PHYTOPLANKTON AS AN INDICATOR OF LONG-TERM CHANGES IN THE AQUATIC ECOSYSTEM OF SEVAN NATIONAL PARK, ARMENIA, 2015–2019

Lake Sevan is the second largest high-mountain lake in the Caucasus, with economic, historical and cultural significance. During the study period of 2015–2019, a total of 209 species of algae and cyanobacteria from eight taxonomic divisions were identified in the phytoplankton of Lake Sevan. Among them, diatoms increased their presence during the study period. The abundance and biomass values increased during the study period, while bloom outbreaks occurred with greater frequency. The Dolichospermum cyanobacterial species causing bloom in the summer, releasing toxins, were noted as posing a threat to the rational use of Lake Sevan. The species number in Big Sevan was generally lower than in Small Sevan. The simultaneous decrease in species richness during the study period from 2015 to 2019 from 53 to 40 species, the Shannon Index from 0.61 to 0.06 and the Saprobic Index from 2.00 to 1.73 reveals a tendency towards restructuring of the ecosystem of Lake Sevan towards simplification. In combination with an increase in quantitative indicators of phytoplankton and the number of species of eutrophic state indicators, these dynamics show an increase in the imbalance of the ecosystem. Redundance Discriminant analysis indicates an increase in water pH and copper concentration as negative factors, and an increase in the total ionic composition and temperature as factors stimulating the development of phytoplankton. Statistical maps revealed points on the coast of Lake Sevan responsible for factors influencing its ecosystem according to various variables, including copper influx. The analysis also revealed sufficient capacity of the ecosystem of Lake Sevan for self-purification during the study period. For the first time, 99 species of diatoms and cyanobacteria with IUCN categories were identified for Lake Sevan, seven of which are considered threatened. The number of threatened species of categories CR and EN decreased between 2015 and 2019, indicating adverse impacts on sensitive ecosystem species. An assessment of the changes occurring in Lake Sevan indicates the need to continue hydroecological monitoring in combination with bioindication and statistical methods and an inventory of all factors affecting the stability of the ecosystem of the high-mountain Lake Sevan.