The Influence of Waters of Lake Baikal on the Spatiotemporal Dynamics of Phytoplankton in the Irkutsk Reservoir

Abstract

On a model natural object, the Lake Baikal–Angara River–Irkutsk Reservoir (IR), we studied changes in the qualitative and quantitative characteristics of phytoplankton communities over three seasons in 2023 depending on seasonal changes in habitat parameters. Of the 151 identified taxa, Chrysophyta (57), Chlorophyta (41) and Bacillariophyta (24) predominated in diversity. Over the entire observation period, the highest values of total biomass and total abundance were detected in the IR in June (hydrological spring) at a water temperature of 10.0–12.7 °C, and the lowest in August, despite the fact that the water warmed up to 20 °C. No mass blooms of Cyanobacteria were observed. Statistical analysis of species abundance profiles revealed that phytoplankton community structure varied across time and space. The direct effect of cold lake waters on the structure of phytoplankton in the reservoir was observed only in early June. In summer and autumn, the structures of phytoplankton in the lake and in the reservoir differed, even at the same water temperature. Low concentrations of phosphates and nitrates, high species diversity, the presence of cold-water species and species with a wide range of temperature preferences formed a dynamic spatiotemporal structure of IR phytoplankton, distinct from other temperate reservoirs, including Lake Baikal. The results obtained are important for understanding the mechanisms of formation of the flora of artificial reservoirs of temperate latitudes and for their monitoring, taking into account seasonal dynamics and the context of global climate warming.

1. Introduction

Phytoplankton are the most important part of the ecosystem, and their responses to environmental changes not only affect the grazing, feeding, growth, reproduction and survival of various aquatic organisms [1], along with biogeochemical cycles [2,3], but can also serve as an indicator of climate change and anthropogenic impacts [4,5,6,7]. Planktonic microalgae have features of seasonal growth, which depend on abiotic conditions in the annual cycle, affecting its composition and quantitative characteristics [8,9,10]. In recent years, climate warming has led to restructuring of the plankton community in large reservoirs [7,11,12,13,14], including a change in species and changes in intraspecific characteristics [15], including in Lake Baikal, where these changes are most pronounced in the southern part of the lake [16]. A significant feature of deep-water lakes of temperate latitudes is the spring bloom, in which the main part of the algae biomass is formed. The study of phytoplankton in different seasons showed that in large reservoirs, spring phytoplankton is significantly (1.5–6 times) greater than summer phytoplankton, and it most often consists of diatoms. For example, in Lake Huron and Erie, spring phytoplankton consisted of filamentous diatoms, primarily Aulacoseira islandica (O.Müller) Simonsen [17]. In coastal seas, such as the Baltic Sea, eutrophication can lead to an increase in the abundance and biomass of summer phytoplankton [11,18], as well as more frequent and intense blooms [19,20,21]. It has also been shown that the species composition of phytoplankton varies depending on the concentrations of nutrients [22]. Observations of seasonal changes help to identify changes in communities of planktonic microalgae and also make it possible to judge not only their biodiversity [8,18,23] but also the appearance of invasive species [24].

