Summer vegetation started with warming up of the upper water layers in July. Blooming spots visible to the naked eye were reported in Mukhor Bay: cyanobacteria developed, first of all, Dolichospermum lemmermannii (P.G.Richt.) Wacklin, L. Hoffm. & Komárek, the number of which was estimated to 1401.1 thousand cells/L. Other cyanobacteria were also registered in noticeable concentrations, such as: D. flosaquae (Bornet & Flahault) P. Wacklin, L. Hoffmann & Komárek, D. scheremetieviae (Elenkin) Wacklin, L. Hoffmann & Komárek and Trichormus variabilis (Kützing ex Bornet & Flahault) Komárek & Anagnostidis, 404.6–612.1 thousand cells/L. In July, pelagic plankton was few in number, represented by negligible concentrations of nanoplanktonic flagellates. In water areas close to Mukhor Bay, we observed onset of cyanobacterial vegetation, the total number of which exceeded 600 thousand cells/L. Elevated concentrations of cyanobacteria persisted until late August in this area. In August, pelagic Baikal phytoplankton basically consists of picoplankton forms (Bondarenko, Guselnikova 1989; Popovskaya 1991; Belykh et al. 2006), the number of which reached more than 220 million cells/L. In August 2024, microphytoplankton was represented by the chrysophyte Dinobryon sociale (Ehrenb.) Ehrenb. 2.5 to 41.4 thousand cells/L and green Monoraphidium contortum (Thur.) Komarkova-Legnerova 1.5 to 10.1 thousand cells/L. M. contortum was the only species among thirteen species found in the plankton at that time and not related to the flagellate forms. The total number of nanoplanktonic phytoflagellates varied from 68 to 240 thousand cells/L. Cyanobacteria continued to vegetate in the nearshore zone by increasing their species number and changing dominants. The highest number was documented in Dolichospermum spiroides (Kleb.) Wacklin, L. Hoffm. & Komárek), up to 540 thousand cells/L. Chroococcus limneticus Lemm., 52.4 thousand cells/L, and D. affine (Lemm.) Wacklin, L. Hoffmann & Komárek, 34thousand cells/L, appeared in plankton.

In the beginning of October, we observed only small algae in the pelagic zone, dominated by Chrysochromulina parva (37.6–378.3 thousand cells/L) and Rhodomonas pusilla (30.7–145.2 thousand cells/L). The amount of nanoplanktonic dinoflagellates was 13–41 thousand cells/L. Their biomass varied along the Strait water area from 61 to 398 mg/m3. Phytoplankton in Mukhor Bay was distinguished from phytoplankton in the pelagic zone by a small number of large-celled algae: the dinoflagellate Ceratium hirundinella (O.F.Müller) Dujardin (1.5–6.4 thousand cells/L) and diatom Asterionella formosa Hass. (3.8–5.8 thousand cells/L). The samples also included semi-decomposed colonies or small fragments of cyanobacteria of the genus Dolichospermum (Ralfs ex Bornet & Flahault) P. Wacklin, L. Hoffmann & J. Komárek (D. lemmermannii, D. spiroides), the number of which ranged within 13.4–72.5 thousand cells/L, their biomass varying from 92 to 188 mg/m³. During our observations maximal biomass values were registered in the littoral zone in July, and two insignificant rises in the pelagic zone in June and October (Fig. 3).

It is considered that the species diversity indices measured during seasonal succession are evidently correlated to the structural transformations in the plankton community. The species diversity in early spring succession is fast growing, then it stabilizes and decreases dramatically in autumn and winter (Andronikova 1996). Similar pattern is generally reported in the pelagic zone where maximum Shannon indices derived for phytoplankton in June (1.88) remain relatively stable in summer (1.70–1.77), dropping in autumn to (1.58). The situation in Mukhor Bay is opposite: maximal indices (2.0) were estimated at the end of spring vegetation, in June and August at the cyanobacterial blooms.

Zooplankton. In 2024, studies were initiated in the first decade of June, when the surface water layer warmed up to 6.4° С in the pelagic zone. Endemic Epischura baikalensis Sars and single specimens of rotifers Synchaeta pachypoda Kutikova et Vassiljeva, and Notholca grandis Voronkov were registered there. In the first decade of July, the temperature of the pelagic water rose to 20° С, remaining constant until late August (Fig. 2). Higher water temperature induced Epischura migration to the hypolimnion, and the dominant position was occupied by Cyclops kolensis Lilljeborg (56 – 96%), the population of which was represented by mature individuals and nauplii.

In the period from late July to early October with abundant algae resources, the community was dominated by cladocerans, D. galeata (22–86%) and B. longirostris (6–78 %), attributed by Gutelmacher (Gutelmacher et al. 1988) to macrofiltrators. A dominant complex of rotifers is represented by 1–4 species, the leading position among which belonged to polyphagous A. priodonta, from 43 to 96% (Table 1). The species dominating a taxonomic group of rotifers the number of which exceeded 5%, were K. longispina, F. terminalis and K. quadrata (Table 1). Quantitative indices of zooplankton reached two peaks in June – October: maximal abundance and biomass in the middle of June owing to E. baikalensis growth, abundance maximum in late August on account of C. kolensis and D. galeata, and biomass due to plentiful Daphnia. The average abundance during the study period amounted to 4.3±1.7 thousand specimens/m³ and the biomass –100±94 mg/m³.

