Achnanthoid diatoms (Bacillariophyta) of relict Lake Mogilnoye (Kildin Island, Barents Sea)

New data on morphology of nine monoraphid diatom species from the order Achnanthales found in late summer in the uppermost brackish-water layer of the unique subpolar meromictic Lake Mogilnoye at salinity to 4 ‰ and temperature about 10 °С are presented. Four taxa, Cocconeis cf. neothumensis var. marina, Planothidium lanceolatum, P. minutissimum and Psammothidium bioretii, are recorded for the first time in the lake. New taxa, Planothidium alekseevae sp. nov., P. asymmetricum sp. nov. and P. delicatulum f. minor f. nova, are described. The lectotype of P. delicatulum is designated.