Impact of the invasive species Sargassum muticum (Phaeophyta) on tide pool macroalgal assemblages of northern Spain

The impact of the invasive seaweed Sargassum muticum (Yendo) Fensholt (Phaeophyta, Sargassaceae) on tide pool macroalgal assemblages of northern Spain was investigated between 2001 and 2004. Invader removal experiments were simultaneously conducted at three semi-exposed rocky-shores within a shoreline length of 200 km. Changes in the abundance of algal functional-form groups, species richness and diversity were compared between manipulated and control rock pools. Leathery macrophytes, especially Bifurcaria bifurcata, slightly increased following removal of S. muticum at 2 of the 3 studied localities. The remaining functional-form groups, diversity and species richness did not appear to be affected. Because of the heterogeneous distribution of S. muticum in northern Spain, the overall effect of the species is likely to be minimal both among localities and among pools within a locality.