Abstract
<jats:p>The paper presents the results of the phytoplankton study in the Lower Ob at three sections (Azov, Kazym Mys and Salemal) of the main channels at the surface and at various layers within the Yamal-Nenets Autonomous Okrug during different periods of the hydrological cycle in 2020–2022. In general, 204 species (236 taxa) of algae from nine divisions were identified, with the largest contribution of diatoms (42.8%) and green (30.1%) algae. The phytoplankton diversity increases from the upper sections of the river (Azov and Kazym Mys) to the lower one (Salemal), and from the under-ice period in March-April to the June flood and maximum development in early August after maximum warming of the water. Based on the results of electron microscopic research, a diverse composition of diatoms, especially centric forms, was revealed.</jats:p>