Boom-bust of Sargassum muticum in northern Spain: 30 years of invasion

The brown seaweed Sargassum muticum is one of the best-studied invasive marine species, although most studies are short-term and focus on the expansion phase of invasion. Here I report 30 years of observations of the invasion of the north coast of Spain by S. muticum at regional (400 km coastline) and local scales (at the site where the invasion was first detected). At the local scale, this long-term series reveals a 'boom-bust' process where the expansion ('boom') phase lasted 12 years and the decline ('bust') was completed in 8 years. This cycle is supported by evidence at the regional scale. The impact of S. muticum on the recipient assemblages was only moderate during the initial 'boom' phase of the invasion and negligible afterwards. In conclusion, 30 years after its first observation in the region, Sargassum muticum can be considered an addition to the algal flora rather than a threat to the native assemblages.