Phylum: Charophyta   Family: Desmidiacea

Staurastrum pingue E. Teiling 1942

This is similar to S. planctonicum but smaller. There is no overlap in cell-body length and the isthmus is also smaller but there is a slight overlap. Coesel & Meesters (2013) consider S. planctonicum a variety of S. pingue and separate them on isthmus breadth: less than 10 µm in S. pingue and greater than 10 µm in var. planctonicum. This precise distinction doesn’t fit data from the Outer Hebrides, so I prefer to follow Van Westen’s (2024) lead and regard them as separate species. He has also identified differences in isthmial apical view. The processes are usually slightly divergent but can be parallel. The degree of armature varies from almost lacking to pronounced teeth-like granules.
Overall cell dimensions: L: 31-38 µm; B: 47-55 µm; Is: 6.9-8.1 µm; L/B: 0.6-0.7.
Less common than S. planctonicum, found in slightly alkaline to slightly acidic waters, such as in the plankton of lochs and marginal vegetation of moorland pools.

References: 
Coesel, P.F.M. & Meesters, K.J. (2013). European Flora of the Desmid Genera Staurastrum and Staurodesmus.
Van Westen, M.C. (2024).Sieralgen in Drenthe.