Phylum: Charophyta   Family: Desmidiaceae

Staurastrum alternans L.A. de Brébisson ex J. Ralfs 1848

First described and illustrated by Brébisson in Ralfs (1848: 132, pl. 21: 7). The original description was poorly figured so the concept of the species presented by West & West (1912: 170, pl 126: 8-9) has been adopted. It’s closely related to S. dilatatum and S. striolatum and is regarded as a problematic complex. The zygospores of this complex are virtually identical.
Cell dimensions: L: 20-32 µm; B: 20-30 µm; Is: 7.0-11.0.
It’s infrequently recorded in oligo-mesotrophic habitats such as moorland pools and loch-margins. It has also been found with bryophytes on wet rocks. It can be found in slightly acidic to slightly alkaline habitats.

References: 
Coesel, P.F.M. & Meesters, K.J. (2013) European Flora of the Desmid Genera Staurastrum and Staurodesmus.
Ralfs, J. (1848) The British Desmidieae.
West, W. & West, G.S. (1912) A Monograph of the British Desmidiaceae, Volume 4.