The increase in the number of reservoirs in the world over the last century is associated with their significant economic and geographical role, and assessment of their environmental condition is very important. Differences in the geographic distributions of lakes and reservoirs determine the geographic features of their catchments. In reservoirs, the structure of the water balance, the period of water exchange, fluctuations in the water level, water inflow and runoff and water temperature change affect the structure and functioning of aquatic organisms [25]. First of all, phytoplankton react to these changes, in which structural and quantitative rearrangements occur. For example, the flow regime of the Wimmera River (Australia) was significantly changed after the construction of a reservoir, and this led to a change in the species structure of algae along the entire length of the river throughout the year. The diversity of Chlorophyta, Cyanobacteria and Chrysophyta taxa gradually augmented from the upstream to the downstream regions under normal flow conditions prior to water release. Conversely, diatom abundance was observed to be higher in the upstream area and to increase downstream following water release [26]. Depending on the type of reservoir, river or lake, which have differences in hydrodynamics and water retention times, the structure of the phytoplankton community also differs. Using the example of the Jiuqiuwan (river-type) and Taihu (lake-type) reservoirs, in the area of the source of the Dongjiang River in the Pearl River basin (China), it has been shown that over time, the number of phytoplankton species in the river-type reservoir has decreased, while in the lake-type reservoir, the number has increased versus that before the flooding of the reservoir [27]. The Irkutsk Reservoir was formed in 1956–1958 as a result of the creation of a dam on the Angara River. The hydroelectric power station (HPP) is located 56 km from Lake Baikal in the city of Irkutsk. The hydrochemical composition of the Irkutsk Reservoir is influenced by the waters of the southern part of Lake Baikal [28]. The waters of the lake belong to the hydrocarbonate class of the calcium group, characterized by very low mineralization (no higher than 100 mg m⁻³), a low content of organic matter and high concentrations of dissolved oxygen [29]. In terms of the composition of planktonic microalgae, the Irkutsk Reservoir is poorer than the underlying sections of the river after the dam [30,31,32]. The temperature regime of the reservoir is heterogeneous. During the open water period, the heated part of the reservoir near the dam is the warmest, with maximum temperatures in August–September of up to 16.2 °C [31]. The bays of the reservoir are also characterized by heterogeneity of the temperature regime and slow warming of deep waters; this is especially noticeable in July–August, when the difference in temperature with depth is 5–8 °C [31,32]. Thus, in Kurminsky Bay in July 1958, the surface temperature reached 19–21 °C, while at a depth of 5 m, it was only 12–13 °C [30]. Further studies have shown that, when including in the dam section of the reservoir, the temperature difference between the surface and bottom layers during the period of most intense heating (July–August) can exceed 14 °C. Observations of changes in the state of phytoplankton in the Irkutsk Reservoir have been carried out sporadically since its creation [30,31,32]. The total number of phytoplankton over the entire observation period has varied from 0.30 × 10⁵ cells L⁻¹ to 2.51 × 10⁵ cells L⁻¹, and the biomass has varied from 0.12 × 10⁵ mg L⁻¹ to 0.85 × 10⁵ mg L⁻¹ [30,31,32]. The maximum growth of phytoplankton during the year can occur in different months; for example, in 1973, it occurred in August, in 1980, in June, in 1982, in August, in 1986, in July and in 1987, in May. The water temperature during these periods was 8.3–16.2 °C. Moreover, the dominant complex of species in the reservoir in the spring was similar to that in Lake Baikal, while in the summer, it was different [31]. The level of Cyanobacteria growth in the reservoir has always remained low [30,31,32].

The dominant phytoplankton complex of the pelagic zone and bays of Lake Baikal includes cold-water species that develop at low temperatures—Aulacoseira baicalensis (K. Meyer) Simonsen at 1–8 °C, Aulacoseira islandica (=A. skvortzowii Edlund, Stoermer & Taylor) at 1–5 °C and Stephanodiscus meyeri Genkal et Popovskaya at up to 10 °C—and species with a wider range of temperature tolerance, for example, Nitzschia graciliformis Lange-Bertalot et Simonsen emend. Genkal et Popovskaya at 1–16 °C, Asterionella formosa Hass. At 8–19 °C, Fragilaria radians (Kützing) Williams et Round at up to 15 °C and Ulnaria acus (Kützing) Aboal at up to 16 °C [33]. Since the Irkutsk Reservoir is connected to Southern Baikal, but is shallow and warms up better, it was interesting to determine the degree of influence of the lake waters on the phytoplankton of the reservoir. In our previous works, we studied the phytoplankton of Southern Baikal and the Irkutsk Reservoir in the hydrological spring (June) [34,35] and summer (August) of 2023 [35,36]. The aim of this work was, based on a comparison of the species structure and abundance of phytoplankton in Southern Baikal and the Irkutsk Reservoir in the seasonal dynamics of changing habitat parameters, to assess the degree of influence of lake waters on the spatiotemporal distribution of phytoplankton in the reservoir.

2. Materials and Methods

2.1. Sampling and Microscopy

Sampling was carried out during expeditions within the framework of the Russian Science Foundation project in Southern Baikal (SB) and the Irkutsk Reservoir (IR) on October 15–17. For comparison, we used the results of the analysis of samples taken on June 22–26 (hydrological spring) [34] and August 17–20 [36] 2023. Additionally, samples were taken at a station 3 km from Listvyanka Village (51°49.033 N 104°54.616 E) after the ice melted, on 1 June, 16 July and 14 September 2023 (Figure 1).

