closterium acerosum Ehrenberg ex Ralfs, 1848	0	0	33	0	0	0	0
Closterium acutum Brébisson, 1848	0	0	33	0	0	0	0
Closterium parvulum Nägeli, 1849	67	0	0	0	0	0	0
Cosmarium punctulatum Brébisson, 1856	0	0	100	0	67	33	0
Mougeotia sp. C. Agardh, 1824	100	0	0	0	0	0	0
Staurastrum gracile Ralfs ex Ralfs, 1848	0	0	67	0	67	0	67
Chlorophyta
Actinastrum aciculare Playfair, 1917	0	33	0	0	0	0	0
Actinastrum hantzschii Lagerheim, 1882	0	100	33	33	0	0	67
Ankistrodesmus fusiformis Corda, 1838	0	67	0	0	0	0	0
Ankistrodesmus spiralis (W.B. Turner) Lemmermann, 1908	0	33	0	0	0	0	0
Ankistrodesmus arcuatus Korshikov, 1953	0	100	33	0	67	100	100
Ankyra ocellata (G. M. Smith) Fott, 1957	33	0	0	0	0	0	0
Binuclearia lauterbornii (Schmidle) Proshkina-Lavrenko, 1966	0	100	0	0	0	0	100
Chlorella vulgaris Beijerinck, 1890	0	100	0	0	0	0	0
Chlorella vulgaris f. globosa V.M. Andreeva, 1975	0	33	0	0	0	0	0
Chlorotetraedron incus (Teiling) Komárek & Kovácik, 1985	0	0	0	0	0	0	33
Closteriopsis acicularis (Chodat) J.H.Belcher & Swale, 1962	0	67	0	0	67	0	0
Closteriopsis longissima (Lemmermann) Lemmermann, 1899	0	0	0	0	33	100	0
Coelastrum microporum Nägeli, 1855	0	0	0	0	0	0	33
Coenochloris aquatica I. Kostikov, T. Darienko, A. Lukesová, & L. Hoffmann, 2002	0	0	0	0	0	0	33
Coenochloris pyrenoidosa Korshikov, 1953	0	0	0	33	0	67	0
Crucigenia fenestrata (Schmidle) Schmidle, 1900	0	33	0	0	0	0	0
Crucigenia quadrata Morren, 1830	0	67	0	0	0	0	0
Desmodesmus armatus (Chodat) E.H. Hegewald, 2000	0	0	67	33	0	100	33
Desmodesmus armatus var. bicaudatus (Guglielmetti) E.H.Hegewald, 2000	0	67	0	0	0	0	0
Desmodesmus communis (E.Hegewald) E.Hegewald, 2000	0	0	33	0	33	0	67
Desmodesmus subspicatus (Chodat) E. Hegewald & A. W. F. Schmidt, 2000	0	33	33	0	0	100	33
Eudorina elegans Ehrenberg, 1832	0	0	0	0	67	0	0
Hyaloraphidium contortum Pascher & Korshikov, 1931	0	67	0	0	0	0	0
Kirchneriella lunaris (Kirchner) Möbius, 1894	0	0	0	0	67	0	0
Lemmermannia tetrapedia (Kirchner) Lemmermann, 1904	0	0	0	0	0	0	33
Monactinus simplex (Meyen) Corda, 1839	100	100	67	67	0	0	67
Monoraphidium contortum (Thuret) Komárková-Legnerová, 1969	0	67	33	0	100	100	100
Monoraphidium convolutum (Corda) Komárková-Legnerová, 1969	0	0	0	0	0	33	0
Monoraphidium griffithii (Berkeley) Komárková-Legnerová, 1969	0	100	33	0	33	100	100
Monoraphidium minutum (Nägeli) Komárková-Legnerová, 1969	0	0	0	0	33	0	0
Monoraphidium pusillum (Printz) Komárková-Legnorová, 1969	0	33	0	0	0	0	0
Mucidosphaerium pulchellum (H.C.Wood) C. Bock, Proschold & Krienitz, 2011	0	100	67	0	100	100	67
Nephrochlamys subsolitaria (G.S.West) Korshikov, 1953	0	100	0	0	0	0	0
Oocystis borgei J. W. Snow, 1903	0	100	0	33	0	33	0
Oocystis lacustris Chodat, 1897	0	100	0	100	0	33	0
Oocystis submarina Lagerheim, 1886	0	0	100	100	67	33	33
Pediastrum duplex Meyen, 1829	100	100	100	100	67	33	33
Phacotus lenticularis (Ehrenberg) Diesing, 1866	0	0	33	33	33	100	0
Pseudopediastrum boryanum (Turpin) E. Hegewald, 2005	0	0	67	0	0	0	0
Raphidocelis sigmoidea Hindák, 1977	0	0	0	0	67	0	100
Scenedesmus semipulcher Hortobágyi, 1960	0	67	67	33	67	100	67
Scenedesmus ellipticus Corda, 1835	0	100	67	100	100	100	33
Scenedesmus obtusus Meyen, 1829	0	0	0	100	0	0	0
Scenedesmus quadricauda (Turpin) Brébisson, 1835	0	100	0	67	0	100	0
Sphaerocystis planctonica (Korshikov) Bourrelly, 1974	0	0	33	33	0	33	67
Stauridium tetras (Ehrenberg) E. Hegewald, 2005	0	0	100	100	0	0	0
Tetradesmus lagerheimii M.J.Wynne & Guiry, 2016	0	100	0	100	0	67	0
Tetradesmus incrassatulus (Bohlin) M. J. Wynne, 2016	0	33	33	0	33	0	0
Tetradesmus obliquus (Turpin) M.J.Wynne, 2016	0	33	33	0	0	0	33
Tetraedron caudatum (Corda) Hansgirg, 1888	0	67	0	0	0	0	67
Tetraedron minimum (A. Braun) Hansgirg, 1889	0	33	0	0	0	0	33
Tetraedron triangulare Korshikov, 1953	0	100	0	0	0	0	0
Volvox globator Linnaeus, 1758	100	0	0	0	0	0	0
Cyanobacteria
Anathece clathrata (West & G. S. West) Komárek, Kaštovský & Jezberová, 2011	0	100	100	33	0	0	100
Aphanizomenon flos-aquae Ralfs ex Bornet & Flahault, 1886	100	100	33	0	33	100	100
Aphanocapsa incerta (Lemmermann) G. Cronberg & Komárek, 1994	0	0	33	33	0	0	33
Aphanocapsa planctonica Komárek & Anagnostidis, 1995	0	0	67	0	0	0	0
Aphanocapsa delicatissima West & G. S. West, 1912	0	0	0	33	0	0	0
Aphanothece elabens (Meneghini) Elenkin, 1936	0	100	100	0	67	0	100
Chroococcus minimus (Keissler) Lemmermann, 1904	0	0	0	0	0	67	0
Chroococcus minor (Kützing) Nägeli, 1849	0	33	0	0	0	0	0
Chroococcus turgidus (Kützing) Nägeli, 1849	0	0	33	0	0	0	33
Chrysosporum bergii (Ostenfeld) E. Zapomelová, O. Skácelová, P. Pumann, R. Kopp & E. Janecek, 2012	100	0	0	0	100	100	33
Coelomoron pusillum (Van Goor) Komárek, 1988	0	0	33	33	0	0	0
Coelosphaerium kuetzingianum Nägeli, 1849	0	0	100	67	0	100	67
Dolichospermum sigmoideum (Nygaard) Wacklin, L. Hoffmann & Komárek, 2009	0	0	0	0	67	0	0
Dolichospermum spiroides (Klebahn) Wacklin, L. Hoffmann & Komárek, 2009	100	33	33	0	0	67	100
Glaucospira laxissima (G. S. West) Simic, Komárek & Dordevic, 2014	0	0	0	0	0	33	0
Gomphosphaeria aponina Kützing, 1836	0	100	67	0	0	67	0
Leptolyngbya angustissima (West & G. S. West) Anagnostidis & Komárek, 1988	0	100	0	0	0	67	33
Leptolyngbya tenuis (Gomont) Anagnostidis & Komárek, 1988	0	100	33	0	0	67	33
Limnococcus limneticus (Lemmermann) Komárková, Jezberová, O. Komárek & Zapomelová, 2010	0	0	33	67	0	0	33
Merismopedia elegans A. Braun ex Kützing, 1849	0	0	33	0	33	0	0
Merismopedia glauca (Ehrenberg) Kützing, 1845	0	33	67	67	33	67	0
Merismopedia minima G. Beck, 1897	0	67	100	33	33	33	33
Merismopedia tenuissima Lemmermann, 1898	0	0	33	0	67	0	0
Merismopedia tranquilla (Ehrenberg) Trevisan, 1845	0	33	0	0	0	0	0
Microcystis aeruginosa (Kützing) Kützing, 1846	0	100	100	0	67	33	100
Microcystis pulverea (H. C. Wood) Forti, 1907	0	100	100	0	0	100	100
Microcystis wesenbergii (Komárek) Komárek ex Komárek, 2009	0	0	67	0	0	0	67
Microcystis ichthyoblabe (G. Kunze) Kützing, 1843	0	33	0	100	0	0	0
Planktolyngbya contorta (Lemmermann) Anagnostidis & Komárek, 1988	0	100	0	0	0	0	0
Pseudanabaena limnetica (Lemmermann) Komárek, 1974	0	0	33	0	33	0	0
Pseudanabaena mucicola (Naumann & Huber-Pestalozzi) Schwabe, 1964	0	0	0	0	0	100	0
Rhabdoderma lineare Schmidle & Lauterborn, 1900	0	0	0	0	0	0	33
Romeria leopoliensis (Raciborski) Koczwara, 1932	0	0	0	0	0	0	100
Snowella atomus Komárek & Hindák, 1988	0	0	0	0	100	0	100
Spirulina laxa G.M.Smith, 1916	0	0	0	0	0	33	0
Synechocystis aquatilis Sauvageau, 1892	0	0	0	67	0	33	33
Woronichinia naegeliana (Unger) Elenkin, 1933	0	0	33	100	33	0	67
Dinoflagellata
Ceratium hirundinella (O. F. Müller) Dujardin, 1841	0	0	0	67	100	100	100
Peridiniopsis quadridens (F. Stein) Bourrelly, 1968	0	0	0	0	0	0	33
Peridinium cinctum (O. F. Müller) Ehrenberg, 1832	0	0	0	0	0	0	100
Euglenophyta
Euglena viridis (O. F. Müller) Ehrenberg, 1830	0	33	0	0	67	0	100
Lepocinclis acus (O. F. Müller) B. Marin & Melkonian, 2003	0	100	0	0	0	0	33
Lepocinclis oxyuris (Schmarda) B. Marin & Melkonian, 2003	0	33	0	0	0	0	0
Monomorphina pyrum (Ehrenberg) Mereschkowsky, 1877	0	33	33	0	0	0	0
Phacus longicauda (Ehrenberg) Dujardin, 1841	0	100	67	0	0	67	67
Trachelomonas hispida (Perty) F. Stein, 1878	0	100	67	100	33	33	67
Trachelomonas oblonga Lemmermann, 1899	0	67	0	0	0	0	0
Trachelomonas oblonga var. attenuata Playfair, 1915	0	33	0	0	0	0	0
Trachelomonas volvocina (Ehrenberg) Ehrenberg, 1834	0	0	33	100	0	0	0
Heterokontophyta
Amphora ovalis (Kützing) Kützing, 1844	0	0	67	0	0	0	33
Asterionella formosa Hassall, 1850	0	0	0	100	0	0	0
Aulacoseira granulata (Ehrenberg) Simonsen, 1979	0	0	0	0	100	0	33
Aulacoseira italica (Ehrenberg) Simonsen, 1979	33	0	0	0	67	0	0
Cocconeis placentula Ehrenberg, 1838	33	0	33	0	67	0	33
Cymbella cistula (Ehrenberg) O. Kirchner, 1878	0	33	0	0	0	0	0
Cymbella tumida (Brébisson) Van Heurck, 1880	0	0	0	0	33	0	0
Cymbopleura lata (Grunow ex Cleve) Krammer, 2003	0	0	0	0	33	0	0
Diatoma vulgaris Bory, 1824	0	0	0	0	0	100	0
Dinobryon divergens O.E. Imhof, 1887	0	0	0	0	0	0	33
Dinobryon sertularia Ehrenberg, 1834	0	0	0	100	0	0	67
Epithemia turgida (Ehrenberg) Kützing, 1844	0	0	33	0	0	67	0
Epithemia adnata (Kützing) Brébisson, 1838	0	0	0	0	67	0	0
Fragilaria capucina Desmazières, 1830	0	0	0	67	0	67	0
Gyrosigma acuminatum (Kützing) Rabenhorst, 1853	0	33	33	0	0	0	0
Melosira inflexa (Roth) Guiry, 2019	67	0	0	0	0	0	0
Navicula cincta (Ehrenberg) Ralfs, 1861	67	0	0	33	0	0	0
Navicula cryptocephala Kützing, 1844	33	33	0	0	0	0	0
Navicula lanceolata Ehrenberg, 1838	67	0	33	0	33	0	0
Navicula minima Grunow, 1880	0	0	0	0	33	0	0
Navicula radiosa Kützing, 1844	0	0	0	33	0	0	0
Navicula rhynchocephala Kützing, 1844	0	0	67	33	0	33	0
Navicula tripunctata (O. F. Müller) Bory, 1822	0	33	33	0	0	0	0
Nitzschia acicularis (Kützing) W. Smith, 1853	0	33	33	100	100	100	0
Nitzschia linearis W. Smith, 1853	0	0	0	33	0	0	0
Nitzschia palea (Kützing) W.Smith, 1856	0	0	33	0	0	0	0
Nitzschia longissima (Brébisson ex Kützing) Grunow, 1862	0	0	0	0	0	67	0
Pseudokephyrion entzii W. Conrad, 1939	0	0	33	0	0	0	67
Sellaphora pupula (Kützing) Mereschkovsky, 1902	67	0	0	0	0	0	0
Staurosira construens Ehrenberg, 1843	0	0	0	0	33	67	0
Staurosira subsalina (Hustedt) Lange-Bertalot, 2004	0	0	0	33	0	0	0
Stephanocyclus meneghinianus (Kützing) Kulikovskiy, Genkal & Kociolek, 2022	0	0	100	0	67	67	100
Stephanodiscus hantzschii Grunow, 1880	0	0	67	0	67	0	33
Tryblionella hantzschiana Grunow, 1862	0	0	67	0	33	0	33
Ulnaria ulna (Nitzsch) Compère, 2001	0	0	0	67	0	100	0
Ulnaria ulna var. spathulifera (Grunow) Aboal, 2003	0	0	0	0	0	33	0
Total:	16	60	61	40	47	49	60

