Spatial patterns of macrobenthic alpha and beta diversity at different scales in Italian transitional waters (central Mediterranean)

Biodiversity is a multidimensional concept encompassing many scales of variation, originally partitioned into three components: alpha, the number of species in a single sampling unit, beta, the variation in species identities from site to site, and gamma, the overall number of species within a defined geographical area. Investigations on macrobenthic assemblages in transitional waters have been traditionally focused on alpha- and gamma-diversity, largely neglecting beta-diversity. The aim of the work was to identify patterns of variation of macrobenthic invertebrates in transitional waters from 18 different Italian lagoons (central Mediterranean) at three different scales (site, lagoon and region), combining classical and multivariate measures for partitioning diversity. Classical alpha, beta and gamma diversity were calculated, together with measures considering also relative species abundances. The highest variability of macrobenthic assemblages was observed at the intermediate scale (among lagoons), with significant variations in terms of both alpha and beta diversity. At the smallest analysed scale (among sites) the variability was mainly in terms of beta diversity, whereas at the biggest scale (among regions), the variability was mainly in terms of alpha diversity. At the intermediate scale (among lagoons) alpha diversity was influenced by the regional species pool, with one exception (Grado-Marano lagoon) where the habitat heterogeneity and beta diversity played a major role. Beta diversity, measured as variability of dispersion with Jaccard resemble measure, was consistent with results obtained with the original formulation of Whittaker. Considering alpha diversity, a different pattern of variability was observed considering relative abundances, whereas considering beta diversity, no significant differences were observed in terms of relative species abundances. The patterns of macrobenthic assemblages among lagoons in terms of both alpha and beta diversity resulted from the complex interaction of different drivers acting in a framework of geographical variability. Alpha diversity was mainly influenced by salinity, but other factors (size, confinement, trophic status and sediment composition) were likely to contribute. Beta diversity was mainly influenced by habitat heterogeneity, deriving from different environmental parameters, such as trophic condition and water confinement. Our results support the usefulness of portioning diversity in alpha, beta and gamma components, and of combining different univariate and multivariate measures of dispersion, to consider patterns of richness and relative abundances separately.