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High-altitude freshwater lakes are experiencing ever-increasing risk of harmful algal blooms (HABs) on the face of climate change and a growing demand on agricultural production. The biggest alpine lake of the Caucasus, Lake Sevan, has "blooming" recently almost every year. Thus, the study of phytoplankton community' development patterns in Lake Sevan is gaining urgency. The aim of the work has been the study of the seasonal dynamics of quantitative and qualitative characteristics of phytoplankton in Lake Sevan. Also, we have tried to determine the features of horizontal distribution of phytoplankton within different seasons with the focus on Cyanobacteria distribution to identify current spatial–temporal features of HABs in Lake Sevan. Seasonal ground data collected from the photic zone of 178 stations in 2016–2018 was analysed and spatially interpolated. The results of analysis of seasonal dynamics of phytoplankton groups abundance have revealed the increased role of Cyanobacteria in the algal "blooms". Particularly, HABs were led by the dominant species of the genus Dolichospermum/Anabaena that are potentially toxic. Univariate analysis of variance with the post hoc Tukey test has proved the significance of changes in the quantitative parameters of phytoplankton development within years with the peak in 2018. Some antagonistic relations between the groups of phytoplankton under the HAB events were also shown through factorial and correlation analysis. Spatial interpolations revealed very limited extents of HABs compared with "blooms" led by Bacillariophyta species. HABs were mainly occurring in the littoral zone in a close proximity to the estuaries of Lake Sevan major tributaries. [ABSTRACT FROM AUTHOR]