Recent observation of seasonal variability of plankton in Maloe More Strait (Lake Baikal)
Nina A. Bondarenko1, Natalia G. Sheveleva1, Olga G. Pen’kova2
1Limnological Institute, Siberian Branch, Russian Academy of Sciences, Irkutsk, 664033, Russia
2Irkutsk State University, Irkutsk, 664003, Russia
Corresponding author: Nataliya G. Sheveleva (shevn@lin.irk.ru)
Academic editor: R. Yakovlev | Received 5 July 2025 | Accepted 1 August 2025 | Published 23 October 2025
http://zoobank.org/38455A8E-57CB-44BB-A2CF-DB7E9B8F70BE
Citation: Bondarenko NA, Sheveleva NG, Pen'kova OG (2025) Recent observation of seasonal variability of plankton in Maloe More Strait (Lake Baikal). Acta Biologica Sibirica 11: 1091–1107. https://doi.org/10.5281/zenodo.17403572
Abstract
In this paper, we describe the structure and quantitative characteristics of phyto- and zooplankton from Maloe More Strait of Lake Baikal studied in June-October, 2024. Maloe More is a popular recreational zone facing higher anthropogenic stress. In summer, phytoplankton in the littoral area of the strait showed continuous biomass maximum, starting from July until late August during cyanobacterial blooms. Two smaller maxima were observed in the strait pelagic zone, the first by the end of spring vegetation of phytoplankton, and second during excessive cyanobacterial growth in August. Elevated water temperatures resulted in prolonged cyanobacterial vegetation with a change of dominants. Restructuring of food resources and temperature fluctuations entailed transformations in the zooplankton structure. In June, the pelagic plankton was dominated by Epischura, a Baikal endemic. As the water temperature rose, the dominant position was occupied by Cyclops kolensis, the population of which was represented by mature specimens and nauplii. In summer, Cladocera reached its abundance peak in the nearshore. The highest number of rotifers was documented during maximal rise of nanoplanktonic flagellate concentrations in autumn.
Keywords
Lake Baikal, intensive cyanobacterial vegetation, phytoplankton, seasonal dynamics, structural changes, temperature, trophic state, zooplankton
Insights into community structures affected by changing environments is one of the basic concerns of ecological scientists. Phyto- and zooplankton play a critical role in linking aquatic food webs, involved in biogeochemical cycles of water bodies. Meanwhile, available knowledge on environmental variables governing their seasonal dynamics requires further improvement, especially with regard to coastal ecosystems (Maberly et al. 2022). It is generally accepted that phytoplankton successions in lakes depend on water temperature, water transparency, light intensity and nutrient availability. However, temperature and phytoplankton (Moore 1982; Riv’er 2012; Dong et al. 2022; and others) are considered critical factors shaping the composition and growth intensity of zooplankton.
Detailed investigations of phyto- and zooplankton of Maloe More Strait were undertaken in 1950s (Kozhova 1959; Vilisova 1959). The authors identified the species composition of plankton and described its seasonal and interannual dynamics. In 1980s, G.I. Popovskaya (1989, 1991) documented first changes in phytoplankton of Maloe More Strait registering excessive growth of cyanobacteria not only in the nearshore of the strait but in its open part in summer. Biomass of phytoplankton increased several times in contrast to the 1950s, which brought the author (Popovskaya 1991) to a conclusion that it was induced by escalating anthropogenic load in this region of Lake Baikal. At present, climate changes and high anthropogenic stress in many freshwater ecosystems, including Lake Baikal, caused structural transformations in aquatic communities and species richness (Izmest’eva et al. 2016; Timoshkin et al. 2016; Sheveleva, Penkova 2020; and others). In Lake Baikal phytoplankton, the structure of the dominant species was changed and number of small mixotrophic flagellates was sharply increased (Bondarenko et al. 2022, 2023). The zooplankton number was increased as well due to the rotifer abundance (Sheveleva, Penkova 2020; Bondarenko et al. 2023).
The present work is aimed at estimating the current diversity and variability of the abundance of plankton from Maloe More Strait studied in June-October 2024. Understanding plankton dynamics is important for appreciating their role in ecosystem health. Plankton promptly responds to environmental challenges due to its short life cycle, hence, the species composition and dynamics most readily reflect changes in their habitat that made us address variations in the trophic state of the strait using phyto- and zooplankton indicators.
Site description
Lake Baikal is the deepest (1642 m), most ancient reservoir on Earth, over 30 million years old, and contains 20% of the world’s surface freshwater. The Maloe More strait is located between the western shore of Lake Baikal and Olkhon Island. It is 75 km long, including the Mukhor Bay, which is 18 km in the widest part and 4 km in the shortest part. Its area is about one thousand km2 and its depth is up to 200 m (Kozhov 1963). This part of Lake Baikal is subject to intense anthropogenic load.
