Phylum: Charophyta   Family: Characeae

Nitella opaca (C. Agardh ex Bruzelius) C. Agardh

Dark Stonewort

A widespread species of lochs, channels and streams, often with a low alkalinity

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