Effects of the spread of the introduced Rhodophyceae Acrothamnion preissiiand Womersleyella setacea on the macroalgal community of Posidonia oceanica rhizomes in the western Mediterranean sea.

In this study, the macroalgal community of Posidonia oceanica rhizomes was described in relation to different levels of development of the two introdu ced 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 sam ples except for Tunisia. The development of algal turfs changed the structu re of epiphytic communities. In the colonised areas, A. preissii and W. set acea formed paucispecific populations. Diversity diminished with the increa sing of turf developments. Turfs trap sediments, forming a stratum that pre vents the development of other algal species. Where the two species were bo th 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. (C) 2000 Adac/Editions scientifiques ct medicales Elsevier SAS.