During sampling, temperature and pH were measured, and water transparency was determined using a Secchi disk (S, m). In the laboratory, frozen water samples were thawed at room temperature. The mineral forms of the biogenic elements were determined after filtration using membrane cellulose acetate filters with 0.45 μm pores (Vladisart, Vladimir, Russia) to remove suspended matter, and we carried out hydrochemical analyses of the water for its Si content, PO₄³⁻, NO₂⁻, NO₃⁻ and NH₄⁺ (Table S1). The methods for the sampling and analysis of hydrochemical parameters were given previously [34].

2.2. Statistical Analysis

Environmental factors and numerical community variables were analyzed for collinearity. Pearson correlation coefficients and their p-values were computed for each pair of explanatory variables using R packages corrplot [37] and Hmisc [38]. The correlation matrix was visualized with the corrplot function using hierarchical clustering to group variables. Exploratory analyses of community composition were performed using R package vegan v.2.5-6 [39]. Environmental variables were centered and scaled to have zero means and standard deviations of one. This standardized environmental matrix was used for ordination. The species abundance data were transformed using the Hellinger procedure and subjected to transformation-based principal component (tbPCA) and redundancy (tbRDA) analyses. For unconstrained ordination, linear regression of explanatory variables was performed by the envfit function of the package vegan followed by the adjustment of permutation-based regression p-values by the FDR procedure. Environmental factors with an adjusted p-value below 0.05 were drawn on the ordination plane. For constrained ordination, eJ9, Jl9 and S9 profiles were excluded as these samples lacked data on environmental variables to use in RDA (Table S1). Both forward selection and backward elimination approaches were tested to produce an RDA model.

To generate the species abundance heatmap, the R package pheatmap [40] was used. The 50 most abundant species were selected, and an abundance table was transformed with log₂(x + 1) and passed to the pheatmap function. UPGMA-assisted clustering of the Bray–Curtis pairwise distance matrices was used to produce sample-wise and OTU-wise clustering trees. The package apcluster [41] was used for cluster analysis of β-diversity. The pairwise distance matrix computed with the Bray–Curtis similarity index was used to generate clusters by affinity propagation, followed by exemplar-based agglomerative clustering.

3. Results and Discussion

3.1. Temporal Monthly Changes in the Phytoplankton Community at the SB Station near the Source of the Angara River During the Open Water Period in 2023

To determine which phytoplankton are transported from the SB to the IR, we examined their composition at St. 9 near the source of the Angara River monthly during the open water period (from June to October) (Figure 1 and Figure 2).

During the research period, the water temperature changed from 4.3 °C to 7 °C (Figure 2A), and increases in the quantitative indicators of phytoplankton and their structure were observed (Figure 2B–G). The total abundance and biomass of phytoplankton varied from 111 × 10³ to 292 × 10³ cells L⁻¹ and from 13 × 10³ to 366 × 10³ mg L⁻¹, respectively (Figure 2B). The abundance levels at the beginning and end of June had similar values, but the biomass at the beginning of June was higher (366 × 10³ mg L⁻¹) due to the growth of large species of Bacillariophyta F. radians and Chrysophyta of the genus Dinobryon (D. cylindricum and D. cylindricum var. palustre). By the end of June, the biomass of small-celled Chlorophyta species such as Mychonastes homosphaera and Koliella cf. variabilis increased, and Microcystis sp. During the entire study period, the number of species at the station was dominated by Bacillariophyta, with the exception of June 1 and August 18, when Chrysophyta of the genus Dinobryon actively developed (Figure 2G). In June at St. 9, the species richness of Bacillariophyta was formed of the large-celled species A. baicalensis, A. islandica, F. radians and U. acus; in July, the number of species increased, and small centric diatom Stephanodiscus minutulus appeared; and in August, large-celled diatoms disappeared, such as spring species of the genus Aulacoseira. The representative of Chlorophyta, M. homosphaera, was found sporadically during open water periods, developing noticeably in August; however, its number did not exceed 50 × 10³ cells L⁻¹ and its biomass 8 × 10³ mg L⁻¹ (Table S1). In July, when the total abundance of phytoplankton was minimal (111 × 10³ cells L⁻¹), the main place in the community composition was occupied by Chlorophyta, dominated by Koliella sp. (23 × 10³ cells L⁻¹) and Monoraphidium griffithii (19 × 10³ cells L⁻¹). The largest contribution to the total biomass, amounting to 68 mg L⁻¹, was made by large-celled Bacillariophyta F. radians (15 × 10³ mg L⁻¹) and A. baicalensis (16 × 10³ mg L⁻¹), despite their low abundance. Species richness increased significantly in the autumn months due to small centric diatoms such as Cyclostephanos makarovae, Cyclostephanos dubius, Discostella pseudostelligera, Stephanocyclus meneghinianus and Thalassiosira pseudonana, which were absent in June and July (Figure 2, Table S1).

