Phylum: Charophyta   Family: Desmidiaceae

Micrasterias pinnatifida  J. Ralfs 1848

A characteristically shaped, relatively small, species.
Cell dimensions: L: 67-71 µm; B: 76-84 µm; Is: 12-14 µm; Th: c.12 µm; L/B: 0.82-0.93.
West & West (1905: 82) note: “This is one of the most abundant species of the genus, more especially in Sphagnum-bogs and pools in upland districts." It is still fairly commonly recorded in the Outer Hebrides, but the habitat preference is slightly alkaline water, which is reasonably free of vegetation.

References: 
Coesel, P.F.M. & Meesters, K.J. (2023 second edition). Desmids of the Lowlands.
West, W. & West, G.S. (1905). A Monograph of the British Desmidiaceae, Volume 2.