Algal vertical zonation and seasonal dynamics along a subtidal cliff on Gorgona Island (Tuscan archipelago, Italy)

Abstract

The infralittoral macroalgal assemblage on a rocky cliff at Gorgona Island (Tuscan Archipelago, Italy) is studied in connection with depth gradient and seasonal dynamics. The structure of algal communities is described using species composition and functional groups. The results of the multidimensional scaling ordination (MDS) applied to sampling‐species matrix indicate the presence along this cliff of three assemblages related to changes in hydrodynamics and illumination: shallow, intermediate and deep. On the contrary the MDS analysis conducted on functional groups does not display a clear zonation because all the functional groups are present at all depths. Moreover, the compact distribution of the samples of the shallow and deep assemblages indicates lack of seasonal change, while temporal fluctuactions in intermediate assemblage are showed by scattering of the sample points. Species number, total cover and the Shannon‐Weaver and Pielou indices are characteristic of well‐structured communities. These parameters are not very different from those of other algal communities studied in the western Mediterranean. The R/P ratio of Capo Zirri is higher than that of the Tuscan Archipelago flora and is comparable with other sciaphilous Mediterranean assemblages.