Macrograzers strongly influence patterns of epiphytic assemblages in seagrass meadows

The influence of factors, both abiotic (light and nutrients) and biotic (meadow structure, herbivory and seagrass shoot length) in the abundance and distribution of epiphytes on the Mediterranean seagrass Posidonia oceanica was investigated by means of a correlational approach over a spatial scale of ca. 500 km and at two different depths (5 and 15 m). Variability was examined from the double perspective of integrative community measures (biomass, species richness and alpha-diversity) and species composition. We assessed the influence of biotic and abiotic factors on integrative community measures using multiple correlation analyses. The influence of these factors in the structure of the whole community was investigated using a distance-based RDA for a linear model. A total of 129 taxa, mostly epiphytic algae, were recorded across all sites and depths. A large part of the variability in species composition (51%) was explained by the variables investigated. In particular, variability caused by differences in grazing pressure was the most important (25%), followed by variables related to nutrient availability (11%), meadow structure (6%), light (5%) and seagrass shoot length (4%). Among integrative community variables investigated, species richness was also best explained by grazing and nutrients.