Phylum: Charophyta   Family: Desmidiaceae

Cosmarium rectangulare A. Grunow 1868

A variable species with some similarity to the poorly known C. pseudonitidulum but that species has two pyrenoids instead of the one found in C. rectangulare.
Cell dimensions: L: 43-48 µm; B: 29-40 µm; Is: 9-13 µm; Th: 22-24 µm; L/B: 1.27-1.44.
Relatively common in moorland pools and loch-margins. Tolerant of acidic to mildly alkaline waters.

References: 
Kouwets, F.A.C. (2025). European Flora of the Desmid Genus Cosmarium.
Van Westen, M.C. (2024). Sieralgen in Drenthe.
West, W. & West, G.S. (1908). A Monograph of the British Desmidiaceae, Volume 3.