Evaluation of benthic macroalgal invasion in a harbour area of the western Mediterranean Sea

Many studies have been performed to investigate invasions in marine habitats, but little is known about the consequences of the simultaneous presence of more alien organisms in the same area. The aims of this work were to quantify the colonization of alien macroalgal species co-occurring in a harbour area of the northwestern Mediterranean Sea and to compare the structure of native phytobenthic communities in invaded and control areas. To achieve these objectives, invasions of four introduced macroalgae (Acrothamnion preissii, Womersleyella setacea, Caulerpa tax folia and C. racemosa) have been quantified in three different habitats (rocks, matte and Posidonia oceanica meadows) throughout one year. A combination of multivariate and univariate analyses was used to detect differences among macroalgal assemblages in control and invaded areas. Results showed that introduced species represented a very high percentage of macroalgal abundance in all the habitats studied. In Posidonia oceanica meadows, the macroalgal assemblage was almost completely represented by introduced turf-forming Rhodophyta. On rocks and matte, invasive species constituted about 50% of the cover of the macroalgal assemblage throughout the year. Both univariate and multivariate analysis showed that, in the study area, the structure of macroalgal assemblages was very different in invaded and control areas. Total percentage cover and diversity were lower in invaded areas than in control areas in all three habitats investigated and temporal modifications in macroalgal assemblages appeared to be linked to the growth cycle of invaders. The results of this study show that the co-occurrence of more alien algae does not seem to limit the total cover of invaders, which reached similar values independently of the dominant species.