Phylum: Charophyta Family: Closteriaceae
Closterium calosporum Wittrock 1869
The British distribution is rather vague: West & West (1904: 138, pl. 16: 1-4) suggest it's a scarce species but can be confused with C. parvulum. They give the Outer Hebrides as one of the locations but it's rarely recorded. The variety brasiliense is much commoner. Found in circumneutral waters.
References:
Wittrock, V.B. (1869) Anteckningar om Scandinavins Desmidiaceer.
West, W. & West, G.S. (1904) A Monograph of the British Desmidiaceae, Volume 1.
Ruzicka, J. (1977) Die Desmidiaceen Mitteleuropas, Volume 1, Part 1.
Brook, A.J. & Williamson, D.B. (2010) A Monograph on some British Desmids.
Coesel, P.F.M. & Meesters, K.J. (2007) Mesotaeniaceae and Desmidiaceae of the European Lowlands.