Phylum: Charophyta   Family: Desmidiaceae

Cosmarium contractum var. subglobosum K. Förster 1965

This taxon is nearly twice as long and broad. It has markedly thick cell-walls and a truncate, retuse apex.
Cell dimensions: L. 35.2-39 µm; B. 19-21 µm; Is. 8.9-9.5 µm; Th. c.18-19 µm; L/B. 1.8-1.9.
It would seem a genuinely rare taxon found in oligotrophic waters in the benthic layer.

References: 
Kouwets, F.A.C. (2025). European Flora of the Desmid Genus Cosmarium.