Thus, at a station near the source of the Angara River during 2023, temporary changes in the structure of phytoplankton communities were observed. At the next stage of research, in order to determine the degree of influence of the waters of Southern Baikal on the Irkutsk Reservoir, a spatial comparison of quantitative and qualitative indicators of phytoplankton in the lake and reservoir was carried out in three seasons (June—which corresponds to the hydrological spring, July, August and September—to the hydrological summer, and October—to the hydrological autumn).

3.2. Seasonal Dynamics of Quantitative Characteristics and Species Structure of Phytoplankton in Southern Baikal and the Irkutsk Reservoir in 2023

Figure 3 shows that during the open water period of 2023, the quantitative characteristics and structure of phytoplankton were changing.

The total abundance and biomass of microalgae in spring (June) at SB stations varied within the ranges of (255–1333) × 10³ cells L⁻¹ and (61–473) × 10³ mg L⁻¹, respectively. It should be noted that at St. 1 (12 km from Kultuk), we found 2779 × 10³ cells L⁻¹ due to the growth of three Cyanobacteria: Cyanodictyon planctonicum, Cyanodictyon sp. and Microcystis sp., while their total biomass had a low value of 5.2 × 10³ mg L⁻¹ (Figure 3B after [34]). The abundance and biomass of phytoplankton in the IR had the highest values for all seasons: (960–2350) × 10³ cells L⁻¹ and (544–1679) × 10³ mg L⁻¹, respectively (Figure 3B).

In summer, the total abundance and biomass of microalgae in SB varied widely, at (190–2779) × 10³ cells L⁻¹ and (26–427) × 10³ mg L⁻¹, respectively. In the IR, including bays, abundance and biomass decreased significantly and had the lowest values over the period of our observations—(186–310) × 10³ cells L⁻¹ and (41–140) × 10³ mg L⁻¹, respectively (Figure 3B according to [36]).

The autumn period was characterized by the smallest differences in water temperature between SB and IR (Table S1). In SB, the abundance varied within the range of (147–427) × 10³ cells L⁻¹, and the biomass of microalgae, due to the predominance of small-celled forms, was significantly lower than in IR and its bays—(12–45) × 10³ mg L⁻¹. The number of microalgae in the IR in autumn varied within the range of (160–494) × 10³ cells L⁻¹, and the biomass at (123–239) × 10³ mg L⁻¹. The highest values of abundance and biomass were noted in Kurminsky Bay—494 × 10³ cells L⁻¹ and 333 × 10³ mg L⁻¹, respectively (Figure 3B).

A total of 151 taxa of microalgae were discovered during the observation period (Table S2), 109 in SB and 129 in IR. We expanded the species list of phytoplankton in IR compared to previous data (118 species [32]) due to greater coverage of stations in the water area reservoirs and a thorough study of silica-scaled chrysophytes within the class Chrysophyceae using scanning QUANTA 200 SEM (FEI Company; Hillsboro, OR, USA) and transmission Leo 906 E TEM (Zeiss, Germany) microscopy [36].

Figure 3C shows the change in departmental structure. The proportion of Chrysophyta in SB and IR varied from 43% in spring to 27% in autumn of the total number of species. The Chlorophyta group was represented by 41 taxa, their number increasing from spring (15%) to autumn (34%). The same trend was noted for Bacillariophyta, the number of which increased due to small-celled centric diatoms from 19% in spring and summer to 22% in autumn.

The species richness of Cyanobacteria, both in SB and IR, was not high (9–11%), increasing slightly by summer in well-warmed bays of the reservoir (St. 11, St. 13 and St. 16).

The predominance of species diversity of microalgae Chrysophyta in the IR distinguishes it from other reservoirs, consisting of Bacillariophyta, Chlorophyta and Cyanobacteria. The predominance of these groups in reservoirs of both temperate [42,43,44] and tropical latitudes [45,46,47] is typical. An analysis of studies in temperate Russian reservoirs of the Volga River—Rybinsk, Sheksna and Kuibyshev—previously showed a high diversity of Chrysophyta [48], but they were not among the three dominant microalgae in those reservoirs.

