Phylum: Charophyta   Family: Closteriaceae

Closterium acutum var. variabile  (Lemmermann) Willi Krieger 1935

Found in both eutrophic and oligotrophic waters as the nominate variety but it would appear to be much scarcer.

References: 
Krieger, W. (1935) Die Desmidiaceen Europas mit Berücksichtigung der aussereuropäischen Arten. 1 Teil, Lief. 2.
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.
Coesel, P.F.M. & Meesters, K.J. (2007) Mesotaeniaceae and Desmidiaceae of the European Lowlands.