According to our research, plankton seasonal changes in the Maloe More straight manifistated as sharp variations of their species and size structures and quantitative parameters. They were caused by species biology and developmental stage, ratios of the taxonomic groups in the biocoenosis, and biotic and abiotic factors. It is well known that seasonal changes of the plankton composition and their abundance depend on the combined influence of the environmental factors and so there is no the only influencing factor. Main factors, light intensity, temperature and biogenic compounds, are generally considered essential contributors to phytoplankton development (Kozhov 1963; Votintsev et al. 1975; Trifonova 1979; Sommer 1986; Talling 1993; Reynolds 2006; Maberly et al. 2022; and others). When studying environmental impacts on seasonal and annual dynamics of Baikal phytoplankton, M.M. Kozhov (1963) arrived at a conclusion that the key factor, initiating spring growth of algae, was increasing illumination in the photosynthesis zone, and the termination of this process was mainly induced by the temperature rise. Multiple consumers that actively graze on algae play an important role in regulating their abundance (Kozhov 1963; Gerasimova, Sadchikov 2024). Studies on deep oligotrophic lakes in the north of Canada (Moore 1981) showed that temperature and light conditions affect initiation of algal bloom and its duration, while nutrient concentrations control only their growth intensity. Summarizing 26-year long observations of phytoplankton in four lake basins of England, J.F. Talling (1993) presented a model correlating seasonal cycles of total phytoplankton abundance and environmental conditions. He assumed that winter minimum of shortwave radiation was critical for the correlation of seasonal cycles and main regular phases of rising phytoplankton numbers occurred at the interphase of time between changes of physical and chemical conditions in such a way that the growth limiting impact of one was compensated by proportional parameters of the other. Every year there are three of such interphases:
a) when the incoming shortwave radiation increases in late winter – early spring in terms of sufficient nutrient quantity; b) when the vertical algal cell circulation is strongly reduced and the light availability for each of them is enhanced owing to summer thermal stratification; c) when the biogenic matter arrival into the upper mixed layer is accelerated by its vertical flux during autumn cooling off. Main phases of phytoplankton depletion are observed during increased sedimentation in late spring, late summer, lower growth rates and intense grazing (Talling 1993). Precisely this situation was registered at the termination of spring vegetation in Maloe More Strait in June 2024.
It is reported that higher summer diversity of taxa and their abundance is, first of all, related to warmer water temperatures (Maberly 2022). In our case, the main driver of summer phytoplankton proliferation was warmer water temperatures alongside with complementary influx of nutrients discharged from the tourist camps. Present period is characterized by climate changes leading to 0.5° C surface air temperature rise over the territory of Russian Federation during the last decade (Third… 2022). Thermal conditions of a hot summer of 2024 enhanced development of planktonic communities. Rise in the proportion of small-celled species was observed in both phyto- and zooplankton of Maloe More Strait. Growth of cyanobacteria, involved in sustaining a long-lasting summer maximum in the seasonal dynamics of the community, contributed to the total phytoplankton abundance increase.
Noticeably lower water temperatures marked the beginning of October. The autumn maximum in the evolution of pelagic phytoplankton, in contrast to spring and summer peaks, was poorly defined. Observations during the same period of the last century showed that phytoplankton was dominated by the diatoms Cyclotella minuta Antip. and Aulacoseira islandica (O. Müll.) Sim., the golden algae Dinobryon and the dinoflagellate Ceratium hirundinellа (Popovskaya 1989, 1991). Whereas C. minuta and the latter two species were encountered by the authors in small quantities in 2024, A. islandica was entirely missing in autumn plankton of the strait at present. Phytoplankton data obtained during forty-year observations in Rappbode Reservoir (Germany) showed that seasonal shift in the community was induced by a compromise between small-celled, fast-growing species capable of utilizing available resources (r-strategists) and large-celled species with more complex and efficient mechanisms to use in nutrient-deficient conditions or get access to previously unexploited mineral nutrients (k-strategists) (Wentzky et al. 2020). In summer, the reservoir was poor in nutrients and dominated by k-strategists. During the rest of the year, the amount of nutrients and turbulence were high and the reservoir was dominated by r-strategists demonstrating maximal growth rates. In Maloe More Strait, the mass propagation of mixotrophic flagellates (r-strategists) as previously reported (Bondarenko et al. 2023) was facilitated by higher organic nitrogen and phosphorus concentrations in the water due to decaying colonies of cyanobacteria excessive growth of which occurred usually in summer and early September.
Key factors affecting the composition and growth intensity of zooplankton include: water temperature, oxygen concentration, qualitative and quantitative parameters of food (Moore 1982; Riv’er 2012; Dong et al. 2022). J. Moore (1982) assumed that temperature, among all, was the critical limiting factor for the growth of all main rotifer and crustacean species in large oligotrophic lakes in northern Canada despite of favorable conditions for feeding and illumination. Whereas food resources specified the range but not the timing of explosive growth of abundance of various zooplankton groups. Our observations in Maloe More Strait showed that structural transformations of zooplankton were related to both nutrient level and water temperature. Baikal water is richly oxygenated throughout the entire water column, the saturation never less that 70-80 % even at the bottom of the deepest parts (Votintsev et al. 1975). Therefore, oxygen concentration is not a limiting factor for the growth of zooplankton in Baikal. From mid-June until the first decade of July at the water temperature from 6.4 to 20° С, the pelagic zone is mainly dominated by copepods with absolute predominance of E. baikalensis in June. Development of Epischura and its habitat layer in Baikal pelagic zone is known to depend significantly on the water temperature (Kozhov 1963; Afanasyeva 1977), the most preferable temperature for crustaceans – 12-15° С. Among copepods, C. kolensis (56-96%) becomes the dominant species from mid-July until late August. Published research (Mazepova 1978; Pislegina 2005; Sheveleva et al. 2025) evidences that pro- lific growth of this cyclopid copepod starts with the water temperature rise.
