Composition and structure of phytoplankton in the Sylvensko-Chusovsky reach Kama Reservoir (Russia)

Based on the materials from studies of phytoplankton in the Sylvensko-Chusovsky reach of the Kama Reservoir in July 2020 and 2022, its taxonomic composition, ecological-geographical structure and quantitative indicators were studied. The large marginal Sylvensko-Chusovsky reach of the Kama Reservoir is formed by the confluence of the Sylva and Chusovaya rivers and includes two districts and three sections (See Fig. 1). The mineralization of water throughout the reach varies significantly, from 225 to 1000 mgL-1, which is associated with the influx of river water. In the Sylva River, the average mineralization values in summer are significantly higher (650 mgL-1) than those of the Chusovaya River (250 mgL-1). Precipitation and surface inflow volumes in the study area (July 2020 and 2022) were below long-term averages. The years of research varied significantly in temperature regime: the water temperature of the Sylvensko-Chusovsky reach in 2020 was 30.3 ± 1.6°С, in 2022 - 20.3 ± 0.6°С. As a result of the study, 267 species and intraspecific taxa of algae from 9 departments were identified in the phytoplankton composition of the Sylvensko-Chusovsky reach of the Kama Reservoir. The basis of the species richness was formed by diatoms (36.0%), green (28.5%), golden (12.5%) algae, and cyanoprokaryotes (11.6%); the share of other algae departments totaled 11% (See Table 1). The species composition of phytoplankton differs in various parts of the reach, which is associated with the flow of river waters from the Sylva and Chusovaya rivers into the bays; the degree of floristic similarity according to the S0rensen-Chekanovsky coefficients is low and ranges from 0.41 to 0.68 (See Fig. 2). As a result of the carried-out work, the floristic list of algae of the Sylvensky Bay was replenished by 41%, previously numbering 69 species and varieties from 5 departments (See Fig. 3). In the ecological and geographical aspect, the algal flora of plankton is represented by typically planktonic species, widespread in water bodies of the globe, indifferent to water salinity, living only in fresh water bodies and preferring neutral waters (See Table 2). In Sylvensky Bay, the ecological structure of algocenoses retained its proportions from 2004 to 2022. During the summer low-water period of algal flora, the most significant contribution is made by cyanoprokaryotes, which appear with diatoms and green sea waves (See Fig. 4). Quantitative indicators of phytoplankton development varied significantly: abundance from 0.9 to 85.0 million cellsL-1, biomass from 0.39 to 13.02 mgL-1. Differences in the temperature regime during the years of the study determined the level of phytoplankton development; in 2022, the abundance and biomass values were three times higher compared to the abnormally hot 2020 (See Table 3). Throughout the entire water area, the dominant complex in number is represented mainly by species indicators of anthropogenic eutrophication (Aphanizomenon flos-aquae, Aphanocapsa holsatica, Aulacoseira granulata, Stephanodiscus binderanus) and there is a massive development of non-heterocyst cyanoprokaryotes (Aphanocapsa holsatica, A. inserta, Anathece clathrata, Planktolyngbya limnetica, A. phanothece bachmannii). Dinophyte algae (Apocalathium aciculiferum, Peridinium cinctum, P. willei, Peridinium sp. and Gymnodinium sp.) played a significant role in the composition of dominant species by biomass, along with diatoms (See Table 4). As a result of the development of smallsized algal species, phytoplankton abundance values have increased more than 20-fold from 2004 to the present, while phytoplankton biomass has increased only twofold. The Shannon species diversity index values (2.96-5.02) indicate a complex structure and high biodiversity of phytoplankton. Higher index values were characteristic of riparian areas in 2022. Simpson's indices, reflecting the degree of expression of dominance of certain species, vary from 0.04 to 0.36 in the Sylvensko-Chusovsky Bay. The decrease of the index in the Chusovsky Bay (0.15 ± 0.03) is associated with a higher diversity of phytoplankton compared to the Sylvensky Bay (0.23 ± 0.05). Based on the composition of structure-forming species and the level of biomass, the trophic status of the Sylvensko-Chusovsky reach is determined as mesotrophic. Since the distribution of phytoplankton across the water area is uneven, the trophic state was assessed from β-oligotrophic in the Sylvensko-Chusovsky area to mesotrophic in the bays in 2020, and from mesotrophic to α-eutrophic level in 2022. The main changes in phytoplankton include a significant increase in the taxonomic diversity of golden algae, an increase in the coenotic role of disazotrophic cyanoprokaryotes and dinophyte algae, which indicates the progressive eutrophication of the waters of the Sylvensko-Chusovsky reach under the combined influence of anthropogenic factors and climate warming. The article contains 4 Figures, 4 Tables, 52 References. The Author declares no conflict of interest.

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