Threat to macroalgal diversity: effects of the introduced green algaCaulerpa racemosain the Mediterranean

Caulerpa racemosais a tropical green alga introduced into the Mediterranean probably as an immigrant from the Red Sea. This study evaluated the modifications caused byC. racemosainvasion on the structure of the benthic macroalgal community near Leghorn (northwestern Mediterranean). To achieve this objective, we studied invasion byC. racemosaof 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 byC. racemosa. Results showed that the invasive algaCaulerpa racemosacompletely 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. racemosaovergrew 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 ofC. racemosadiminished, the macroalgal community did not return to initial conditions. Instead, the structural changes increased in the following season, indicating the importance ofC. racemosainvasion on a local scale.