Nitzschia humerosa sp. nov., a new freshwater diatom species from Northern Alaska, USA

Re-examination of the diatom collection of Leonard Freese from Northern Alaska revealed the presence of a previously unknown species of the genus Nitzschia characterised by small tri- or biundulate valves. Additional diagnostic characters of this species include small capitate valve apices, irregularly oriented and often fused fibulae, presence of a central nodule, raised virgae and square areola openings on the external valve surface. This species, described here as Nitzschia humerosa sp. nov., has been found only in a few lakes and ponds of the Alaskan Coastal Plain with slightly alkaline fresh water. We compare N. humerosa to the most similar species, N. lacuna R.M. Patrick & Freese, which has been commonly reported from various areas of the Northern Hemisphere under the names of N. thermalis var. minor Hilse or N. homburgiensis Lange-Bertalot. Notably, N. lacuna was originally described from the same lake in the Point Barrow area as N. humerosa. The absence of illustrations of such morphologically distinct species as N. humerosa in diatom floras suggests that it may be either rare or geographically restricted, in contrast to other Nitzschia species from the High Arctic, which appear to have circumpolar or even cosmopolitan distributions.