Phylum: Charophyta   Family: Closteriaceae

Closterium abruptum  West 1892

A common acidophile found in peaty pools, often with Sphagnum, and vegetated loch margins. It can be found with striae and, seemingly, without. Formae still exist that separate these taxa but, in my opinion, it's just a variable feature within the species.

References: 
Coesel, P.F.M. & Meesters, K.J. (2007) Mesotaeniaceae and Desmidiaceae of the European Lowlands.
Brook, A.J. & Williamson, D.B. (2010) A Monograph on some British Desmids.
John, D.M. & Williamson, D.B. (2009) A Practical Guide to the Desmids of the West of Ireland.