Phylum: Ochrophyta   Family: Laminariaceae

Saccharina latissima  (Linnaeus) C.E. Lane, C. Mayes, Druehl & G.W. Saunders, 2006

Sugar Kelp

Broad strap-like fronds with a wavy edge. The thin, smooth stipe emanates from a branched holdfast which is tiered. Classed as a brown seaweed, it can be green, golden or yellow depending on age and the light. A very common alga of the sublittoral zone, sometimes exposed on low spring tides

References: 
Fish & Fish (2011) - A Student's Guide to the Seashore;
Bunker, et al - Seaweeds of Britain and Ireland