Phylum: Charophyta   Family: Desmidiaceae

Cosmarium moniliforme  Ralfs 1848

Possibly widespread in nutrient poor condition, but not proving common in the Outer Hebrides. This species and its two varieties have some overlap in terms of size and quotient ratio, so not always easy to determine

References: 
Kouwets, F.A.C. (in manuscript) European Flora of the Desmid Genus Cosmarium
Coesel, P.F.M. & Meesters, K.J. (2007) Mesotaeniaceae and Desmidiaceae of the European Lowlands
West, W. & West, G.S. (1908) A Monograph of the British Desmidiaceae, Volume 3