The coastal algocenosis of the Listvennichnyi Bay of Lake Baikal since 2011 has been undergoing changes because of excess anthropogenic level. We studied species composition and abundance of the bay littoral diatom community in the spring and summer of 2016–2017 along a section located at a depth of 2–8 m. In the studied period, Fragilaria capucina (Kütz.) J.B. Petersen dominated the studied area. The abundant development of this taxon (yielding than 50% of the total number of microalgae) and associated species of diatoms in the spring decreased the number of microphytobenthos to extremely high, previously unrecorded values. Inhibition of the development of littoral endemic species was noted; however, the endemic Hannaea baicalensis Genkal, Popovskaya et Kulikovskiy dominated at depth of 2 m with numbers comparable with F. capucina. It happened firstly at the beginning of ecological degradation of the coastal zone in 2011, when the development of this and other Baikal endemics fell to solitary cells. The subdominant complex of benthic diatoms included frequently occurring, mainly mesosaprobic species. Changes in the dominant complex of diatom communities together with the high abundance of microphytobenthos indicate a steady trend of increasing water trophic level and the further development of eutrophication in the Listvennychnyi Bay.
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