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 Mediterranea n probably as an immigrant from the Red Sea. This study evaluated the modif ications caused by C. racemosa invasion on the structure of the benthic mac roalgal community near Leghorn (northwestern Mediterranean). To achieve thi s objective, we studied invasion by C, racemosa of algal assemblages on 2 d ifferent sub strata by monitoring the structure of the phytobenthic communi ty before and after the invasion of the alga and comparing our results with assemblages not invaded by C, racemosa. Results showed that the invasive a lga Caulerpa racemosa completely covered the surface of both types of subst ratum 6 mo after the start of the invasion. With rapid horizontal elongatio n of the stolons (up to 2 cm d(-1)) C, racemosa overgrew native macroalgal species within 4 mo. As a consequence, phytobenthic community structure dee ply changed: species cover, number and diversity greatly decreased. Further more, the relative importance among vegetation layers of the macroalgal ass emblage changed: turf and encrusting species were deeply affected while ere ct species survived after 1 yr of the invasion. During the period from Dece mber to May, when both cover and biomass of C. racemosa diminished, the mac roalgal community did not return to initial conditions. Instead, the struct ural changes increased in the following season, indicating the importance o f C. racemosa invasion on a local scale.