Seasonal dynamics of phytoplankton and some hydrochemical indicators of the Peipsi-Pskov Lake

The Peipsi-Pskov Lake is the largest freshwater body in Europe, ranking fourth in area and fifth in volume. It is characterized by shallow water and a high level of trophy. The water in the lake is poorly mineralized and has little transparency due to suspended sediments and the development of plankton. Phytoplankton acts as a primary link in trophic chains, quickly reacts to changes in the aquatic environment and serves as a convenient object in monitoring studies. The average concentrations of total nitrogen in the lake during the observation period were in the range of 525–818 µg/cubic dm. The content of ammonium, nitrate, and nitrite nitrogen in the samples was mostly below the detection limits. The values of total phosphorus varied from 20 µg/cubic dm to 54 µg/cubic dm, and its concentrations were lower than the sensitivity of the method during the flood recession. The maximum values of total nitrogen and phosphorus were recorded in August: in Lake Peipsi — 1.12 mg/cubic dm and 0.09 mg/cubic dm, in Lake Pskov — 1.59 mg/cubic dm and 0.14 mg/cubic dm, respectively. BOD5 values ranged from 1.96 mg/cubic dm in autumn to 4.26 mg/cubic dm in summer. During the growing season of 2020, 244 species taxa of phytoplankton from 8 phylums were identified in the Peipsi-Pskov Lake: Chlorophyta, Bacillariophyta, Cyanobacteria, Chrysophyta, Euglenophyta, Cryptophyta, Dinophyta and Xanthophyta. Floristic complex was characterized as сhlorophyta-diatom-cyanobacterial. The number of phytoplankton varied between 2.1 and 16.2 million cells/l depending on the season. The average number was 7.6 million cells/l. The biomass values ranged from 0.9 g/m3 to 3.6 g/cubic m. The average biomass was 2.3 g/cubic m. According to the ecological and geographical characteristics of the lake, widespread freshwater forms of microalgae predominated, preferring stagnant-flowing, slightly alkaline waters. Saprobiological analysis showed that the waters of the Peipsi-Pskov Lake were classified as moderately polluted, class III of water purity quality.