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

Abstract

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 Acrothamnion preissii and Womersleyella setacea. In this work algal assemblages on rocky bottoms and rhizomes of Posidonia oceanica were studied during one year in three localities of the northwestern Mediterranean Sea (Leghorn, Vada Shoals and Gorgona Island) characterised by different sedimentation rate and composition of particulate matter. Dead matte of P. 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 rocks they reached high cover at Leghorn and Vada but not on Gorgona Island. The dead matte of P. oceanica showed patterns similar to those found on rocky bottoms. Turfs seem positively influenced by sedimentation while neither the nature of particulate matter nor other polluting substances had any important result on their spread. The development of turfs negatively affects species number and diversity of invaded communities, while cover did not show significant variations. Womersleyella setacea replaced Acrothamnion preissii on rocky bottoms and on dead matte, but not on Posidonia oceanica rhizomes where Acrothamnion preissii was dominant everywhere. In spite of the occurrence 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.