Recent studies have investigated spatial and temporal variability of coralligenous assemblages on coastal rocky cliffs, while structure and variability of platform banks have been rarely investigated. In the northern Adriatic continental shelf, coralligenous biogenic reefs are scattered on sandy and muddy bottoms, and may be separated by a few tens of meters to tens of kilometres. Their benthic assemblages were investigated by photographic sampling in two main areas about 100 km away: off Chioggia-Venice and Grado-Trieste. Within each area six outcrops, 1-2 km away, were sampled. Assemblages on reefs closer to the coast were dominated by algal turfs and boring sponges, while offshore they were generally characterised by the richest and most diverse communities. Contributions to the total species richness increased with the investigated spatial scale up to areas, while variation in species diversity monotonically decreased by increasing distance. Dominant species, including the main reef builders (i.e. encrusting calcified Rhodophyta), spatially changed following a geographical pattern. Among others, coralline algae (e.g. Lithophyllum incrustans), sponges (e.g. Chondrosia reniformis) and colonial ascidians (e.g. Polycitor adriaticus) were the main species responsible for the observed spatial differences, in terms of species replacement (β diversity).
