Phylum: Charophyta Family: Desmidiaceae
Cosmarium thwaitesii Ralfs
Widspread but not at all common. Thought originally to be associated with circumneutral water, it is now regraded as more tolerant of acidic conditions. Many of the early records are regarded as doubtful through misidentification, so I have taken Kouwets' pending work for reference
References:
Kouwets, F.A.C. (in manuscript) European Flora of the Desmid Genus Cosmarium