* Abbreviation of the lake names as in Table 1.

).

Table A2. Species richness of phytoplankton in floodplain lakes of the Irtysh River, 2023–2024.

	Abbreviation, Station Number and Sampling Year *
Phyla name	BA- 1-24	BA- 2-24	BA- 3-24	SH- 1-24	SH- 2-24	SH- 3-24	KU- 1-23	KU- 2-23	KU- 3-23	KU- 1-24	KU- 2-24	KU- 3-24	OR- 1-23	OR- 2-23	OR- 3-23	OR- 1-24	OR- 2-24	OR- 3-24	SI- 1-23	SI- 2-23	SI- 3-23
Charophyta	2	2	1	0	0	0	3	2	2	0	0	0	1	2	1	0	0	1	1	1	0
Chlorophyta	3	3	4	25	24	21	11	14	6	14	9	9	9	11	11	14	14	15	12	14	14
Cyanobacteria	3	3	3	11	10	13	12	11	14	6	7	6	4	9	7	10	10	12	12	13	14
Dinoflagellata	0	0	0	0	0	0	0	0	0	1	1	0	1	1	1	1	1	1	2	2	3
Euglenophyta	0	0	0	6	4	5	0	2	4	2	2	2	0	2	1	1	1	1	4	2	2
Heterokontophyta	3	4	4	2	0	3	9	2	8	5	7	6	2	9	11	6	7	8	3	5	5
Total	11	12	12	44	38	42	35	31	34	28	26	23	17	34	32	32	33	38	34	37	38

* Abbreviation of the lake names as in Table 1.

shows the distribution of planktonic algal species by phylum at each station surveyed. Some algae species recorded in the phytoplankton of floodplain lakes are shown in Figure A1.

The highest species richness of phytoplankton communities (60–61) was identified in Shoptykol (2024), Stary Irtysh (2023) and Kurkol (2023) lakes. In Baskol, the number of species was almost four times less. Phytoplankton communities of other lakes occupied an intermediate position in terms of the number of species.

Twelve species of algae were widespread: Monactinus simplex, Monoraphidium contortum, M. griffithii, Mucidosphaerium pulchellum, Pediastrum duplex, Scenedesmus ellipticus (Chlorophyta), Aphanizomenon flos-aquae, Aphanothece elabens, Microcystis aeruginosa, M. pulverea (Cyanobacteria), Nitzschia acicularis (Heterokontophyta). Within the water areas of each of the lakes, the number of widespread species was larger: in Kurkol, from 23 (2024) to 28 (2023); in Orlovskoye, from 27 to 34; in Stary Irtysh, 32; in Baskol, 12; and in Shoptykol, 38.

