Phylum: Charophyta   Family: Desmidiaceae

Cosmarium paludis-veteris F.A.C. Kouwets 2025

A recently described species that accommodates these small smooth-walled cells that have a semicell that is depressed-trapeziform. Many have been misidentified in the past.
Kouwets’ cell dimensions: L: 17-20 µm; B: 17-21.5 µm; Is: 4.5-6 µm; Th: c.10 µm; L/B: 0.92-1.0.
Mine – four cells: L: 16.8-22.5 µm; B: 17.3-22.9 µm; Is: 5.3-6.6 µm; L/B: 0.98-1.0.
Of the few records currently known from the Outer Hebrides, it would seem to enjoy neutral to slightly alkaline mesotrophic waters. It is quite like to be more widespread

References: 
Kouwets, F.A.C. (2025). European Flora of the Desmid Genus Cosmarium.