Article

Posidonia oceanica (L.) Delile leaves are one of the most important substrates for the establishment of epiphytic algae in Mediterranean coastal areas. To date most of the literature on epiphytic algae living on Posidonia oceanica focuses on their importance as primary producers and on the seasonal or low scale spatial variations of the community. The effects of nutrient enrichment on the biomass of seagrasses epiphytes have also been the subject of numerous publications. Nevertheless, studies considering the floristic composition of the epiphyte communities and high scale spatiotemporal (hundreds kilometres; several years) variations are scarce. The goal of this paper is to compare floristic inventories of Posidonia oceanica leaves algal epiphytes made in different regions of the Western Mediterranean. Compared inventories are those published between 1971 and 1999 by several authors and related to nine different meadows located in Spain, France, Corsica, Italy and Sicily. New records, made in the Revellata Bay (Calvi, Corsica, France) between 2002 and 2005 are presented and have been included in the comparison. Floristic inventories have been compared at the species level and at a higher taxonomical level using group-average clustering and non-metric Multidimensional Scaling (MDS) based on Bray-Curtis similarity measures. Results revealed that the overall diversity is huge: at least 199 taxa may occur on Posidonia oceanica leaves in the whole Western Mediterranean. However, in each meadow, the species richness is much lower (between 39 and 97 species) and the specific composition of Posidonia oceanica epiflora appears quite variable among meadows. Only 11 species are constantly represented and thus really characterise the association at the species level. The variability is mostly due to the Ceramiales (Rhodophyceae) which represent more than 50 % of the total diversity but Chlorophyceae can also be responsible by characterising shallow and/or eutrophicated meadows. Nevertheless, the similarities observed between French and Corsican meadows on one hand, between Southern Italian and Sicilian meadows on the other hand, as well as the apparent isolation of the Spanish meadow, allow to think that a geographical correlation in species composition exists. Some important features explaining the variability in species composition are discussed (e.g. seasons, depths, observational strategy and occurrence of occasional taxa). At the opposite of the high variability at species level, the general structure of floras at a higher taxonomical level (order) is generally preserved. The number of species belonging to each order thus appears as a strong characteristic of the association that could be used as a diagnostic tool to assess the ecological state of a Posidonia oceanica meadow in the Western Mediterranean. Shifts in the spectra of orders possibly reveal some degree of perturbation. Concerning the Revellata Bay, our results show that the specific composition and seasonal variation of Posidonia oceanica epiphytic macroflora does not diverge from other localities of the same part of the Western Mediterranean. If compared to other meadows, the epiphytic species richness encountered in the Revellata Bay appears average.