CHANGES AND DEGRADATION IN A POSIDONIA-OCEANICA BED INVADED BY THE INTRODUCED TROPICAL ALGA CAULERPA-TAXIFOLIA IN THE NORTH-WESTERN MEDITERRANEAN

A study of the impact of Caulerpa taxifolia on the Posidonia oceanica beds was carried out in two similar areas, one with and one without the alga. In the invaded site, nearly all substrates around the living P. oceanica were occupied by dense C. taxifolia meadows. During the winter, few differences were recorded in comparison with the Reference site. Early in spring, C. taxifolia patches started to grow in the P. oceanica beds of low densities. Certain consistent changes were then observed in invaded P. oceanica beds: a change in the arrangement of tannin cells; a decrease in number, width, and longevity of leaves; chlorosis and necrosis, and finally death of shoots. The impact of C. taxifolia was correlated with its seasonal development: the peak of disturbance was recorded in densely invaded P. oceanica beds in summer and autumn when the growth and the terpenoid synthesis of C. taxifolia were at a maximum. This introduced alga seems to be able to occupy the P. oceanica biotope. After three years of competition, the sparse seagrass beds appear as the most threatened whereas dense ones show a better resistance.