We conducted five surveys: one at the end of spring phytoplankton vegetation in June, three in summer (first decade of July, late July and August) and one in early October 2024. Plankton was sampled at two nearshore sites in Mukhor Bay and two sites in the pelagic zone of Maloe More Strait (Fig. 1).
Figure 1. Map of the study area. 1, 2 – pelagic zone; 3, 4 – Mukhor Bay.
To assess the current state of plankton in the strait, we compared original records with the published data.
Collection and processing of phytoplankton samples. 1 L samples were collected from a 0–1 m water layer by Ruttner water sampler and fixed in Utermóhl’s solution and concentrated by sedimentation. The 0.1 ml concentrate was examined by triple counting using a light microscope Peraval (Carl Zeiss). Net- and nanoplankton were counted and identified at ×720 magnification. Biomass of algae was assessed with account of individual volumes of their cells (Makarova, Pichkily 1970). Species with maximal contribution to the total biomass were specified as dominant. The species diversity was estimated according to Shannon’s index (Н) calculated by the biomass. Algae were identified following Starmash 1985; Komarek, Anagnostidis 1998; Genkal et al. 2020; and others.
Collection and processing of zooplankton samples. Juday plankton net (diameter 37.5 cm, mesh size 100 µm) was used for sampling in the upper 25-m pelagic layer and surface-to-bottom littoral layer. The laboratory processing was carried out using the standard method, the individual weights of the Baikal organisms were used to calculate the biomass (Kozhova, Melnik 1978). Average abundance, Ntotal (thousand individuals/m3), abundance of three main taxonomic groups (rotifers, cladocerans, and copepods), and the total biomass (Βtotal, mg/m3) were calculated. The species diversity was estimated according to Shannon’s index (Н), calculated by the abundance and biomass. Each taxonomic group included species with relative abundance and biomass ≥ 5% that were considered dominant (Lazareva et al. 2001). For species identification, we used guides and keys (Kutikova 1970; Einsle 1996; Korovchinsky et al. 2023).
The trophic state was assessed by determination of the number of species indicators of eutrophication (SE); biomass ratios of crustaceans and rotifers Bcr/Brot, as well as cyclopoid and calanoid copepods Bcycl/Bcal; number of cladocerans and copepods Nclad/Ncop, crustaceans and rotifers Ncr/Nrot; waverage (Btotal/Ntotal) – average individual weight of a specimen in the community; Bzoo/Bphyto – biomass ratio of consumers and producers. The trophic state was determined according to the Mäemets trophic coefficient E = K (x+1)/(A+V) (y+1) (Andronikova, 1996), where K – number of Rotifera species, A – number of Copepoda species, V – number of Cladocera species, х – number of meso-eutrophic species, y – number of oligo-mesotrophic species (Andronikova 1996).
Temperature. In the first decade of June, the nearshore water was well warmed, whereas the temperature in the pelagic zone was low (Fig. 2). Higher values were observed both in the nearshore and pelagic zones of the Strait in the first decade of July with their maxima registered in Mukhor Bay, 21.5–22.5° С. Temperatures remained elevated at all sampling sites until late August, and in early October they dropped to 12.8–14.6° С.
Phytoplankton. In the end of spring vegetation in June, the phytoplankton of the pelagic zone of the Strait was represented by common species recently observed in Lake Baikal: dominant green Koliella longiseta (Vischer) Hind, 173.4–280.8 thousand cells/L, and diatom Ulnaria acus (Kützing) M. Aboal, number of which ranged from 21.6 to 88.9 thousand cells/L. Nanoplanktonic dinoflagellates, Biecheleria sp. and Woloszynskia cf. pascheri, are plentiful, 22.2–31.2 thousand cells/L. The total biomass amounted to 301–404 mg/m3 (Fig. 3). In Mukhor Bay, the biomass was 214–418 mg/m3, the microplanktonic forms were scanty dominated by the diatom Nitzschia graciliformis Lange-Bertalot et Simonsen emend Genkal et Popovskaya (11–110 thousand cells/L). Nanoplanktonic flagellates, represented by haptophyte Chrysochromulina parva Lackey (25.4–1227.6 thousand cells/L) and cryptophyte Rhodomonas pusilla (Bachm.) Javorn. (33.9–135.2 thousand cells/L), reached high concentrations.
Figure 2. Changes in water temperature during the study period, 2024.
Figure 3. Dynamics of phytoplankton biomass, 2024: 1 – pelagic zone; 2 – Mukhor Bay.