Phylum: Charophyta   Family: Desmidiaceae

Cosmarium obtusatum (W. Schmidle) W. Schmidle 1898

There are a number of similar looking species but this is one of the smallest with fewer side granules visible in face view.
Cell dimensions: L: 43-68 µm; B: 41-58 µm; Is: 14-20; Th: 23-31; L/B: 1.0-1.3.
Described as rather common and widely distributed on a European scale, but not proving too common in the Outer Hebrides. It prefers the tychoplankton of circumneutral waters.

References: 
Coesel, P.F.M. & Meesters, K.J. (2023 second edition). Kouwets, F.A.C. (2025). European Flora of the Desmid Genus Cosmarium.
Van Westen, M.C. (2024). Sieralgen in Drenthe.