According to the results of cluster analysis, phytoplankton communities were characterised by a low level of similarity (Figure 3). The most unique composition of planktonic algae species (with similarity less than 20%) was identified in Baskol (2024), Kurkol (2024), and Orlovskoye (2023). Within the water areas of the lakes, phytoplankton communities, as a rule, were similar in species composition, except for one of the sites (st.1, Figure 1) of Lake Orlovskoye in 2023. Despite the differences in the period of material collection (2023 and 2024), more than 50% of the total species were recorded in the phytoplankton communities of the Stary Irtysh and Shoptykol lakes.

In total, the species composition of planktonic algae in floodplain lakes differed significantly between 2023 and 2024 (Figure 4).

4.2.2. Quantitative Variables and Dominants

Quantitative variables of phytoplankton communities everywhere reached high and very high levels (

Table A3. Phytoplankton abundance (thou. cells L⁻¹) of floodplain lakes of the Irtysh River, 2023–2024.

№	Abbreviation	Charophyta	Chlorophyta	Cyanobacteria	Dinoflagellata	Euglenophyta	Heterokontophyta	Total
1	BA-1-24	3.3	6101.7	2040.0	0.0	0.0	145.0	8290.0
2	BA-2-24	1.7	50,233.3	1303.3	0.0	0.0	100.0	51,638.3
3	BA-3-24	0.0	13,055.0	845.0	0.0	0.0	6.7	14,450.0
4	SH-1-24	0.0	6505.0	30,180.0	0.0	250.0	40.0	36,975.0
5	SH-2-24	0.0	6563.3	19,875.0	0.0	145.0	0.0	26,583.3
6	SH-3-24	0.0	3441.7	10,970.0	0.0	18.3	20.0	14,450.0
7	KU-1-23	10.0	7793.3	237,886.7	0.0	0.0	328.3	246,018.3
8	KU-2-23	3.3	6653.3	173,416.7	0.0	23.3	143.3	180,240.0
9	KU-3-23	10.0	3616.7	76,816.7	0.0	11.7	170.0	80,625.0
10	KU-1-24	0.0	436.7	495.0	3.3	6.7	1200.0	2141.7
11	KU-2-24	0.0	448.3	1505.0	10.0	6.7	2023.3	3993.3
12	KU-3-24	0.0	543.3	1525.0	0.0	10.0	1691.7	3770.0
13	OR-1-23	0.0	163.3	473.3	11.7	0.0	15.0	663.3
14	OR-2-23	10.0	2008.3	5000.0	60.0	13.3	230.0	7321.7
15	OR-3-23	20.0	1281.7	2280.0	50.0	10.0	326.7	3968.3
16	OR-1-24	0.0	490.0	3405.0	1.7	50.0	358.3	4305.0
17	OR-2-24	0.0	1018.3	3926.7	1.7	10.0	226.7	5183.3
18	OR-3-24	0.0	1313.3	4853.3	15.0	15.0	250.0	6446.7
19	SI-1-23	40.0	1703.3	15,938.3	80.0	41.7	146.7	17,950.0
20	SI-2-23	10.0	2483.3	15,996.7	220.0	18.3	240.0	18,968.3
21	SI-3-23	0.0	2048.3	16,176.7	220.0	120.0	455.0	19,020.0

and

Table A4. Phytoplankton biomass (10⁻³ mg L⁻¹) of floodplain lakes of the Irtysh River, 2023–2024.

№	Abbreviation	Charophyta	Chlorophyta	Cyanobacteria	Dinoflagellata	Euglenophyta	Heterokontophyta	Total
1	BA-1-24	71.8	13,078.5	2365.4	0.0	0.0	316.0	15,831.6
2	BA-2-24	35.9	109,693.3	1178.4	0.0	0.0	1020.0	111,927.6
3	BA-3-24	0.0	27,938.7	944.9	0.0	0.0	26.2	28,909.8
4	SH-1-24	0.0	5113.8	12,229.6	0.0	9055.4	87.0	26,485.8
5	SH-2-24	0.0	6332.7	5992.9	0.0	5014.7	0.0	17,340.3
6	SH-3-24	0.0	2649.0	5332.9	0.0	592.1	31.0	8604.9
7	KU-1-23	63.7	8212.9	94,248.2	0.0	0.0	813.1	103,337.9
8	KU-2-23	4.5	5648.2	96,845.7	0.0	740.4	197.6	103,436.5
9	KU-3-23	13.5	3416.3	39,862.4	0.0	572.9	382.3	44,247.4
10	KU-1-24	0.0	592.6	584.5	504.1	116.5	3365.6	5163.3
11	KU-2-24	0.0	439.4	1560.8	1512.3	116.5	4786.1	8415.0
12	KU-3-24	0.0	605.7	1406.5	0.0	174.7	4588.4	6775.4
13	OR-1-23	0.0	106.6	171.1	417.9	0.0	11.2	706.7
14	OR-2-23	13.5	688.6	1167.9	2149.0	52.7	247.9	4319.6
15	OR-3-23	27.1	451.7	302.9	1790.8	64.5	460.0	3097.0
16	OR-1-24	0.0	309.6	396.1	129.1	2547.0	3769.2	7150.0
17	OR-2-24	0.0	745.9	976.4	129.1	293.4	3033.2	5180.0
18	OR-3-24	0.0	2707.2	959.1	1162.1	13.8	3516.2	8358.3
19	SI-1-23	54.1	1353.4	4305.2	4022.9	1240.7	309.1	11,285.5
20	SI-2-23	13.5	1487.0	3771.3	9963.3	774.7	317.6	16,327.4
21	SI-3-23	0.0	635.5	2480.4	6324.4	607.3	441.5	10,489.1

). Lake Kurkol was especially prominent in 2023 (

Table 3. Quantitative variables of phytoplankton of floodplain lakes of the Irtysh River (mean with standard error), 2023.

Phylum	Kurkol	Orlovskoye	Stary Irtysh
Abundance, thou. cells L−1
Charophyta	7.7 ± 2.2	10.0 ± 5.7	16.6 ± 12
Chlorophyta	6181.1 ± 1364.8	1151.1 ± 536.5	2083.8 ± 225.3
Cyanobacteria	162,546.6 ± 46,675.8	2584.4 ± 1315.5	16,045.5 ± 79.8
Dinoflagellata	0.0 ± 0.0	40.5 ± 14.7	173.3 ± 46.6
Euglenophyta	11.6 ± 6.7	11.6 ± 1.6	60.0 ± 30.7
Heterokontophyta	213.8 ± 57.7	190.5 ± 92.1	266.6 ± 78.1
Total	168,961.1 ± 48,076.8	3984.4 ± 1922.1	18,646.1 ± 348.3
Biomass, mg L−1
Charophyta	0.0 ± 0.0	0.01 ± 0.01	0.02 ± 0.02
Chlorophyta	5.87 ± 1.48	0.42 ± 0.17	1.16 ± 0.26
Cyanobacteria	76.88 ± 18.53	0.55 ± 0.31	3.52 ± 0.54
Dinoflagellata	0.00 ± 0.00	1.45 ± 0.53	6.77 ± 1.73
Euglenophyta	0.44 ± 0.22	0.04 ± 0.02	0.87 ± 0.19
Heterokontophyta	0.46 ± 0.18	0.24 ± 0.13	0.40 ± 0.04
Total	83.67 ± 19.71	2.71 ± 1.06	12.70 ± 1.83

): the abundance of algoplanktocenosis was 9.1–42.4, and the biomass was 6.6–30.9 times higher than in other lakes.

In 2024, the most abundant phytoplankton was observed in the Shoptykol and Baskol Lakes (

Table 4. Quantitative variables of phytoplankton of floodplain lakes of the Irtysh River (mean with standard error), 2024.

