Possible effects of the invasive algae Caulerpa cylindracea on the benthic assemblages

In the Mediterranean Sea, the spread of the invasive algae Caulerpa cylindracea appears to be one of the most severe threat to the biodiversity of the coralligenous benthic communities. The magnitude of its effects may be modulated by the features of recipient communities, considering the high variability in species composition and relative abundance that characterize these biogenic reefs. The structure of benthic assemblages, taxa abundances and species diversity were investigated using a manipulative approach at three study sites in Elba Island and applying a multifactorial experimental design. Benthic assemblages’ structure, abundance and diversity were investigated in relation to the possible effects of both presence and absence of C. cylindracea and of the gorgonian Paramuricea clavata. Being C. cylindracea widely spread in the study sites, its absence in the specifically intended plots was maintained by periodical manual removals, while appropriate plots with and without P. clavata were selected for proper experimental setup. The coverage of P. clavata was comparatively low and, except a slightly but significant increase of benthic species richness in presence of the gorgonian, no other effects of its presence on the benthic assemblages among sites nor in relation to the presence of C. cylindracea were detected. Overall, Corallinaceae encrusting algae, turfs and sediment dominated in term of percent cover both in presence and absence of C. cylindracea. The effects of the presence of C. cylindracea on the structure of benthic assemblages appeared moderate and site specific. That is probably due to high heterogeneity of environmental conditions and different invasiveness of C. cylindracea at the investigated spatial scales.