Figure 3D shows that SB and IR phytoplankton differed in the range of dominant species across all seasons.

Cluster analysis of the abundance of phytoplankton species (Figure 4A) showed that in all seasons, the communities of IR stations form clusters, separate from the clusters of SB stations.

At the same time, St. 9 (eJ9, J9, Jl9, A9, S9, O9), located near the source of the Angara River (3 km from Listvyanka Village), does not fall into the cluster of reservoir stations at any time. But the profile of St. 10, located in the reservoir at a distance of 26 km from the source, is grouped with the spring profiles of Southern Baikal. These observations indicate that the direct influence of lake waters and the transfer of phytoplankton species from the lake to the reservoir occurred only in the spring.

Figure 4B shows that divisions occurred in community groups due to changes in species, changes in the abundance of the same species and changes in dominant species. The divisions occurred into four large clusters. The first cluster combined spring samples from SB and IR; the second, summer–autumn samples from IR. Summer and autumn SB samples formed two more separate clusters. This may indicate that the phytoplankton of the reservoir develop in relative independence from the phytoplankton of the lake in the summer and autumn periods.

3.3. Correlation of Parameters of the Aquatic Environment with Quantitative Indicators of Phytoplankton and Its Structure

We have shown differences in the degrees of correlation between environmental parameters and quantitative characteristics of phytoplankton in different seasons of the year (Figure 5A). For example, in spring (June), biomass is positively correlated with water temperature, with a coefficient of 0.85, in summer (July), it does not depend on temperature, and in autumn (October), it is again positively correlated, but with a lower correlation coefficient (0.53) than in spring.

During the open water period, environmental parameters had different relationships (Figure 5A). In June, there was a high positive correlation between water transparency and the concentration of phosphates and nitrates, as well as between water temperature and general indicators of community productivity (abundance, biomass). There was a negative correlation between these two groups of parameters. In August and October, the interdependence of these two groups of factors became less pronounced, but was partially preserved.

If we consider the codependency of environmental factors throughout the entire period of open water (Figure 5B), then the concentrations of phosphates and nitrates had a positive correlation. In addition, PO₄³⁻ and NO₃⁻ concentrations were positively correlated with water clarity. These three measures were negatively correlated with temperature. The silicon concentration, on the contrary, had a strong positive correlation with water temperature. No strong or moderate correlation of phytoplankton quantitative parameters with parameters of the aquatic environment was observed (Figure 5B). Concentrations of nitrites and ammonium in all periods considered were at the detection limits of the methods used, and, accordingly, their connection with the growth of phytoplankton was not identified (Table S1).

The results of the unconstrained ordination of all community profiles (Figure 5C) support the results of the cluster analysis (Figure 4A) and show that in all seasons, IR stations cluster separately from SB stations. The sample taken near the source of the Angara River (St. 9) on 1 June (eJ9) is grouped with samples from IR stations (J10–J17) on June 24–25 (Figure 5C), which may indicate a direct influence of lake waters on the reservoir. In the results of the cluster analysis of the full species profiles (Figure 5D), eJ9 falls into the group J7–10 + Jl9, and this group is somewhat distant from J1–6 (SB)/J11–17. That is, it turns out that in June, the IR profiles are similar to the profiles of Baikal communities with St. 1–6. However, in the remaining months of observation, no such connection was detected.

We used the following continuous environmental variables to build a model of factors influencing the community structure: phosphate, nitrite anion concentrations, concentration of ammonium cation, water transparency, water temperature and pH. The adjusted total explained variance of a community composition matrix was only 28%. By using the “forward selection” approach, a model was generated that included the phosphate anion concentration, water temperature, transparency and pH as significant quantitative model variables (Figure 5D). The adjusted total explained variance of the model was 25.5%. The ordination pattern obtained with the constrained approach was similar to that of the unconstrained technique (Figure 5C).

3.4. Changes in the Concentration of Nutrients and the Growth of Different Groups of Microalgae

If the species composition and abundance of phytoplankton depend on temperature, and temperature does not depend on the presence of microalgae, then interactions with other environmental parameters are more complex for microalgae. Water transparency depends not only on suspended matter but also on the presence of algae themselves in the environment. On the one hand, nutrients are necessary for the growth of algae, and on the other hand, they are susceptible to grazing, and in different ways by different groups of microalgae. Therefore, when interpreting the correlation between the concentration of nutrients and the growth of phytoplankton, it is necessary to consider the species composition of the dominant species.

