Phylum: Charophyta   Family: Closteriaceae

Closterium closterioides var. intermedium (J. Roy & J.P. Bisset) J. Ruzicka 1973

The only different between this variety and the nominate is size, being less than half. These smaller cells could be confused with large cells of the similar Closterium navicula.
Cell dimensions: L: 90-130 µm; B: 19-25 µm; Ap: 8-10 µm; L/B: 4.9-6.
An acidophile that tolerated very slightly alkaline conditions. Found in peaty pools and loch-margins with vegetation. It's much scarcer than the nominate.

References: 
Brook, A.J. & Williamson, D.B. (2010). A Monograph on some British Desmids.
Coesel, P.F.M. & Meesters, K.J. (2023 second edition). Desmids of the Lowlands.
Van Westen, M. (2024). Sieralgen in Drenthe.