Sediment disturbance and loss of beta diversity on subtidal rocky reefs.

How changes in environmental complexity and heterogeneity affect beta diversity is poorly known. We investigated patterns of beta diversity in subtidal assemblages of algae and invertebrates in the northwest Mediterranean in relation to inclination of the substratum and sedimentation. Vertical and horizontal substrata supported distinct assemblages under low, but not under heavy, ambient loads of sediment. To test the hypothesis that sediment reduced the dissimilarity between assemblages, sedimentation was increased experimentally in plots established on vertical and horizontal surfaces at sites experiencing low ambient levels of sedimentation. Patterns were compared to those occurring at unmanipulated sites and at sites exposed to heavy loads of sediment about 2 km apart. After one year, assemblages on vertical substrata were indistinguishable from those occurring on flat surfaces at manipulated sites and both converged toward those occurring at sites exposed to heavy loads of sediment. Control sites still supported distinct assemblages on vertical and horizontal substrata by the end of the experiment. Similar effects of sediment were observed on recovering assemblages in experimental clearings. These results show that sediment increased similarity in assemblages overriding the influence of habitat complexity on beta diversity at small and large spatial scales.