Long-term changes of phytoplankton in the lake, not affected by anthropogenic impact (Lake Krivoe, North Karelia)

Abstract

First studies of the lake ecosystem were performed in 1968–1969 in framework of the International Biological Program, as on a model lake for the observations on the natural dynamic processes. Further work had continued in 1972 and annually from 2002 to 2014. During all study period Cyanobacteria and Chrysophyta have been permanent representatives in phytoplankton. Cyanobacteria were mostly represented by small-cell species from Chroococcales, so their share in the total biomass was low, despite the high abundance. Algae from genera Dinobryon and Uroglena, had dominated in Spring and remained in the plankton during entire season. A distinctive feature of the lake plankton was a low development of diatoms (less than 10% in abundance and biomass) and the absence of the spring outbreak of diatoms in both: under ice and in open water. Cryptomonads (Cryptopyta), which also had been represented throughout of season, became practically new, fairly widespread group in lake phytoplankton. Detailed studies carried out in 2002–2014, have shown that the spring maximum of phytoplankton rarely exceeded the summer one and reached in average 0.46 mg/L. The percentage of each size class of algae in the total biomass of the plankton was determined. Algae with size to 40 microns, which are the most consumed ones by filter feeding zooplankton, constituted a significant share in the Spring time. However the most favorable trophic conditions for zooplankton were during the summer. The development of single-celled Cryptomonad, which reached about 30% of the total biomass, has increased the trophic importance of phytoplankton. Species composition and abundance of phytoplankton in the lake may indicate a conservation of the lake as a clean oligotrophic reservoir.