Morphological variability of 9 representatives of pennate diatoms (Achnanthes impexa Lange-Bert., Achnanthidium helveticum (Hust.) Monnier et al., Ach. kriegeri (Krasske) Hamilton et al., Ach. subatomoides (Hust.) Monnier et al., Karayevia laterostrata (Hust.) Round et Bukht., Nupella imperfecta (Schimanski) Lange-Bert., N. tenuicephala (Hust.) Lange-Bert., Psammothidium levanderi (Hust.) Czarn., P. marginulatum (Grunow) Bukht. et Round) in a deep ultraoligotrophic cold-water Lake Elgygytgyn (Chukotka, Russia) and waterbodies of its basin was studied. It is shown that qualitative (shape of a valve, axial and middle field and quantitative (length and width of a valve, number of stria in 10 µm) features of the algae vary to a greater extent than it is described in literature. A broadened diagnosis of Ach. kriegeri and a new combination: Nupella impexa (Lange-Bert.) Genkal et Kharitonov comb. nov. are presented.

Keywords: Bacillariophyta, Pennatophyceae, Achnanthes s.l., morphology, Lake Elgygytgyn (Chukotka, Russia)

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