Phylum: Cyanobacteria   Family: Oscillatoriaceae

Plectonema tomasinianum Bornet ex Gomont

Fairly common species found as free-floating caespitose masses or turfs on rocks

References: 
John, D.M., Whitton, B.A. & Brook, A.J. (2011) - The Freshwater Algal Flora of the British Isles