Effects of sedimentation on subtidal macroalgal assemblages: an experimental study from a mediterranean rocky shore

Abstract

The effects of sedimentation on the development and structure of macroalgal assemblages were investigated from June 1992 to December 1993 on a rocky subtidal shore South of Livorno (Ligurian Sea). Experimental procedures were specifically designed to address: (1) which species recruit in patches of bare rock under different sedimentation rates; (2) whether small-scale variations of sediment deposition influence the local diversity of the assemblage; (3) whether the effects of sedimentation are independent from the time of the year when succession begins. Depositional environment was manipulated in the field by means of transparent Plexiglas panels which on average reduced the amount of sediment sinking over the algal assemblage by 35%. Possible effects on the penetration of light and flow microenvironment were assessed. Small-scale variations of sedimentation rates significantly influenced the local structure and diversity of the algal assemblage. Such effects were particularly evident during the recolonisation of patches of bare rock and were dependent upon the time at which succession was initiated. The erect algae were the ones more affected by depositional environment and showed different responses to the experimental reduction of sediment supplies. All the plots were quantitatively dominated by a filamentous turf mostly consisting of Polysiphonia setacea, whose growth was potentially enhanced by reduced sedimentation rates. At the end of the experiments, the overall diversity and evenness of the assemblage were lower in treatments with lessened sediment supplies. It is suggested that small-scale spatial variability of the depositional environment may affect the within-habitat diversity of algal assemblages either through direct effects on individual species or on their propagules, and through indirect effects mediated by competitive outcomes.