High concentrations of phosphates (0.015–0.023 mg L⁻¹) and nitrates (0.29–0.41 mg L⁻¹) were observed in the cold (3.6–4.5 °C) SB waters in spring (Table S1). Silicon concentrations during this period varied over a wide range, as a rule, depending on the dominant microalgae in abundance and biomass. For example, at St. 1 and St. 2, higher silicon concentrations were noted (0.52 and 0.53 mg L⁻¹, respectively), with the dominance of Chlorophyta M. homosphaera, which does not consume silicon. At St. 4 and St. 5, the drop in silicon concentration (to 0.17 and 0.20 mg L⁻¹, respectively) was associated not only with the growth of Bacillariophyta A. baicalensis, N. gracilliformis, F. radians and U. acus but also with the formation of siliceous stomatocysts of the chrysophyte D. cylindricum. During the same period, in warmer (7.6–11.5 °C) IR waters, the concentrations of phosphates (0.007–0.016 mg L⁻¹) and nitrates (0.04–0.24 mg L⁻¹) were lower than in SB due to more active growth of microalgae. The silicon concentration also dropped to 0.40 mg L⁻¹ (Table S1) as a result of its consumption by the dominant Bacillariophyta A. formosa, A. islandica, N. gracilliformis, F. radians and U. acus (Figure 3D from [34]).

At SB stations in summer, the temperature in the surface layers of water mainly varied between 10 and 16 °C. The abundance and biomass were dominated by Chrysophyta genus Dinobryon and Chlorophyta M. homosphaera, which is not involved in silicon consumption, and the growth of which led to a decrease in the concentrations of phosphates (to 0.007 mg L⁻¹) and nitrates (to 0.06 mg L⁻¹). St. 9 (3 km from Listvyanka) differed from other SB stations in the summer, where at the lowest water temperature (6.3 °C), the highest concentrations of nutrients (phosphates—0.023 mg L⁻¹ and nitrates—0.35 mg L⁻¹, respectively) and the lowest values of the abundance and biomass of microalgae were observed (according to [36]). Summer water temperatures in the IR reached the highest values for the period of our observations (16.6–18.3 °C), large species of spring Bacillariophyta had low numbers (Figure 3A,D according to [36]) and the silicon concentration had the highest values, up to 0.91 mg L⁻¹ for the entire period of our study. The drop in concentrations to the lowest values for the entire period of our study for phosphates (up to 0.008 mg L⁻¹) and nitrates (up to 0.06 mg L⁻¹) (Table S1) was associated with their consumption by small-celled Cyanobacteria (dominated by C. planctonicum, Microcystis sp., Dolichospermum lemmermannii) and Chlorophyta (Koliella longiseta, M. griffithii and Chlorella sp. dominated). The abundance of large-celled species was low, with only Dinobryon sociale and D. sociale var. americanum.

In autumn, water temperature (Figure 3A) and nutrient concentrations (Table S1) were generally comparable between SB and IR. At St. 3 and St. 7, the highest concentrations of phosphates (0.016 and 0.015 mg L⁻¹, respectively) and nitrates (0.30 and 0.29 mg L⁻¹, respectively) were observed; the productivity of microalgae was not high, but the maximum abundance of the small-celled cyanobacterium C. planctonicum was noted (at St. 7—144 × 10³ cells L⁻¹) (Figure 3B). In addition, these two stations had the highest transparency (9.5 and 9 m, respectively). The abundance of phytoplankton in SB, as in the summer, was formed by small-celled Cyanobacteria—Microcystis sp., C. planctonicum, Synechocystis limnetica—and Chlorophyta—Coenococcus planctonicus, Coenocystis sp., Lindavia minuta, which were dominant among Bacillariophyta.

