Distribution and dominance of two introduced turf-forming macroalgae on the coast of Tuscany, Italy, northwestern Mediterranean Sea in relation to different habitats and sedimentation

The occurrence of filamentous turf-forming macroalgae assemblages has been widely reported in tropical and temperate oceans. In the Mediterranean Sea the development of dense and persistent turfs has recently started and has been linked to the spread of the two introduced filamentous Rhodophyta Acro thamnion preissii and Womersleyella,ella setacea. In this work algal assemb lages on rocky bottoms and rhizomes of Posidonia oceanica were studied duri ng one year in three localities of the northwestern Mediterranean Sea (Legh orn, Vada Shoals and Gorgona Island) characterised by different sedimentati on rate and composition of particulate matter. Dead matte of R oceanica was also studied on Vada Shoals. Results showed that turfs are able to develop on all habitats investigated, but different patterns are found. Turfs were abundant on rhizomes of P oceanica in all localities studied while on rock s they reached high cover at Leghorn and Vada but not on Gorgona Island. Th e dead matte of P oceanica showed patterns similar to those found on rocky bottoms. Turfs seem positively influenced by sedimentation while neither th e nature of particulate matter nor other polluting substances had any impor tant result on their spread. The development of turfs negatively affects sp ecies number and diversity of invaded communities, while cover did not show significant variations. Womersleyella setacea replaced Acrothamnion preiss ii on rocky bottoms and on dead matte, but not on Posidonia oceanica rhizom es where Acrothamnion preissii was dominant everywhere. In spite of the occ urrence of competition between the two algae, it did not seem to limit the development of turfs as they reached similar cover values independently of the dominant species.