Macroalgal competition and invasive success: testing competition in mixed canopies of Sargassum muticum and Saccharina latissima

The invasive fucoid Sargassum muticum is currently consolidating its occupation of subtidal fringe habitat in Strangford Lough, Northern Ireland. The native laminiarian Saccharina latissima is the dominant species historically associated with the invaded habitat. The competitive interaction between Sargassum and Saccharina and its ultimate outcome is therefore of considerable consequence for the subtidal communities in Strangford Lough. To examine competition between adult macroalgae, a replacement series methodology was used to quantify intra- and inter-specific competition for light in the process of canopy over-growth. Competitive effects were not apparent for Saccharina, whereas Sargassum grew better mixed with Saccharina and at low densities, suggesting intra-specific competition is strong in this species. These results suggest increases in Sargassum will not affect adult stands of Saccharina.