Phylum: Charophyta   Family: Desmidiaceae

Cosmarium dubovianum J. Lütkemüller 1910

Similar in many respects to Cosmarium hornavanense but smaller with little overlap. The isthmus is smaller without overlap. In face view, 8-11 rather large granules can be observed along one side of a semicell. Kouwets (2025) gives L x B: 52-64 x 43-51 µm. Van Westen (2023) as C. hornavanense var dubovianum (Lütkemüller) Ružicka 1949, gives L x B: 63-82 x 49-61 µm, presumably considering Ružicka 's forma Messikommeri. The Outer Hebridean cell dimensions straddle these, suggesting a continuous cline.
Cell dimensions: L: 65-72 µm; B: 54-58 µm; Is: 17.6-21.6 µm; L/B:1.18-1.25.
The utter confusion over several similar taxa makes it difficult to assess distribution and abundance. With more clarity from Kouwets’ Flora (2025) a better understand should emerge.

References: 
Kouwets, F.A.C. (2025). European Flora of the Desmid Genus Cosmarium.
Van Westen, M.C. (2024). Sieralgen in Drenthe.