Phylum	Baskol	Shoptykol	Kurkol	Orlovskoye
Abundance, thou. cells L−1
Charophyta	1.7 ± 1.0	0.0 ± 0.0	0.0 ± 0.0	0.0 ± 0.0
Chlorophyta	23,130.0 ± 13,699.5	5503.3 ± 1030.9	476.1 ± 33.7	940.5 ± 240.8
Cyanobacteria	1396.1 ± 348.1	20,341.6 ± 5550.3	1175.0 ± 340.0	4061.6 ± 423.5
Dinoflagellata	0.0 ± 0.0	0.0 ± 0.0	4.4 ± 2.9	6.1 ± 4.4
Euglenophyta	0.0 ± 0.0	137.7 ± 66.9	7.7 ± 1.1	25.0 ± 12.5
Heterokontophyta	83.8 ± 40.7	20.0 ± 11.5	1638.3 ± 239.1	278.3 ± 40.5
Total	24,611.7 ± 13,610.3	26,002.7 ± 6508.8	3301.6 ± 583.5	5311.6 ± 621.5
Biomass, mg L−1
Charophyta	0.04 ± 0.02	0.00 ± 0.00	0.00 ± 0.00	0.00 ± 0.00
Chlorophyta	50.24 ± 30.04	4.70 ± 1.08	0.55 ± 0.05	1.25 ± 0.74
Cyanobacteria	1.50 ± 0.44	7.85 ± 2.20	1.18 ± 0.30	0.78 ± 0.19
Dinoflagellata	0.00 ± 0.00	0.00 ± 0.00	0.67 ± 0.44	0.47 ± 0.34
Euglenophyta	0.00 ± 0.00	4.89 ± 2.44	0.14 ± 0.02	0.95 ± 0.80
Heterokontophyta	0.45 ± 0.30	0.04 ± 0.03	4.25 ± 0.44	3.44 ± 0.22
Total	52.22 ± 30.09	17.48 ± 5.16	6.78 ± 0.94	6.90 ± 0.93

), where the abundance was 4.9–14.1 times higher than in other communities. The maximum biomass of planktonic algae was recorded in Baskol, with a 3.4–8.9-fold excess of these variables relative to other lakes.

Cyanobacteria most often formed the basis of phytoplankton abundance (Figure 5). Chlorophyta dominated with a significant margin. In 2024, the phytoplankton of Lake Kurkol was dominated by species of the Heterokontophyta phylum, while in Baskol, it was dominated by Chlorophyta.

The composition of algae phyla, which dominated in terms of biomass, was more heterogeneous (Figure 6). In 2023, Cyanobacteria formed the basis of the variable in the phytoplankton of Lake Kurkol, and Dinoflagellata in the Orlovsky and Stary Irtysh. In 2024, species of the Heterokontophyta phylum dominated in Kurkol and Orlovsky Lakes, and Chlorophyta dominated in Baskol. Cyanobacteria dominated in Shoptykol, with Euglenophyta and Chlorophyta playing a subordinate role.

The complex of dominant species most often included Microcystis pulverea, M. aeruginosa, Aphanizomenon flos-aquae (Cyanobacteria) and Ceratium hirundinella (Dinoflagellata) (

Table 5. Composition of dominant species in phytoplankton communities of floodplain lakes of the Irtysh River, 2023–2024.

Lake *	Year	Species Name	Share, %		Lake *	Year	Species Name	Share, %
			Abundance	Biomass				Abundance	Biomass
BA	2024	V. globator	92.5	95.2	OR	2024	C. kuetzingianum	11.0	0.4
SH	2024	M. aeruginosa	13.3	27.3			M. pulverea	21.7	0.8
		M. pulverea	16.5	1.5			F. capucina	1.5	14.1
		P. longicauda	0.1	19.9			U. ulna	0.5	14.9
KU	2023	M. aeruginosa	50.8	81.6	SI	2023	A. clathrata	10.9	0.2
		M. pulverea	21.1	0.3			Ap. flos-aquae	10.8	0.3
	2024	A. formosa	18.8	15.2			M. aeruginosa	10.1	11.8
		D. sertularia	30.3	45.8			M. pulverea	13.4	0.1
OR	2023	M. pulchellum	13.9	3.5			C. hirundinella	0.4	22.6
		S. atomus	19.4	1.5			P.cinctum	0.3	29.4
		C. hirundinella	1.0	53.6

* Abbreviation of the lake names as in Table 1.

). In the phytoplankton of Baskol, the absolute dominant was the colonial green alga Volvox globator, which led to very high values of total abundance and biomass.

4.2.3. Structural Variables

The average number of species varied widely, with the minimum values in the phytoplankton of Lake Baskol and its almost fourfold excess in Shoptykol (

Table 6. Structural variables of phytoplankton of floodplain lakes of the Irtysh River (mean with standard error), 2023–2024.

Lake	Year	Average Number of Species	Shannon-Ab	Shannon-Bi	Average Mass (Volume) of the Cell, mg 10−6
Baskol	2024	11.6 ± 0.3	0.84 ± 0.38	0.62 ± 0.30	2.05 ± 0.08
Shoptykol	2024	42.6 ± 2.0	3.99 ± 0.09	3.29 ± 0.18	0.66 ± 0.04
Kurkol	2023	33.3 ± 1.2	2.39 ± 0.18	1.26 ± 0.20	0.51 ± 0.05
	2024	25.6 ± 1.4	3.24 ± 0.06	2.66 ± 0.06	2.11 ± 0.18
Orlovskoye	2023	27.6 ± 5.3	3.42 ± 0.30	2.53 ± 0.27	0.81 ± 0.14
	2024	34.6 ± 2.1	3.92 ± 0.06	3.36 ± 0.24	1.32 ± 0.19
Stary Irtysh	2023	36.3 ± 1.2	3.86 ± 0.09	3.11 ± 0.13	0.68 ± 0.09

). According to the Shannon index, phytoplankton of floodplain lakes were characterised by high diversity. The exception was the Baskol Lake community, where the pronounced dominance of Volvox globator caused very low values of the index. Low values of the Shannon index were also recorded for the phytoplankton of Kurkol in 2023. The value of the size index (average cell volume) in phytoplankton communities changed by an order of magnitude. Between 2023 and 2024, the values of the Shannon index and the size index increased in Lakes Kurkol and Orlovskoye.

4.3. Assessment of the Ecological State of Floodplain Lakes by Phytoplankton and Chemical Variables

4.3.1. Ratio of Indicator Species

Table A5. Species composition and ecological preferences of phytoplankton species of algae and cyanobacteria in floodplain lakes of the Irtysh River, 2023–2024.

