Phylum: Charophyta   Family: Closteriaceae

Closterium tumidulum  F. Gay 1884

Similar to Closterium incurvum but that species lacks the tumidity, has more curvature and has less crystals. Zygospores don’t sit in a flat plain, hence the concave sides. Quadrate in face view, more triangular in oblique view. Sizes below reflect the orientation measured.
Cell dimensions: L: 90-141 µm; B: 12-22 µm; Ap: 1.9-3.7; Arc: 134-155º; µm; L/B: 6-8.
Zygospore dimensions: 36-41 x 32-36 µm.
Infrequently recorded but can be locally common in acidic and mildly alkaline conditions. Moorland lochs with peaty margins and ephemeral water bodies, usually in winter.

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.