Boom-bust of <i>Sargassum muticum</i> in northern Spain: 30 years of invasion

The brown seaweed <i>Sargassum muticum</i> 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 <i>S. muticum</i> 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 <i>S. muticum</i> 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, <i>Sargassum muticum</i> can be considered an addition to the algal flora rather than a threat to the native assemblages.