Phylum: Charophyta Family: Desmidiaceae
Staurastrum polymorphum Brébisson 1848
As the name suggests, a variable species. The spiny tips to the processess and the intramarginal bifurcate granules in apical view are good identification features. Found in peaty pools and bogs but less often in the plankton.
References:
Ralfs, J. (1848) The British Desmidieae.
West, W., West, G.S. & Carter, N. (1923) A Monograph of the British Desmidiaceae, Volume 5.
Coesel, P.F.M. & Meesters, K.J. (2013) European Flora of the Desmid Genera Staurastrum and Staurodesmus.