Sediment deposition dampens positive effects of substratum complexity on the diversity of macroalgal assemblages

•The study evaluated the interaction between substrate complexity and sedimentation.•Sedimentation rate and substrate complexity were experimentally manipulated.•Substratum complexity enhanced species richness and architecturally complex species.•Substrate-related differences among assemblages decreased under high sedimentation.•The increase in sediment deposition reduced both alpha and beta diversity.

A three-year experimental study was performed to evaluate the interactive effects of topographic complexity and sedimentation in determining the structure of rocky macroalgal assemblages. The following hypotheses were tested: i) the structure of macroalgal assemblages varies according to the complexity of the substratum; ii) high sediment deposition reduces variations in the structure of assemblages among substrata characterized by different complexity. At natural levels of sediment deposition, greater substratum complexity enhanced species richness and favored the development of assemblages dominated by architecturally complex species, such as large corticated Rhodophyta. Under high sediment deposition, turfs became the main component of macroalgal assemblages, although different filamentous forms responded differently to substratum complexity. In addition, high sediment deposition increased the abundance of the invasive Chlorophyta, Caulerpa cylindracea, on low complexity substrata, but decreased it on high complexity substrata. These results show that an increase in sediment deposition can dampen variations between assemblages associated to substrata characterized by different complexity, with consequent reduction of both alpha (i.e., species loss) and beta diversity (i.e., decreased small-scale variation in community structure).