Threat to macroalgal diversity: effects of the introduced green alga Caulerpa racemosa in the Mediterranean

Caulerpa racemosa is a tropical green alga introduced into the Mediterranean probably as an immigrant from the Red Sea. This study evaluated the modifications caused by C. racemosa invasion on the structure of the benthic macroalgal community near Leghorn (northwestern Mediterranean). To achieve this objective, we studied invasion by C. racemosa of algal assemblages on 2 different substrata by monitoring the structure of the phytobenthic community before and after the invasion of the alga and comparing our results with assemblages not invaded by C. racemosa. Results showed that the invasive alga Caulerpa racemosa completely covered the surface of both types of substratum 6 mo after the start of the invasion. With rapid horizontal elongation of the stolons (up to 2 cm d–1) C. racemosa overgrew native macroalgal species within 4 mo. As a consequence, phytobenthic community structure deeply changed: species cover, number and diversity greatly decreased. Furthermore, the relative importance among vegetation layers of the macroalgal assemblage changed: turf and encrusting species were deeply affected while erect species survived after 1 yr of the invasion. During the period from December to May, when both cover and biomass of C. racemosa diminished, the macroalgal community did not return to initial conditions. Instead, the structural changes increased in the following season, indicating the importance of C. racemosa invasion on a local scale.