Composition and spatio temporal variability of the epiphytic macroalgal assemblage of Fucus vesiculosus Linnaeus at Clare Island, Mayo, western Ireland

The composition and the patterns of spatial and temporal variability of the epiphytic assemblages of Fucus vesiculosus of Clare Island, on the western coast of Ireland, were investigated for an annual cycle. Specimens of Fucus were collected on seven sampling dates from three sites of the island (Portlea, Kinnacorra and Portnakilly). Data of cover of the most common epiphytic species were collected and analysed by multivariate and univariate techniques. Elachista fucicola, Polysiphonia lanosa, Porphyra umbilicalis, Spongonema tomentosum and Ulva compressa were the most common species. Spatial and temporal variation was detected both for the whole assemblage and for the most abundant species. In general, the assemblage was quantitatively more abundant and diverse in spring-summer than in autumn-winter. In spring-summer, there was a clear differentiation among the assemblage of Portnakilly and the assemblages of the other two sites. Individual species were also generally more abundant in spring-summer and their distribution at the three sites was often not consistent in time; P. lanosa was the only epiphyte for which a consistent effect of site was found. Spatial variation on a scale of meters to tens of meters was the most striking pattern of distribution of the epiphytic assemblage; significant effects related to this spatial scale were detected both for the whole assemblage and for individual species. Phenological patterns of the epiphytic species determining availability of propagules and limited dispersal, leading to aggregated patterns of distribution, are considered the main factors responsible of such patchiness. The importance of the incorporation of small spatial scales in sampling designs analysing the distributional patterns of epiphytic assemblages is discussed.