Although silicon concentrations were comparable between SB and IR in autumn, in contrast to SB, large species of Chrysophyta, Bacillariophyta and Dinophyta predominated in IR. Also in IR, a high content of benthic diatoms was observed, the number of which exceeded 50% of the total number of species and amounted to up to 147 × 10³ cells L⁻¹. At the same time, at all IR stations, including bays, transparency was low, at 1.5–2 m, with the exception of the very first station of the reservoir, where transparency was 7 m. Cyanobacteria were practically absent. The abundance was formed by Chlorophyta—Coenocystis sp., M. griffithii, Koliella variabilis—as well as small centric diatoms C. dubius, C. makarovae, Discostella pseudostelligera, S. minutulus and T. pseudonana. The abundance of Chrysophyta in IR compared to SB was higher due to D. divergens, and Bacillariophyta due to N. graciliformis, A. formosa and U. acus. The IR bays differed in composition, both among themselves and from the central part of the reservoir. In Kurminsky Bay, the dominant species were Bacillariophyta A. formosa, A. granulata, N. graciliformis, U. acus and A. islandica. In Elovy Bay, Chlorophyta C. planctonicus and Bacillariophyta A. formosa were dominant. In Ershovsky Bay, Coenocystis sp., N. graciliformis, U. acus, A. granulata and Chrysophyta Dinobryon divergens were dominant. All bays were dominated by small centric diatoms, the maximum abundance of which was noted in Kurminsky Bay—120 × 10³ cells L⁻¹.

In autumn, the concentrations of phosphates (0.008–0.012 mg L⁻¹) and nitrates (0.06–0.10 mg L⁻¹) in the bays were not high, as in other parts of the IR, due to their consumption by actively developing microalgae. Silicon concentrations in the bays were comparable to other IR stations and varied from 0.61 to 0.76 mg L⁻¹.

Thus, in autumn, despite similar habitat parameters, the species compositions in SB and IR differed. Small-celled Cyanobacteria and Chlorophyta continued to develop in SB, while large-celled Chrysophyta, Bacillariophyta and Dinophyta species predominated in IR, which was reflected in higher phytoplankton biomass values in IR compared to SB (Figure 3D).

3.5. Comparison of the Obtained Data with Previous Studies

As our studies showed, the species compositions of phytoplankton in SB and IR throughout all three seasons had both similarities and differences (Figure 6). Studies conducted in the Irkutsk Reservoir previously [30,31,32] also showed significant differences in both quantitative indicators and species structure of phytoplankton throughout the year. Comparison of the species structure of phytoplankton in SB and IR in the same seasons revealed differences in the growth of the dominant Baikal species. As shown above, their distribution is influenced by a complex of factors, the most significant of which is water temperature. A comparison of the phytoplankton communities of SB and IR (not including bays) in different seasons showed that the largest number of common species is observed in summer (August), when the difference in water temperature evens out. The number of common species is lowest in spring and autumn (Figure 6).

In spring, these differences are most pronounced in SB, with a spring peak in the growth of cold-loving Bacillariophyta species, some of which do not develop in IR. At the same time, a peak in the growth of heat-loving Bacillariophyta species not found in SB was observed in IR. The spring growth of Bacillariophyta in IR was also shown previously [32]. However, analysis of seasonal dynamics in IR in different years indicates significant variability may occur in spring [32], summer [30,31] and autumn [30]. As a rule, the growth of large species of Bacillariophyta in spring and early summer is typical for many reservoirs in both temperate [42,43,49] and tropical zones [50].

Previous studies [30,31,32] showed that in spring, A. baicalensis is capable of developing in the waters of the reservoir, and its abundance depends on its abundance in Lake Baikal. In our study, A. baicalensis did not reach great growth in Baikal, and it did not spread far with the water flow in IR. If we evaluate the level of phytoplankton growth in 2023 on the productivity scale [51], then in SB, the year was unproductive, since the biomass did not exceed 500 × 10³ mg L⁻¹. At the same time, IR should be classified as highly productive, since the biomass exceeded 1600 × 10³ mg L⁻¹. This high productivity was ensured by the absolute dominance of three diatom species—A. formosa, N. graciliformis and A. islandica. At St. 11, these three species accounted for 80% of the total abundance. A. islandica is an oligotrophic cold-water species that replaced A. baicalensis, developing maximally at temperatures of 7.66–11.55 °C (Sts 11–17). Also, S. meyeri developed at these stations, reaching its maximum values during the observation period at the same temperatures (Figure 3D), which corresponds to its autecology.

As was shown earlier, the cosmopolitan species A. formosa has a temperature optimum of 10–15 °C; temperatures above this inhibit the growth of this species [30], making it a typical representative of summer phytoplankton for Baikal [52]. In our study, the peak of its development occurred at the end of June, falling within the range of these temperatures in the warmer IR waters of Sts 11–17 at temperatures of 7.66–11.55 °C.

