Phylum: Charophyta   Family: Desmidiaceae

Staurastrum planctonicum E. Teiling 1946

This is similar to S. pingue but larger. There is no overlap in cell-body length and the isthmus is also broader but there may be 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 then as separate species. The processes are usually slightly divergent but can be parallel.
Cell dimensions: L: (body): 40-55 µm L: (overall): 50-55 µm; B: 76-85 µm; Is: 9.5-11.2 µm; L/B: (overall): 0.6-0.7.
Fairly common in circumneutral to slightly acidic waters, such as in the plankton of lochs and marginal vegetation of moorland pools.

References: 
Van Westen, M.C. (2024). Sieralgen in Drenthe.