Phylum: Charophyta   Family: Desmidiacea

Cosmarium subcostatulum  P.F.M. Coesel & K.J. Meesters 2023

This species needs to be compared with the similar looking C. triomphalum, which has the same habitat requirements. The main distinguishing character of the latter is two supraisthmial granules. It also has a series of intramarginal granules. C. subcostatulum has a small cluster of granules above the isthmus, which sometimes shows as a protuberance when view apically.
Cell dimensions: L: 20-27 µm, B: 18-24 µm, Is: 6.5-8 µm; 13-14 µm, L/B: 1.1-1.2.
A reasonably common species found in the tychoplankton of circumneutral waters.

References: 
Coesel, P.F.M. & Meesters, K.J. (2023 second edition). Desmids of the Lowlands.
Van Westen, M. (2024). Sieralgen in Drenthe.