Phylum: Cyanobacteria   Family: Rivulariaceae

Rivularia atra Roth ex Bornet & Flahault, 1886

A relatively common brackish water species, found in the upper tidal zone

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