	Ecological Preferences of Phytoplankton Species *
Species	HAB	TEMP	OXY	HAL	pH	D	AUT-HET	TRO	S Total
Charophyta
Closterium acerosum Ehrenberg ex Ralfs, 1848	B	–	aer	–	acf	–	–	om	–
Closterium acutum Brébisson, 1848	P-B	–	st-str	–	ind	–	–	m	2.05
Closterium parvulum Nägeli, 1849	P-B	–	–	i	ind	–	–	m	2.0
Cosmarium punctulatum Brébisson, 1856	P-B	–	–	hb	ind	–	–	m	1.3
Mougeotia sp. C.Agardh, 1824	B	–	–	–	–	–	–	–	1.0
Staurastrum gracile Ralfs ex Ralfs, 1848	P-B	–	st	i	acf	–	–	m	–
Chlorophyta
Actinastrum aciculare Playfair, 1917	P	–	–	–	–	–	–	–	–
Actinastrum hantzschii Lagerheim, 1882	P-B	–	st-str	i	–	–	–	–	2.3
Ankistrodesmus fusiformis Corda, 1838	P-B	–	st-str	i	–	–	–	e	2.0
Ankistrodesmus spiralis (W.B.Turner) Lemmermann, 1908	P	–	–	–	–	–	–	e	2.1
Ankistrodesmus arcuatus Korshikov, 1953	P-B	–	st-str	i	–	–	–	–	2.1
Ankyra ocellata (G.M.Smith) Fott, 1957	Ep	–	–	oh	–	–	–	–	–
Binuclearia lauterbornii (Schmidle) Proshkina-Lavrenko, 1966	–	–	–	–	–	–	–	–	1.8
Chlorella vulgaris Beijerinck, 1890	P-B,pb,S	–	–	hl	–	–	–	e	3.1
Chlorella vulgaris f. globosa V.M.Andreeva, 1975	–	–	–	–	–	–	–	–	–
Chlorotetraedron incus (Teiling) Komárek & Kovácik, 1985	P-B	–	st-str	i	–	–	–	–	1.9
Closteriopsis acicularis (Chodat) J.H.Belcher & Swale, 1962	P-B	–	st-str	i	–	–	–	e	1.9
Closteriopsis longissima (Lemmermann) Lemmermann, 1899	P	–	st-str	i	–	–	–	e	1.8
Coelastrum microporum Nägeli, 1855	P-B	–	st-str	i	ind	–	–	e	2.3
Coenochloris pyrenoidosa Korshikov, 1953	P	–	–	hl	–	–	–	–	–
Crucigenia fenestrata (Schmidle) Schmidle, 1900	P-B	–	st-str	–	–	–	–	e	1.8
Crucigenia quadrata Morren, 1830	P-B	–	st-str	i	acf	–	–	e	1.9
Desmodesmus armatus (Chodat) E.H.Hegewald, 2000	P-B	–	st-str	–	–	–	–	e	1.9
Desmodesmus armatus var. bicaudatus (Guglielmetti) E.H. Hegewald, 2000	P-B	–	st-str	–	–	–	–	e	2.2
Desmodesmus communis (E.Hegewald) E.Hegewald, 2000	P-B	–	st-str	–	–	–	–	e	2.0
Desmodesmus subspicatus (Chodat) E.Hegewald & A.W.F.Schmidt, 2000	P	–	–	–	–	–	–	e	–
Eudorina elegans Ehrenberg, 1832	P	–	st-str	i	–	–	–	–	2.3
Hyaloraphidium contortum Pascher & Korshikov, 1931	P-B	–	–	i	–	–	–	–	–
Kirchneriella lunaris (Kirchner) Möbius, 1894	P-B	–	st-str	i	–	–	–	e	–
Lemmermannia tetrapedia (Kirchner) Lemmermann, 1904	P-B	–	st-str	i	ind	–	–	e	2.0
Monactinus simplex (Meyen) Corda, 1839	P-B	–	st-str	–	–	–	–	–	2.0
Monoraphidium contortum (Thuret) Komárková-Legnerová, 1969	P-B	–	st-str	i	–	–	–	–	–
Monoraphidium convolutum (Corda) Komárková-Legnerová, 1969	P-B	–	st-str	–	–	–	–	e	2.2
Monoraphidium griffithii (Berkeley) Komárková-Legnerová, 1969	P-B	–	st-str	i	–	–	–	e	2.5
Monoraphidium minutum (Nägeli) Komárková-Legnerová, 1969	P-B	–	st-str	i	–	–	–	–	–
Monoraphidium pusillum (Printz) Komárková-Legnorová, 1969	P	–	–	–	–	–	–	–	1.0
Mucidosphaerium pulchellum (H.C.Wood) C.Bock, Proschold & Krienitz, 2011	P-B	–	st-str	i	ind	–	–	–	1.8
Nephrochlamys subsolitaria (G.S.West) Korshikov, 1953	P	–	–	–	–	–	–	–	–
Oocystis borgei J.W.Snow, 1903	P-B	–	st-str	i	ind	–	–	e	1.7
Oocystis lacustris Chodat, 1897	P-B	–	st-str	hl	–	–	–	–	–
Oocystis submarina Lagerheim, 1886	P-B	–	st	i	–	–	–	–	–
Pediastrum duplex Meyen, 1829	P	–	st-str	i	ind	–	–	e	–
Phacotus lenticularis (Ehrenberg) Diesing, 1866	P	–	st	–	–	–	–	–	–
Pseudopediastrum boryanum (Turpin) E.Hegewald, 2005	P-B	–	st-str	i	ind	–	–	e	2.1
Raphidocelis sigmoidea Hindák, 1977	P	–	st-str	–	–	–	–	e	1.5
Scenedesmus semipulcher Hortobágyi, 1960	P	–	–	–	–	–	–	e	2.2
Scenedesmus ellipticus Corda, 1835	P-B, S	–	st-str	–	–	–	–	–	1.7
Scenedesmus obtusus Meyen, 1829	P-B	–	st-str	–	–	–	–	e	1.8
Sphaerocystis planctonica (Korshikov) Bourrelly, 1974	P	–	–	i	–	–	–	e	1.0
Stauridium tetras (Ehrenberg) E.Hegewald, 2005	P-B	–	st-str	i	ind	–	–	om	–
Tetradesmus lagerheimii M.J.Wynne & Guiry, 2016	P-B	–	st-str	i	ind	–	–	e	2.15
Tetradesmus incrassatulus (Bohlin) M.J.Wynne, 2016	P-B	–	st-str	–	–	–	–	e	–
Tetradesmus obliquus (Turpin) M.J.Wynne, 2016	P-B, S	–	st-str	i	ind	–	–	ot	2.4
Tetraedron caudatum (Corda) Hansgirg, 1888	P-B	–	st-str	i	ind	–	–	e	2.0
Tetraedron minimum (A.Braun) Hansgirg, 1889	P-B	–	st-str	i	alf	–	–	e	2.1
Tetraedron triangulare Korshikov, 1953	P-B	–	st-str	i	–	–	–	e	2.0
Volvox globator Linnaeus, 1758	P	–	–	–	–	–	–	–	2.0
Cyanobacteria
Anathece clathrata (West & G.S.West) Komárek, Kaštovský & Jezberová, 2011	P-B	–	–	hl	–	–	–	me	1.8
Aphanizomenon flos-aquae Ralfs ex Bornet & Flahault, 1886	P-B	–	–	hl	alb	–	–	m	1.95
Aphanocapsa incerta (Lemmermann) G.Cronberg & Komárek, 1994	P-B	–	–	i	–	–	–	me	2.2
Aphanocapsa planctonica Komárek & Anagnostidis, 1995	P-B	–	–	i	–	–	–	o-e	–
Aphanocapsa delicatissima West & G.S.West, 1912	P-B	–	–	i	–	–	–	m	–
Aphanothece elabens (Meneghini) Elenkin, 1936	P-B	–	–	–	–	–	–	ot	–
Chroococcus minimus (Keissler) Lemmermann, 1904	P-B	–	–	hl	–	–	–	e	–
Chroococcus minor (Kützing) Nägeli, 1849	B,S	–	–	–	–	–	–	e	1.4
Chroococcus turgidus (Kützing) Nägeli, 1849	P-B,S	–	aer	hl	alf	–	–	e	0.8
Chrysosporum bergii (Ostenfeld) E.Zapomelová, O.Skácelová, P.Pumann, R.Kopp & E.Janecek, 2012	P	–	–	–	–	–	–	e	–
Coelomoron pusillum (Van Goor) Komárek, 1988	P	–	–	–	–	–	–	e	1.8
Coelosphaerium kuetzingianum Nägeli, 1849	P	–	–	i	–	–	–	m	1.6
Dolichospermum sigmoideum (Nygaard) Wacklin, L.Hoffmann & Komárek, 2009	P	–	–	i	–	–	–	e	1.7
Dolichospermum spiroides (Klebahn) Wacklin, L.Hoffmann & Komárek, 2009	P-B	–	st-str	i	–	–	–	e	1.3
