Phylum: Charophyta Family: Closteriaceae
Closterium idiosporum var. punctatum (Skuja) Willi Krieg 1935
The vegetative cells can only be reliably separated when found with the zygospores; rectangular in var. punctatum, elliptical in the nominate.
References:
Coesel, P.F.M. & Meesters, K.J. (2007) Mesotaeniaceae and Desmidiaceae of the European Lowlands