The structure of phytoplankton in the Votkinsk Reservoir, Russia (2020–2025)

Abstract

Using data from phytoplankton studies conducted in the Votkinsk Reservoir from July 2020 to 2025, its taxonomic composition, ecogeographic structure, and quantitative indicators were studied. The algal flora includes 379 intraspecific algal taxa from 9 divisions identified, with diatoms and green algae accounting for 61% of the algal flora. The annual list of planktonic algae included between 193 and 301 species and subspecies, with the main species composition remaining relatively constant over several years. In terms of ecogeography, the reservoir’s phytoplankton algae are represented by typically planktonic species, widespread in water bodies worldwide, indifferent to water salinity, and found only in freshwater bodies with neutral waters. The average phytoplankton abundance was 59.6±8.3 million cells/L and was determined by the development of cyanoprokaryotes, predominantly with small cell sizes. The average biomass was 8.89±0.93 mg/L, consisting mainly of cyanoprokaryotes and diatoms. Over the study period, phytoplankton abundance and biomass fluctuated by two orders of magnitude and exhibited increasing trends. The dominant assemblage was formed by cyanoprokaryotes, primarily anheterocystous small-celled representatives of the genera Aphanocapsa, Anathece and Planktolyngbya, diatoms, cryptophytes, and dinoflagellates. In all years of the study, dominant species were Aphanizomenon flos-aquaе and Aulacoseira granulata. Ongoing changes in phytoplankton, particularly the increased coenotic role of small-sized disazotrophic cyanoprokaryotes and dinophytes, indicate progressive eutrophication of the waters of the Kama Reach of the Votkinsk Reservoir due to the combined influence of anthropogenic factors and global warming. The community index Q and saprobity index characterize of the Votkinsk Reservoir as β-mesosaprobic, moderately polluted, corresponding to water quality class III.