Glaucospira laxissima (G.S.West) Simic, Komárek & Dordevic, 2014	P	–	st	–	–	–	–	–	–
Gomphosphaeria aponina Kützing, 1836	P-B	–	st-str	hl	alf	sx	–	–	–
Leptolyngbya angustissima (West & G.S.West) Anagnostidis & Komárek, 1988	B,Ep,S	warm	st-str,aer	–	–	–	–	om	–
Leptolyngbya tenuis (Gomont) Anagnostidis & Komárek, 1988	B.S	–	st-str	i	alf	–	–	om	1.1
Limnococcus limneticus (Lemmermann) Komárková, Jezberová, O.Komárek & Zapomelová, 2010	P-B	–	–	i	acf	–	–	me	1.8
Merismopedia elegans A.Braun ex Kützing, 1849	P-B,Ep	–	–	i	ind	–	–	me	–
Merismopedia glauca (Ehrenberg) Kützing, 1845	P-B	–	–	i	ind	–	–	e	–
Merismopedia minima G.Beck, 1897	B,S	–	aer	–	–	–	–	e	–
Merismopedia tenuissima Lemmermann, 1898	P-B	–	–	hl	–	–	–	e	–
Merismopedia tranquilla (Ehrenberg) Trevisan, 1845	P-B	–	–	i	ind	–	–	–	2.3
Microcystis aeruginosa (Kützing) Kützing, 1846	P-B	–	–	hl	acf	–	–	me	2.2
Microcystis pulverea (H.C.Wood) Forti, 1907	P-B,S	–	–	i	–	–	–	e	–
Microcystis wesenbergii (Komárek) Komárek ex Komárek, 2009	P-B	–	–	–	–	–	–	–	2.3
Microcystis ichthyoblabe (G.Kunze) Kützing, 1843	P	–	–	i	–	–	–	e	–
Planktolyngbya contorta (Lemmermann) Anagnostidis & Komárek, 1988	P-B	–	–	–	alf	–	–	–	–
Pseudanabaena limnetica (Lemmermann) Komárek, 1974	P-B	–	–	–	alf	–	–	e	2.2
Pseudanabaena mucicola (Naumann & Huber-Pestalozzi) Schwabe, 1964	B,Ep	–	–	i	–	–	–	e	2.1
Rhabdoderma lineare Schmidle & Lauterborn, 1900	P	–	–	hb	–	–	–	–	–
Romeria leopoliensis (Raciborski) Koczwara, 1932	P	–	st	–	–	–	–	e	1.5
Snowella atomus Komárek & Hindák, 1988	P	–	–	–	–	–	–	me	–
Spirulina laxa G.M.Smith, 1916	P	–	st	–	alf	–	–	e	3.6
Synechocystis aquatilis Sauvageau, 1892	P-B	warm	–	–	alb	–	–	–	–
Woronichinia naegeliana (Unger) Elenkin, 1933	P	–	st	–	–	–	–	e	1.8
Dinoflagellata
Ceratium hirundinella (O.F.Müller) Dujardin, 1841	P	–	st-str	i	–	–	–	e	1.3
Peridiniopsis quadridens (F.Stein) Bourrelly, 1968	P	–	–	–	–	–	–	–	1.4
Peridinium cinctum (O.F.Müller) Ehrenberg, 1832	P-B	–	st-str	i	–	–	–	–	1.4
Euglenophyta
Euglena viridis (O.F.Müller) Ehrenberg, 1830	P-B,S	eterm	st-str	mh	ind	–	–	–	1.5
Lepocinclis acus (O.F.Müller) B.Marin & Melkonian, 2003	P	eterm	st	i	ind	–	–	–	2.4
Lepocinclis oxyuris (Schmarda) B.Marin & Melkonian, 2003	P-B	–	st-str	mh	ind	–	–	–	2.3
Monomorphina pyrum (Ehrenberg) Mereschkowsky, 1877	P-B	eterm	st-str	mh	ind	–	–	–	–
Phacus longicauda (Ehrenberg) Dujardin, 1841	P-B	–	st	i	ind	–	–	–	2.8
Trachelomonas hispida (Perty) F.Stein, 1878	P-B	eterm	st-str	i	acf	–	–	–	2.2
Trachelomonas oblonga Lemmermann, 1899	P	eterm	st-str	i	–	–	–	–	2.4
Trachelomonas oblonga var. attenuata Playfair, 1915	–	–	–	–	–	–	–	–	2.4
Trachelomonas volvocina (Ehrenberg) Ehrenberg, 1834	P-B	eterm	st-str	i	ind	–	–	–	2.0
Heterokontophyta
Amphora ovalis (Kützing) Kützing, 1844	B	temp	st-str	i	alf	sx	ate	e	1.5
Asterionella formosa Hassall, 1850	P	temp	st-str	i	alf	sx	ate	me	1.35
Aulacoseira granulata (Ehrenberg) Simonsen, 1979	P-B	temp	st-str	i	alf	es	ate	e	2.0
Aulacoseira italica (Ehrenberg) Simonsen, 1979	P-B	cool	st-str	i	ind	es	ate	me	1.45
Cocconeis placentula Ehrenberg, 1838	P-B	temp	st-str	i	alf	es	ate	me	1.35
Cymbella cistula (Ehrenberg) O.Kirchner, 1878	B	–	st-str	i	alf	sx	ats	e	1.2
Cymbella tumida (Brébisson) Van Heurck, 1880	B	temp	st-str	i	alf	sx	ats	me	2.2
Cymbopleura lata (Grunow ex Cleve) Krammer, 2003	B	–	–	i	ind	–	–	ot	1.0
Diatoma vulgaris Bory, 1824	P-B	temp	st-str	i	alf	–	–	–	2.4
Dinobryon divergens O.E.Imhof, 1887	P-B	–	st-str	i	ind	–	–	–	1.2
Dinobryon sertularia Ehrenberg, 1834	P-B	–	–	i	–	–	–	–	1.3
Epithemia turgida (Ehrenberg) Kützing, 1844	B	temp	st-str	i	alf	–	–	–	1.1
Epithemia adnata (Kützing) Brébisson, 1838	B	temp	st-str	i	alb	–	–	–	1.2
Fragilaria capucina Desmazières, 1830	P-B	temp	st-str	i	ind	–	–	–	–
Gyrosigma acuminatum (Kützing) Rabenhorst, 1853	B	temp	st-str	i	alf	–	–	–	–
Melosira inflexa (Roth) Guiry, 2019	P-B	eterm	str	mh	alf	–	–	–	2.0
Navicula cincta (Ehrenberg) Ralfs, 1861	B	temp	st-str	hl	alf	–	–	–	–
Navicula cryptocephala Kützing, 1844	P-B	temp	st-str	i	ind	–	–	–	2.4
Navicula minima Grunow, 1880	P-B	temp	st-str	hl	alf	–	hce	e	1.0
Navicula radiosa Kützing, 1844	B	temp	st-str	i	ind	sx	–	–	–
Navicula rhynchocephala Kützing, 1844	B	temp	st-str	hl	alf	–	–	–	1.3
Navicula tripunctata (O.F.Müller) Bory, 1822	P-B	temp	st-str	i	alf	es	–	e	–
Nitzschia acicularis (Kützing) W.Smith, 1853	P-B	temp	st	i	alf	es	ats	om	1.4
Nitzschia linearis W.Smith, 1853	B	temp	st-str	i	alf	–	–	–	–
Nitzschia palea (Kützing) W.Smith, 1856	P-B	temp	st-str	i	ind	–	–	–	2.0
Nitzschia longissima (Brébisson ex Kützing) Grunow, 1862	–	–	–	mh	alf	–	–	–	–
Pseudokephyrion entzii W.Conrad, 1939	–	–	–	–	–	–	–	–	1.5
Sellaphora pupula (Kützing) Mereschkovsky, 1902	B	eterm	st-str	hl	ind	sx	ate	me	1.9
Staurosira construens Ehrenberg, 1843	P-B	temp	st-str	i	alf	–	–	–	1.0
Staurosira subsalina (Hustedt) Lange-Bertalot, 2004	P-B	–	st-str	hl	alf	–	–	–	–
Stephanocyclus meneghinianus (Kützing) Kulikovskiy, Genkal & Kociolek, 2022	P-B	temp	st-str	hl	alf	sp	hne	e	2.8
Stephanodiscus hantzschii Grunow, 1880	P	temp	st-str	i	alf	sx	–	–	–
Tryblionella hantzschiana Grunow, 1862	B	–	st-str	hl	alf	–	ate	e	2.6
Ulnaria ulna (Nitzsch) Compère, 2001	P-B	temp	st-str	i	alf	es	ate	e	2.4
Ulnaria ulna var. spathulifera (Grunow) Aboal, 2003	B	–	st-str	i	alf	–	ats	e	1.7

