Effects of geographic distance, water circulation and environmental conditions in shaping the biodiversity of Mediterranean rocky coasts

cological connectivity is important for ef- fective marine planning and biodiversity conservation. Our aim was to identify factors important in influencing variation in benthic community structure on shallow rocky reefs in 2 regions of the Mediterranean Sea with contrasting oceanographic regimes. We assessed beta (β) diversity at 146 sites in the littoral and shallow sub- littoral from the Adriatic/Ionian Seas (eastern region) and Ligurian/Tyrrhenian Seas (western region) using a null modelling approach to account for variation in spe- cies richness. The distance decay relationship between species turnover within each region and geographic distance by sea was determined using generalised lin- ear models. Mantel tests were used to examine correla- tions between β diversity and connectivity by ocean currents, estimated from Lagrangian dispersal simula- tions. Variation in β diversity between sites was parti- tioned according to environmental and spatial compo- nents using a distance-based redundancy approach. Species turnover along a gradient of geographic dis- tance was greater by a factor of 3 to 5 in the western region than the eastern region, suggesting lower con- nectivity between sites. β diversity was correlated with connectivity by ocean currents at both depths in the eastern region but not in the western region. The influnce of spatial and environmental predictors of β diver- sity varied considerably between regions, but was simi- lar between depths. Our results highlight the interaction of oceanographic, spatial and environmental processes influencing benthic marine β diversity. Persistent cur- rents in the eastern region may be responsible for lower observed β diversity compared to the western region, where patterns of water circulation are more variable.