Phylum: Charophyta   Family: Desmidiaceae

Staurodesmus convergens ( J. Ralfs) S. Lillieroth 1950

The length of the spines varies considerably, and the cell-shape alters accordingly.
Cell dimensions (minus spines): L: 39-50.8 µm; B: 43-57.1 µm; Is: 10.6-15.8 µm; Sp: 3-15 µm; L/B: 0.85-0.95.
Common in shallow, slightly acid, mesotrophic water, also in the tychoplankton.

References: 
Coesel, P.F.M. & Meesters, K.J. (2013). European Flora of the Desmid Genera Staurastrum and Staurodesmus.
Van Westen, M.C. (2024). Sieralgen in Drenthe.