* HAB—habitat, ecological preference for a habitat type; TEMP—temperature, temperature regime indicators; OXY—oxygen, oxygen regime and water mass mobility indicators; HAL—halobity, ratio to chloride concentration in water; pH—ratio to active reaction in water, proton concentration; D—indicator taxon resistance category to organic pollution according to Watanabe [89]; AUT-HET—nutrition type categories; TRO—water trophic state indicator categories; S total—species-specific saprobity index values according to Sládeček [90].

demonstrates that in phytoplankton communities, indicators of substrate, oxygen conditions, salinity, water pH, trophic state of the water body according to Barinova [8] and Van Dam et al. [67], and water quality classes according to the EU classification were most fully represented [66]. With a single number of indicator phytoplankton species for which temperature preferences, Watanabe saprobity and nutritional type were established, we excluded them from the analysis (

Table A6. Number of indicator species in phytoplankton of floodplain lakes of the Irtysh River, 2023–2024.

	Abbreviation, Station Number and Sampling Year *
Group of indicators	BA-1-24	BA-2-24	BA-3-24	SH-1-24	SH-2-24	SH-3-24	KU-1-23	KU-2-23	KU-3-23	KU-1-24	KU-2-24	KU-3-24	OR-1-23	OR-2-23	OR-3-23	OR-1-24	OR-2-24	OR- 3-24	SI- 1-23	SI- 2-23	SI- 3-23
Habitat
Ep	0	0	1	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0
B	1	3	3	3	3	5	5	2	6	1	2	2	0	3	3	4	5	3	1	2	2
P-B	6	6	4	32	27	28	23	23	24	16	17	16	9	22	21	18	17	24	21	23	27
P	3	3	3	6	6	6	5	6	4	10	6	5	8	9	7	9	9	9	10	10	7
total	10	12	11	41	36	39	33	31	34	27	25	23	17	34	31	31	31	36	32	35	36
Temperature
cool	1	0	0	0	0	0	0	0	0	0	0	0	0	1	1	0	0	0	0	0	0
temp	0	2	2	1	0	3	6	2	7	4	6	4	2	7	8	6	5	7	1	2	4
eterm	1	2	1	3	3	4	0	1	3	2	2	2	0	2	1	0	0	1	3	1	2
warm	0	0	0	1	1	1	0	0	0	1	1	0	0	0	0	1	1	1	0	1	1
total	2	4	3	5	4	8	6	3	10	7	9	6	2	10	10	7	6	9	4	4	7
Oxygen
aer	0	0	0	0	1	1	3	1	1	0	0	1	0	1	0	0	1	0	0	0	2
str	1	1	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0
st-str	4	6	6	26	22	23	17	13	18	14	15	14	9	20	20	16	18	21	15	20	20
st	0	0	0	2	2	3	3	4	3	4	3	3	4	2	3	5	3	3	5	4	2
total	5	7	6	28	25	27	23	18	22	18	18	18	13	23	23	21	22	24	20	24	24
Salinity
hb	0	0	0	0	0	0	1	1	1	0	0	0	1	1	0	0	0	1	0	0	1
i	4	4	3	24	20	20	14	18	19	18	17	14	10	20	18	19	19	20	16	19	24
hl	1	3	3	6	6	6	7	3	8	2	3	3	0	4	5	4	3	7	5	4	5
mh	1	1	0	2	0	1	0	0	1	0	0	0	0	1	1	0	1	1	1	1	1
oh	0	0	1	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0
total	6	8	7	32	26	27	22	22	29	20	20	17	11	26	24	23	23	29	22	24	31
pH
acf	0	0	0	3	3	2	2	4	3	2	2	1	0	3	2	0	0	2	3	2	3
ind	3	3	3	10	8	9	6	7	10	6	6	6	3	7	4	5	3	6	4	6	9
alf	1	3	1	6	3	5	10	2	9	3	5	4	2	7	8	6	8	10	3	3	5
alb	1	1	1	1	1	1	0	0	1	1	1	0	0	1	2	1	1	2	1	1	2
total	5	7	5	20	15	17	18	13	23	12	14	11	5	18	16	12	12	20	11	12	19
Watanabe
sx	0	1	1	2	1	1	2	1	3	2	1	1	0	1	2	0	1	1	0	1	1
es	1	1	0	1	0	1	2	0	1	2	2	1	2	4	4	2	2	2	0	0	2
sp	0	0	0	0	0	0	1	1	1	0	0	0	0	1	1	1	0	1	1	1	1
total	1	2	1	3	1	2	5	2	5	4	3	2	2	6	7	3	3	4	1	2	4
Autotrophy-Heterotrophy
ats	0	0	0	1	0	1	1	0	0	1	1	1	1	1	2	1	2	1	0	0	0
ate	1	2	1	0	0	0	1	1	3	2	2	1	1	3	4	1	1	1	1	0	3
hne	0	0	0	0	0	0	1	1	1	0	0	0	0	1	1	1	0	1	1	1	1
hce	0	0	0	0	0	0	0	0	0	0	0	0	0	1	0	0	0	0	0	0	0
total	1	2	1	1	0	1	3	2	4	3	3	2	2	6	7	3	3	3	2	1	4
Trophic State
ot	0	0	0	1	2	1	1	2	1	0	0	0	0	2	1	0	0	0	2	1	1
om	0	0	0	2	2	3	3	1	2	2	2	2	1	1	1	3	3	1	0	2	0
m	2	2	1	1	1	1	3	3	4	0	1	2	1	2	2	2	2	3	3	3	1
me	1	2	1	2	2	2	2	4	4	3	3	1	1	4	6	0	0	1	3	3	6
e	3	3	3	19	13	14	14	11	11	11	8	8	8	15	12	14	15	18	14	13	16
o-e	0	0	0	0	0	0	1	1	0	0	0	0	0	0	0	0	0	0	0	0	0
total	6	7	5	25	20	21	24	22	22	16	14	13	11	24	22	19	20	23	22	22	24
Class of Water Quality
Class 2	3	3	2	2	2	5	5	4	6	5	4	4	3	10	8	4	6	7	8	11	11
Class 3	5	6	5	24	21	22	13	13	13	13	11	9	8	11	10	13	14	16	16	16	21
Class 4	0	0	0	1	1	1	2	2	3	0	0	0	0	1	2	2	1	1	3	2	1
Class 5	0	0	0	1	1	1	0	0	0	0	0	0	0	0	0	0	0	1	0	0	0
total	8	9	7	28	25	29	20	19	22	18	15	13	11	22	20	19	21	25	27	29	33

* Abbreviation of the lake names as in Table 1.

).

Concerning the substrate, planktonic algae species prevailed in the Kurkol (2024), Orlovskoye (2023), and Baskol Lakes (locally); in other cases, plankto-benthic algae species prevailed (Figure 7).

Most of the indicator species of algae belonged to those inhabiting slowly flowing and stagnant waters, as well as medium and slightly oxygenated waters (Figure 8).

Regarding pH, the majority of species were indifferent or preferred alkaline waters (Figure 9). Alkalibionts, which inhabit mainly waters with a pH more than 7, and acidophiles, inhabitants of acidic waters, were represented by a small number of species.

Among the algal species that serve as indicators of salinity, indifferent algae prevailed, with a smaller number of halophiles preferring waters with a high concentration of dissolved salts (Figure 10). Mesohalobes and oligohalobes, inhabitants of brackish and fresh waters, were locally found.

The most significant number of species was represented by eutraphents and meso-eutraphents (Figure 11), for which conditions with high and high levels of organic and nutrient content are optimal. Indicators of moderately polluted and clean waters (ot, om, m) were less represented in the phytoplankton communities of all lakes.

In the composition of phytoplankton communities, indicators of four classes of water quality were recorded, among which species preferring a moderate level of organic pollution, of the third quality class, prevailed everywhere (Figure 12).

4.3.2. Quantitative and Structural Variables

In both years of the study, the abundance and biomass of phytoplankton communities in floodplain lakes were high (Table 4 and Table 5), corresponding to the level of β-eutrophic water bodies with a high content of organic substances [66].

A key characteristic of the structure of phytoplankton communities is the average individual cell mass (volume). Reducing the size of algal cells is one of the universal ecological responses to global warming and pollution of aquatic ecosystems [64,65]. In 2023, the dominance of Cyanobacteria in the phytoplankton of Stary Irtysh, Kurkol, and Orlovskoye led to low values of this variable (Table 6). In 2024, the average mass (volume) of individual cells in the last two lakes increased by an order of magnitude, indicating an improvement in water quality.

Multivariate statistical analysis of the RDA (Figure 13) demonstrates that nitrate nitrogen was a key factor in explaining the variability of Cyanobacteria biomass. Species of this phylum made the main contribution to the growth of the saprobity index S values, which is an integral characteristic of the level of organic pollution in aquatic ecosystems.

The primary factor explaining the variability in green algae biomass was phosphate and TDS levels (Figure 13). The biomass of the remaining algal phyla (Heterokontophyta, Euglenophyta and Dinoflagellata) was higher under favourable oxygen conditions. The relationship between phytoplankton structure and water temperature was found to be insignificant.

5. Discussion

Floodplain lakes are characterised by a high heterogeneity of external conditions [68], which determines the uniqueness of the species composition of phytoplankton communities in each of them. In total, 149 species and forms were recorded in their composition, which emphasises the importance of floodplain lakes in maintaining biological diversity [2,3]. In each of the lakes, the species composition of phytoplankton communities was relatively homogeneous, which is due to their small sizes and depths. The composition of indicator algae species, among which not only plankton, but also plankto-benthic forms prevailed, reflected the shallow water status of the lakes.

A high proportion of eutraphents and meso-eutraphents [69], quantitative variables of phytoplankton [62], dominance of Cyanobacteria and Chlorophyta [70], composition of dominant species (Table 6), PI, and mineral nitrogen content [62] characterised an increased level of organic pollution of the surveyed floodplain lakes of the Irtysh River.

Cyanobacteria are the main participants in water blooms and indicators of eutrophication of aquatic ecosystems [70]. It has been shown that the growth of Cyanobacteria is most often limited by phosphorus [71,72]. According to [73], Cyanobacteria actively increase biomass at a total phosphorus (TP) content of approximately 20 μg L⁻¹, and more significantly at TP levels more than 30–50 μg L⁻¹. During the period of our studies, the average phosphate content in the floodplain lakes varied within the range of 19–210 μg L⁻¹ (except Orlovskoye Lake, where the amount did not exceed 10 μg L⁻¹), which in terms of phosphorus is 6–68 μg L⁻¹ (Table 2). The maximum biomass of Cyanobacteria was recorded at phosphate concentrations ranging from 50 to 130 μg L⁻¹, corresponding to phosphorus levels from 16.3 to 42.4 μg L⁻¹. Our results generally corresponded to the data of the works cited above.

