Phylum: Charophyta   Family: Desmidiaceae

Cosmarium humile  C.F.O. Nordstedt 1889

A small desmid with three undulate sides and four undulate apices, it’s not readily confused with similar taxa. It has a variable central protuberance, which is not thought to be taxonomically significant.
Cell dimensions: L: 13-18 µm; B: 13-16.1 µm; Is: 4-5.6 µm; Th: 7.5-10.9 µm; L/B: 1.00-1.17.
Common in mildly acidic to neutral waters such as moorland pools, disused peat-cuttings, and shallow lochs.

References: 
Kouwets, F.A.C. (2025). European Flora of the Desmid Genus Cosmarium. Part 1: Text. Part 2: Plates.
Van Westen, M.C. (2024). Sieralgen in Drenthe.