Phylum: Charophyta   Family: Desmidiaceae

Staurodesmus convergens (Ehrenberg ex Ralfs) S. Lillieroth 1950

Common in shallow, slightly acid, mesotrophic water, also in the tychoplankton. The length of the spines varies considerably and the cell-shape alters accordingly.

References: 
Lillieroth, S. (1950) Über Folgen kulturbedingter Wasserstandsenkungen für Makrophyten- und Planktongemeinschaften in seichten Seen des südschwedischen Oligotrophiegebietes. Eins Studie mit besonderer Berücksichtigung der angewandten Limnologie.
West, W. & West, G.S. (1912) A Monograph of the British Desmidiaceae, Volume 4.
John, D.M. & Williamson, D.B. (2009) A Practical Guide to the Desmids of the West of Ireland.
Coesel, P.F.M. & Meesters, K.J. (2013) European Flora of the Desmid Genera Staurastrum and Staurodesmus.