<i>Discostella lacuskarluki</i> (Manguin ex Kociolek &amp; Reviers) comb. nov.: a common nanoplanktonic diatom of Arctic and boreal lakes

<i>Cyclotella lacuskarluki</i> Manguin ex Kociolek &amp; Reviers is a small diatom known only from the original drawing that provides little information on the morphology of this taxon. As Manguin’s original materials are lost, recently collected samples from Lake Karluk, Alaska, the type locality of the species, were studied with light and scanning electron microscopy to determine its identity. As a result of this investigation, <i>C. lacuskarluki</i> is transferred here to the genus <i>Discostella</i>. <i>Discostella lacuskarluki</i> is 2.5–7 µm in diameter, has a shallow mantle, a stellar pattern of alveoli in the valve centre that may be reduced or completely absent, bifurcating costae and 4–5 marginal fultoportulae opening externally by short thickened tubes with round openings. We observed this species in numerous lakes across Alaska, in Lake Tahoe, Nevada, in several lakes across Canada, and in tundra and tree-line lakes of Northeastern Siberia. Although direct comparison with other <i>Discostella</i> species is difficult because the small size of these diatoms requires high-resolution imagery to reveal ultrastructural details, it appears that <i>D. lacuskarluki</i> is conspecific with <i>D. tatrica</i> Procházková et al. recently described from lakes of Central Europe and also reported from the European part of Russia. It is also conspecific with <i>D. nipponica</i> (Skvotzov) Tuji &amp; D. M. Williams known from Japan. The apparent wide distributional range of this species indicates that it has been overlooked and commonly reported as <i>D. pseudostelligera</i> or <i>D. stelligera</i> in arctic, boreal and alpine lakes.