Phylum: Charophyta   Family: Desmidiaceae

Cosmarium contractum E.O.O. von Kirchner 1878

A complex group that Kouwets (2025) has made good progress in sorting out. There is still much ongoing research.
Cell dimensions: L: 31.2-39.9 µm; B: 24-26.8 µm; Is: 6.5-7.6 µm; Th: c.17; L/B: 1.28-1.54.
The status for the Outer Hebrides is unclear as it has spawned many varieties and forms over the years. It would seem to be relatively common in shallow lochans and with submerged Sphagnum and other aquatics. An acidophile found in oligo-mesotrophic waters.

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