Relatively high water temperature (from 21° С to 19° С in July and August) and abundant algae food facilitated growth of thermophilic rotifers and cladocerans that constituted the structure-forming core (Table 1). Abundant food stimulates phytophagous rotifers, K. longispina, K. quadrata and F. terminalis (Monakov 1998) to form big clusters (Riv’er 1986). During the whole open water period, A. priodonta, was constantly dominant, its abundance rising from July until October (from 0.01 to 0.2 thousand specimens/m3), that coincided with the phytoplankton peak. All Asplanchna are macrophages (Gilyarov 1977), being able to consume and utilize large cells and even whole colonies of algae. In summer 2024, D. galeata – coarse mesh filter-feeder able of consuming vegetative 0.32–1.0 µm plankton (Gutelmacher et al. 1988; Monakov 1998), outcompeted cladocerans in abundance in Maloe More Strait (44–86%).
Water cooling to 12° С in early October led to increase of Epischura proportion (57%): relatively multiple nauplii of the summer-autumn generation and junior copepodite stages in the plankton. The abundance of Cyclops kolensis remained at a high level. The leading position of D. galeata in the cladoceran group was occupied by B. longirostris (22 and 49%, respectively). During this period, rotifers had three dominant species, relatively large percentage in abundance belonged to K. quadrata 49%, A. priodonta 23% and K. longispina 15%.
There were two peaks in the abundance and biomass of pelagic zooplankton. The first peak was assigned to the growth and abundance of winter-spring Epischura, the second peak in late August was related to abundance of thermophilic C. kolensis and D. galeata species.
Zooplankton, in turn, was capable of regulating algal and cyanobacterial growth. Absence of algal blooms, despite of high nutrient loads, is attributed to intensive zooplankton (filter feeders) grazing (Gerasimova, Sadchikov 2024). Vigorous consumers of algae and cyanobacteria are large-sized Daphnia, such as, Daphnia longispina O.F. Müller (Gerasimova, Sadchikov 2024). Under high nutrient concentrations, algal blooms are supressed by abundant Daphnia. In some cases, colonial algae grow to such an extent that become hard to consume, resistant to herbivorous zooplankton grazing. In such periods, zooplankton is not able to control algal blooms and recover water quality, that was registered in Maloe More Strait in July 2024. By August, cladocerans attained large numbers and the water temperature dropped, diminishing the concentrations of cyanobacteria.
It seems a challenging task to select an indicator representing trophic status of water for such a unique basin as Lake Baikal. The phytoplankton sustains a more distinct tendency for higher trophic levels during intensive cyanobacterial vegetation, first of all, in Mukhor Bay. Following Kitaev’s classification (Kitaev 2007), indices estimated for Mukhor Bay are consistent with either eutrophic or mesotrophic (rarely, oligotrophic) type of water bodies in different periods, whereas the strait pelagic zone is described as an oligotrophic zone in spring and August. In July, the trophic status of the pelagic zone is higher. It was earlier stated (Bondarenko et al. 2024) that the trophic state of the pelagic zone becomes higher during years of prolific spring vegetation of psychrophilic Baikal complex as well.
This study was focused on seasonal fluctuations of the species composition and quantitative parameters of phyto- and zooplankton of Maloe More Strait (Lake Baikal) with the account of temperature variations of the environment. The authors collected and analyzed samples from the nearshore and pelagic zones of the strait during 4 months in 2024. It was found out that these indices varied throughout seasons and the strait area. Phytoplankton biomass in the nearshore zone demonstrated maximum during intensive cyanobacterial vegetation in July-August at the water temperature rise up to 21.5–24° С. We also observed two smaller peaks in the pelagic zone of the strait: at the end of spring phytoplankton vegetation and during cyanobacterial blooms in summer. Food resources and temperature range affected the qualitative composition of zooplankton and its quantitative characteristics. During low water temperature periods, the pelagic plankton was dominated by Epischura, during cyanobacterial blooms and water warming up we registered abundance maximum of cladocerans. Bursts of cyanobacterial growth in the nearshore zone were accompanied by maximal increase of cladoceran and thermophilic rotifer numbers. In the pelagic and littoral zones the highest numbers of rotifers was registered when nanoplanktonic phytoflagellates reached maximal concentrations. The majority of structural parameters of plankton from Mukhor Bay obtained during the open water period in 2024, is consistent with the values typical for mesotrophic and eutrophic basins. Based on the quantitative and structural parameters of plankton obtained during surveying the strait pelagic zone in June, August–October, this zone is described as an oligotrophic area. In July, during cyanobacterial blooming the trophic level of the pelagic zone was higher.
This study was carried out under the State Project № 0279-2021-0007. Thanks are offered to E.M. Timoshkina for your help in finding papers on the seasonal dynamics of plankton and translating the article into English.
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