Abstract
The species composition of phytoplankton and quantitative parameters of its abundance are indicators of ecological conditions in a water body. This paper deals with the analysis of phytoplankton in the Irkutsk Reservoir, which was formed more than 50 years ago on the Angara River flowing out of Lake Baikal. The level of phytoplankton development in the reservoir is directly dependent on the abundance of diatoms in Lake Baikal. The dominant assemblage of algal species is stable and consists mainly of diatoms, chlorophytes, and chrysophytes. Cyanobacteria are represented by a small number of species with low abundance. According to the composition of indicator species, the water in the lower reaches of the Irkutsk Reservoir can be classified as clean. The saprobity index in the open-water period is 1.2, increasing to between 1.3 and 1.5 in small bays. The values of phytoplankton biomass and chemical parameters of water recorded in 2008 indicate that the Irkutsk Reservoir is an oligotrophic water body. Its trophic status has not changed significantly since the time of impoundment (1956–1958).