Phylum: Charophyta   Family: Desmidiaceae

Cosmarium decedens (P.F. Reinsch) M. Raciborski 1889

A distinct shape, which is almost twice as long as broad. It needs to be compared with the very similar Cosmarium tatricum Raciborski 1885. That latter species tends to have a narrower apex in relation to the breadth of the semicells’ base. It also has a lower L/B ratio, but there is overlap. Intermediates exist which are difficult to determine.
Cell dimensions: L: 36-48 µm; B: 18.8-24.7 µm; Is: 13.4-19.1 µm; Th: c.16 µm; L/B. 1.9-2.07.
An Arctic-alpine species that prefers sub-atmophytic habitats such as wet rocks. This is proving to be much scarcer than the similar Cosmarium tatricum.

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