It was previously noted that in the spring of 1980, N. graciliformis was actively developing in SB, reaching 2 × 10⁵ cells L⁻¹, and remained dominant in IR. In summer, the growth of N. graciliformis began to decline. It was replaced by A. formosa and A. islandica [31]. In our study, N. graciliformis reaches its peak growth in spring in the IR. At the same time, A. formosa and A. islandica were its subdominants, remaining in the summer phytoplankton in insignificant quantities (less than 10 × 10³ cells L⁻¹).

In summer, species diversity increases significantly in both SB and IR and the number of common species increases (Figure 6). The role of small-celled Cyanobacteria and Chlorophyta is increasing. At the same time, Cyanobacteria C. planctonicum, colonial cyanobacteria typical of surface waters of mesotrophic lakes in the summer, dominate in abundance. This species of Cyanobacteria was previously shown to be part of the dominant group in the coastal areas of SB [53], together with two representatives of summer phytoplankton—Microcystis sp. and D. lemmermannii. These species, while actively developing in SB, significantly reduced their quantitative indicators in IR. Thus, C. planctonicum had the maximum abundance in SB at St. 1 (840 × 10³ cells L⁻¹), at other stations its abundance was significantly lower (Figure 3D) and at IR stations it did not exceed 68 × 10³ cells L⁻¹. Importantly, the active growth in summer in SB of small-celled Cyanobacteria and Chlorophyta, which have the characteristics of R-strategists, was noted previously [52,53]. The total biomass of this ultrananoplanktonic group of algae, which includes Chlorophyta, Chlorococcales and Cyanobacteria with a size of no more than 4 microns, first described by O.M. Kozhova (1964) [30] as “green bacteria”, during the year varied from 1 × 10³ mg L⁻¹ to 238 × 10³ mg L⁻¹ with a maximum in July–September. Changes in the community structure towards small cells under conditions of summer warming of waters, as a rule, indicate ecological instability [43,49,54]. However, since in our studies, the quantitative indicators of this group of planktonic algae were low and did not exceed 60 × 10³ mg L⁻¹ (among which M. homosphaera predominated), we cannot consider them indicators of deterioration in water quality. Among Chlorophyta, as before, Koliella longiseta and M. griffithii predominated [32]. In the present study, the summer growth of Chlorella sp. and the number of large-celled species has decreased significantly, leaving only Dinobryon sociale and D. sociale var. americanum. The total abundance and biomass of microalgae in IR, including bays, decreased significantly in summer from 186 × 10³ cells L⁻¹ to 310 × 10³ cells L⁻¹ and 41 × 10³ mg L⁻¹ to 140 × 10³ mg L⁻¹, respectively [36].

In autumn in IR, we observed the growth of the phytoplankton community, previously characteristic of the summer period [32]. In addition, we recorded a second peak in the growth of large-celled microalgae, mainly Bacillariophyta. The quantitative indicators of IR did not exceed those previously discovered. As before, Cyanobacteria species did not reach high values, unlike other reservoirs of the Angara cascade [31,55,56].

4. Conclusions

For the first time, we carried out simultaneous studies of the southern part of Lake Baikal and the Irkutsk Reservoir during three seasons of the year, which made it possible to identify the dynamics of phytoplankton growth, assess the influence of environmental parameters on the distribution and growth of qualitative and quantitative indicators of phytoplankton and trace the relationship between the drop in nutrient concentrations and the levels of growth of various microalgae. During the most productive period in the growth of phytoplankton in the reservoir, in spring (June), the main abundance and biomass are created by diatoms. During this season, water temperature has a major influence on the spatial distribution of algae. The most sensitive to changes in the temperature regime in the phytoplankton community were silica-scaled chrysophytes and diatoms. In the summer and autumn periods, the flow of water with a low content of phosphates and nitrates from the oligotrophic lake into the reservoir limited the growth of Cyanobacteria and small-celled Chlorophyta; they were present both in the lake and in the reservoir, but their numbers and biomass were not high. The influence of Southern Baikal on the Irkutsk Reservoir primarily lies in the direct transfer of water from the lake to the Angara River, on which a dam was built and a reservoir was formed. The high diversity of species, both cold-water and those with broader temperature preferences, forms a spatiotemporal structure of the reservoir’s phytoplankton that differs from those of other temperate reservoirs.