For Cyanobacteria, ammonium nitrogen is the preferred form [74]. Excess ammonium ions can potentially inhibit photosynthesis, as has been established for Synechocystis [75]. During our research period, ammonium nitrogen accounted for 51.0–95.6% of the total content in 75% of the analysed water samples. In the remaining 25% of samples, 62.0–88.8% of the total nitrogen content was in the form of nitrate. RDA analysis showed that the variability of Cyanobacteria biomass in floodplain lakes was determined not by ammonium but by nitrate nitrogen, which was less available. This may be because the efficiency of algae assimilation of a particular form of nitrogen is species-specific. For example, in the San Francisco Estuary, Microcystis aeruginosa has a positive correlation with nitrate nitrogen, phosphorus, and total nitrogen [76], which is broadly consistent with our findings.

Green algae, along with Cyanobacteria, make a significant contribution to water blooms. The variability of Chlorophyta biomass in the floodplain lakes of the Irtysh River was determined by phosphates (Figure 13). The positive effect of phosphates on green algae was demonstrated by Felisberto et al. [77].

The composition of species dominating phytoplankton communities is one of the indicators of the ecological state of water bodies. It is known that, with a high level of organic pollution, species of the genus Microcystis develop in mass in phytoplankton [78]. Many species of Chlorophyta, including Volvox, prefer nutrient-rich waters [78]. Representatives of the Dinoflagellata (e.g., Ceratium, Peridinium) can significantly degrade water quality in water bodies during the period of mass reproduction [79,80]. The dominance of Microcystis aeruginosa, M. pulverea (Shoptykol, Kurkol, Orlovskoye, Stary Irtysh), Volvox globator (Baskol), Ceratium hirundinella (Stary Irtysh, Orlovskoye) was another sign of their high trophic status.

The structure of phytoplankton in floodplain lakes, which experience regular disturbances in the form of abrupt changes in physical and chemical conditions, is primarily determined by the hydrological regime [5]. During low water levels, the relative content of nutrients increases, and the dominance of Cyanobacteria, specifically the genera Microcystis, Aphanizomenon, and Dolichospermum, in the phytoplankton increases.

A similar picture was observed in the Kurkol and Orlovskoye Lakes, which were studied during both the low-water period of 2023 and the high-water period of 2024. In Kurkol, Cyanobacteria dominated in terms of abundance and biomass in 2023, and Heterokontophyta dominated in 2024 (Figure 5 and Figure 6). In Orlovskoye, from 2023 to 2024, the share of Cyanobacteria in the total abundance of phytoplankton increased, but their role in biomass formation decreased; biomass dominance shifted from dinoflagellates to heterokontophytes. There was also a complete change in the composition of the dominant species (Table 6). Changes in the structure of phytoplankton communities from 2023 to 2024 reflected the values of the Shannon index and average cell mass, which increased (

Table 7. Water quality classes and assessment of the trophic state of floodplain lakes of the Irtysh River based on phosphorus content [60,61].

Variable	Baskol	Shoptykol	Kurkol		Orlovskoye		Stary Irtysh
	2024	2024	2023	2024	2023	2024	2023
P-PO4, mg L−1	0.068 ± 0.002	0.008 ± 0.002	0.029 ± 0.008	0.006 ± 0.001	0.009 ± 0.001	0.003 ± 0.000	0.012 ± 0.002
Class of water quality	3	2	2	2	2	1	2
Trophic state	hypereutrophic	eutrophic	eutrophic	meso-trophic	eutrophic	oligo-trophic	eutrophic

).

The results of chemical analyses of water generally confirmed this conclusion. According to the classification [66], the content of easily oxidizable organic substances (PI) varied from a high level (15–30 mgO L⁻¹) in Baskol, Shoptykol (2024), and Kurkol (2023) to medium (7.5–15.0 mgO L⁻¹) in Stary Irtysh and Orlovskoye (Table 2). The mineral nitrogen content was characteristic of β-eutrophic and α-eutrophic waters. The content of easily oxidizable organic substances and total nitrogen in Kurkol and Orlovskoye Lakes in 2024 was lower compared to 2023. Lower nutrient content was also recorded during high-flow periods compared to low-flow periods in a floodplain eutrophic reservoir located in southern Vietnam [81].

The phosphorus content was relatively low, except for Baskol. According to [60,61], the trophic state of Baskol was assessed as hypereutrophic (Table 7). In 2023, the phosphorus content characterised the eutrophic state of the Stary Irtysh, Kurkol and Orlovskoye Lakes. In 2024, the phosphorus content in the water of the last two lakes decreased to the mesotrophic and oligotrophic levels, respectively.

Thus, changes in chemical and biological variables were largely synchronous, which once again confirmed the high indicator value of phytoplankton communities. A significant difference in the phytoplankton community variables in the low-water period of 2023 and the high-water period of 2024 revealed an improvement in the trophic state of the lakes in the latter case.

The absence of a statistically significant effect of temperature on the phytoplankton of floodplain lakes (Figure 13) is due to the small gradient of values of this variable during the study period (Table 2). As shown in Section 4.1, the atypical decrease in water temperature in Lake Orlovskoye was short-term and, as follows from the results obtained, did not have a significant effect on the structure of phytoplankton communities.

Temperature is one of the key predictors of variability in the species composition and quantitative variables of phytoplankton communities [82]. However, as the analysis of the literature cited below shows, its influence is often not obvious and is more significant with a favourable combination of other factors, in particular, the availability of nutrients. In the Shershnevskoye Reservoir (Chelyabinsk, Russia), under conditions of a moderate continental climate, the maximum development of phytoplankton was observed during four months, from May to August, at a temperature of 18.4–21.0 °C [83]. Cyanobacteria and green algae occupied a dominant position. In ponds with different levels of anthropogenic load (Samara Region, Russia), the seasonal dynamics of phytoplankton varied [84]. With an increase in water temperature from 16–18 °C in May to 26.1–28.7 °C in July, the quantitative variables of phytoplankton decreased approximately twofold under conditions of low anthropogenic load, and increased eightfold in a pond polluted by agricultural runoff. Under conditions of a moderate marine climate (the Netherlands), the maximum phytoplankton abundance in a shallow eutrophic lake was recorded in May, followed by a decline [85]. In a small urbanised eutrophic lake (Tolyatti, Russia), no significant differences were found between the phytoplankton assemblages in June and July [86]. Marked differences in the seasonal dynamics of phytoplankton in two neighbouring tropical high-mountain lakes (Mexico) were not associated with temperature, but depended on the pH value and trophic status [87]. According to our unpublished data, in Sorbulak (a reservoir for municipal and industrial wastewater of Almaty, Southeast Kazakhstan), the phytoplankton abundance linearly increased from 653.7 thous. cells L⁻¹ in April to 14,363.0 thous. cells L⁻¹ in August and slightly decreased to 14,017.1 thous. cells L⁻¹ in September. The water temperature during the observation period varied from 15.6 °C in April to 27.8 °C in mid-August and to 23.4 °C in September. From April to the end of July, green algae dominated, with cyanobacteria lagging somewhat behind. In August and September, the ratio of the phylum in the total abundance of phytoplankton was reversed.

Thus, the analysis of literary and own data showed that the seasonal succession of phytoplankton varies significantly depending on the water body. An increase in water temperature in the spring stimulates the development of phytoplankton, but its further succession is largely determined by the availability of nutrients or other limiting factors. With regard to the floodplain lakes we examined, the amount of nutrients and, as a consequence, the features of seasonal succession depend largely on hydrological conditions, i.e., on the volumes and timing of the onset of floods in a given year. In turn, the flooding of the Irtysh River floodplain is determined not only by the climatic conditions of the year, but also by anthropogenic factors, namely, the volumes of water released from the Upper Irtysh cascade of reservoirs. For a deeper understanding of the features of the formation of phytoplankton communities of the floodplain lakes of the Irtysh River, depending on natural and anthropogenic factors, expanded studies are needed, covering observations of different seasons and years. This approach is extremely important, especially for revealing complex, region-specific and constantly changing effects of global and local abiotic factors on phytoplankton as the basis of primary productivity of aquatic ecosystems [88].

6. Conclusions

The phytoplankton of the floodplain lakes on the Irtysh River were characterised by high species richness, which emphasises their role in maintaining biological diversity. According to the complex of chemical parameters and the structure of phytoplankton communities, all lakes were assessed as polluted. The hydrological regime had a significant impact on the ecological state of floodplain lakes: its improvement occurred during the spring flood, and deterioration occurred during the low-water period. The results obtained are highly novel, as they were the first to be performed on water bodies in this region. The combination of chemical and biological methods we used may be useful in assessing the ecological state of aquatic ecosystems in other regions.