Structure and diversity patterns of coralligenous cliffs across three ecoregions in the Central-Western Mediterranean Sea

Aim: Coralligenous reefs are the main marine bioconstructions in terms of spatial distribution within the Mediterranean basin. Two distinct reef morphologies can be distinguished based on the surface and topographical features of the seafloor: cliffs developing vertical slopes and banks found on gently steep or horizontal bottoms. Despite their importance for monitoring and conservation efforts, observations regarding the variability of biogeographical patterns are scarce. Here, we aimed to assess the differences in the composition and structure of these cliffs across ecoregions and estimate the relative role of abiotic environmental features, geographic location, and connectivity in shaping diversity patterns. Location: The study was carried out in the Central-Western Mediterranean Sea. Samples were collected at 65 sites across the Algero-Provençal Basin, the Ionian Sea and the Tyrrhenian Sea. Methods: We assessed the composition and structure of coralligenous cliffs through photographic samplings collected by scuba divers. Patterns in α- and β-diversity were associated with 9 abiotic environmental variables, latitudinal and longitudinal gradients, and connectivity measures using Generalized Additive (GAM) and Conditional Autoregressive (CAR) models. Results: Coralligenous cliffs were primarily composed of algae and displayed a high degree of variability. The Partition Around Medoids (PAM) clustering method successfully delineated seven distinct clusters with a non-uniform distribution within the studied ecoregions. The α-diversity increased in eastern and northern sites and with phosphate concentration, while decreased with temperature, chlorophyll and nitrates concentration. β-diversity at the site level increased with temperature, while it was negatively affected by northward current speed and chlorophyll concentration. Moveover, β-diversity increased within connected sites. Main Conclusions: Coralligenous cliff diversity responds both to the physico-chemical features of the habitat and between-habitats connectivity. However, our findings suggest that small-scale abiotic and biotic processes could contribute to explaining the variability observed. These findings can significantly enhance the monitoring and conservation efforts of this Mediterranean endemic ecosystem.