Effets de l'expansion des Rhodophyceae introduites Acrothamnion preissii et Womersleyella setacea sur les communautés algales des rhizomes de Posidonia oceanica de Méditerranée occidentale

Abstract

In this study, the macroalgal community of Posidonia oceanica rhizomes was described in relation to different levels of development of the two introduced Rhodophyceae Acrothamnion preissii and Womersleyella setacea. Fifteen P. oceanica meadows were chosen in the western Mediterranean. In each meadow, two samples of five shoots were made at 4 and 15 metres deep. Sixty-five macroalgal species were found: seven Chlorophyta, eight Fucophyceae and 50 Rhodophyta. A. preissii was distributed from the Balearic Islands to Tuscan Archipelago and along the Ustica Island. W. setacea was present in all samples except for Tunisia. The development of algal turfs changed the structure of epiphytic communities. In the colonised areas, A. preissii and W. setacea formed paucispecific populations. Diversity diminished with the increasing of turf developments. Turfs trap sediments, forming a stratum that prevents the development of other algal species. Where the two species were both present, A. preissi reached higher values of dominance. The development of algal turfs seems not limited by the competition between A. preissii and W. setacea. Turf cover was the same independently of the dominant species.