Phylum: Charophyta Family: Desmidiaceae
Staurodesmus dejectus var. robustus (Messikommer) Coesel 1993
This variety is proving to be quite scarce in the Outer Hebrides. It occurs in mildly acidic, meso-oligotrophic water. Interestingly, it has been found with moss on wet rocks.
References:
Coesel, P.F.M. (1993) Taxonomic notes on Dutch desmids II.
Coesel, P.F.M. & Meesters, K.J. (2013) European Flora of the Desmid Genera Staurastrum and Staurodesmus.