Phylum: Charophyta   Family: Closteriaceae

Closterium tumidulum  F. Gay 1884

First described by Gay (1884: 339) but not illustrated. 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: 90-125 µm; B: 12-17 µm; Ap: 1.9-3.7; Arc: 134-155º; µm; L/B: 6-8.
Zygospore dimensions: 36-41 x 32-36 µm.
Generally scarce but can be locally common in acidic and mildly alkaline conditions. Moorland lochs with peaty margins and ephemeral water bodies, usually in winter. Very similar to Closterium incurvum. but that species lacks the tumidity and has less crystals.

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.
Gay, F. (1884). Note sur les Conjugées du Midi de la France.
Van Westen, M. (2024). Sieralgen in Drenthe.