Phylum: Cyanobacteria   Family: Nostocaceae

Nostoc caeruleum var. planctonicum (V.S. Poretzky & V.K. Tschernov) B.A. Whitton 2011

First described by Poretzky & Tschernov (1929: 553, figs 1-10). It was later moved to its varietal status by Whitton in John, Whitton, & Brook (2011). This is one of the aquatic taxa.
Cell dimensions: L: 5-7.5 µm; B: 3.2-6 µm; Is: Colonies up to 4.5 mm.
These aquatic species are scarcer than the terrestrial ones but that could be due to under-recording. This one is usually found free-floating.

References: 
Poretzky, V.S. & Tschernov, V.K. (1929) Bulletin du Jardin Botanique.
Whitton, B.A. in John, D.M., Whitton, B.A. & Brook, A.J. (2011 second edition). The Freshwater Algal Flora of the British Isles.