Phylum: Charophyta Family: Desmidiaceae
Cosmarium subgranatum (Nordstedt) Lütkemüller
Occasional in circumneutral waters. There are a number of similar species to add confusion.
References:
Coesel, P.F.M. & Meesters, K.J. (2007) Mesotaeniaceae and Desmidiaceae of the European Lowlands