Alterations of the structure of Posidonia oceanica beds due to the introduced alga Caulerpa taxifolia

Abstract

The impact of Caulerpa taxifolia on the structure of shallow Posidonia oceanica beds was studied in permanent quadrats from 1995 to 2005 at the invaded site of Cap Martin and the control site of Cap d’Antibes (French Riviera, France). The cover of C. taxifolia, shoot density, number of orthotropic and plagiotropic shoots and proportion of ramifications of P. oceanica were measured yearly. The cover of C. taxifolia in the invaded zone rapidly reached a maximum of infestation in 2000 with 93% of the quadrats covered by the alga. In 2001 an unexplained phenomenon led to a sharp decrease in the infestation and in the following years the colonisation remained low. Within the 10 years of the study, P. oceanica did not disappear from the permanent quadrats, but we observed a drastic change in the structure of the meadow invaded by C. taxifolia. Between 1999 and 2000 a decrease in the shoot density observed at both sites was probably related to the warm temperature event recorded in 1999 (from 636 to 143 shoots m-2 at the invaded site and from 488 to 277 at the control site). At the invaded site, the seagrass never recovered its initial density even after a sharp decrease in C. taxifolia. The orthotropic/plagiotropic shoot ratio was strongly modified at the invaded site, where plagiotropic shoots became dominant because of an increase in their ramification.

Keywords

• caulerpa taxifolia
• posidonia oceanica
• competition
• ecological impact