Sargassum muticum is a brown seaweed which has regularly alternating branches with flattened oval blades and spherical gas bladders. It is highly distinctive and olive-brown in colour. Wireweed is an invasive species from the Pacific that appeared on the Isle of Wight in 1973, having spread to Britain from France. It can accumulate high biomass and may quickly become a strong competitor for space and light, subsequently it competes with native species such as seagrasses and is considered a nuisance in harbours, beaches and shallow waters.

Source:

http://www.marlin.ac.uk/speciesinformation.php?speciesID=4296

http://www.issg.org/database/species/ecology.asp?si=727&fr=1&sts