Observations in Mukhor Bay started when the surface water temperature was 15.1° С. The plankton fauna was characterized by a high species diversity – 25 species, 15 crustaceans, 10 of which were cladocerans. Two rare species were encountered in the Bay – Daphnia cristata Sars and Diaphanosoma brachyurum (Lievin). The complex of dominants included from 5 to 9 species (Table 1). Permanent constituent of the dominating core were D. galeata, E. graciloides and A. priodonta. Copepods dominated in abundance in the middle of June on account of C. kolensis population, and in late July owing to thermophilic cyclopoid copepods, Th. Crassus + M. leuckarti, attaining high abundance. Just in late July we registered abundance maximum of this taxonomic group (Fig. 4).

The development of Cladocerans was characterized by two peaks, the highest observed in late August. It should be noted that D. galeata outnumbered B. longirostris from 2.5 to 9 times. Being relatively diverse, the rotifers were inferior to the crustaceans in abundance (Table 1). A group of foraging dominants (Monakov 1998; Lazareva 2004) included predators A. priodonta, B. hudsoni, S. stylata, and filter-feeders – phytophagous K. longispina, P. dolichoptera, K. quadrata, E. dilatata, F. terminalis (Table 1). The highest abundance of rotifers was observed in early October (Fig. 4). The open water period was characterized by two peaks in zooplankton abundance with the highest value (53.0 thousand specimens/m³) registered in late August that coincided with the maximal biomass (3.76 g/m³).

Table 1. Structure of a dominant zooplankton complex (% of the taxonomic group number) in Maloe More Strait (June-August 2024)

A      B

Figure 4. Relative abundance (proportion of taxonomic group in total abundance, %) of zooplankton, А – pelagic zone; В – Mukhor Bay.

Trophic state of Maloe More Strait. To gain a better understanding of the present trophic state of the Strait, the authors used different structural coefficients (Table 2), however, the results obtained were ambiguous and inconsistent. Due to the dominance of small-sized forms, biomass of the pelagic phytoplankton remained within the limits of the oligotrophic status. Oligotrophy of Maloe More pelagic zone is also confirmed by dominant E. baikalensis and C. kolensis species, zooplankton biomass (0.015–0.240 mg/m3) and the following informative coefficients (Table 2): dominance (abundance and biomass in the taxonomic group) of copepods compared to cladocerans and rotifers (R:Cl:Cop); biomass ratio of crustaceans and rotifers (B crus/B rot); low value of cladoceran number compared to copepods N clad/N cop; w – mean individual weight of zooplankton organisms dominated by crustaceans. In oligotrophic lakes, phytoplankton biomass is sometimes lower than the biomass of zooplankton, in eutrophic waters it is higher, and similar in mesotrophic water bodies (Andronikova 1996). According to this criterion and based on average values measured during three seasons, the pelagic part of the strait may be referred to as an oligotrophic zone only in June when the zooplankton is mainly represented by Epischura.

Because of biomass is a trophic parameter of water-reservoirs, Shannon’s index ranges based on the zooplankton biomass was proposed to characterize different aquatic objects (Andronnikova, 1996). Based on the zooplankton productivity indices, the water bodies were rated as follows: 2.6-4.0 – oligotrophic; 2.1-2.5 – mesotrophic; 1.0-2.0 – eutrophic. All the indices we obtained for Mukhor Bay were belonged to the eutrophic type, the pelagic zone of Maloe More Strait is an area of extreme conditions. So Shannon’s index is inappropriate to describe the pelagic zone of Lake Baikal.

In Mukhor Bay, zooplankton biomass is higher even during intense proliferation of cyanobacteria when the total biomass of phytoplankton reaches or exceeds 1 g/m3. This is typical for oligotrophic waters, and it is only in June that the biomass of phyto- and zooplankton is comparable, as in mesotrophic lakes. The majority of structural characteristics of zooplankton from Mukhor Bay are consistent with those typical for mesotrophic and eutrophic water bodies: abundance/biomass ratio of taxonomic groups (R:Cl:Cop based on N and B, %), the biomass ratio of predator/filter feeders (B3/B2), ratio of Cladocera and Copepoda numbers (N clad/ N cop), as well as Shannon’s index based on the biomass (Нв) and trophic state index (Е). Moreover, dominant complexes included indicator species of eutrophic waters (Andronikova 1996): K. quadrata, F. terminalis, C. kolensis, B. longirostris, Th. crassus. Average seasonal zooplankton biomass amounted to 1450±571 mg/m3, which, following S.P. Kitaev’s classification (2007), complies with the mesotrophic status of the strait. Non-informative parameters describing the strait were as follows: biomass ratio of cyclopoid and calanoid copepods and (Bcycl/Bca), mean seasonal zooplankton biomass 1450±571 mg/m3) and ratio of crustaceans and rotifers (Bcr/ Brot), the latter is indicative of large-sized species dominance of the strait plankton community.