Phylum: Charophyta   Family: Desmidiaceae

Cosmarium isthmium W. West 1890

There are a number of these 'dumbbell-like' cells, which probably comprise a complex. This species should be compared with the similar but larger C. swieri Kouwets 2025, both species enjoy the same habitat-type.
Single cell dimensions: L: 47.5 µm, B: 27.6 µm, Is: 12.4 µm; L/B: 1.73.
The most likely habitat is oligo-mesotrophic lochs and lochans, also boggy pools. The single Outer Hebridean find was from the shallow, small, loch with aquatics. A very rare desmid with probably an Atlantic-subarctic distribution.

References: 
Kouwets, F.A.C. (2025). European Flora of the Desmid Genus Cosmarium. Part 1: Text. Part 2: Plates.