Effects of mean intensity and temporal variance of sediment scouring events on assemblages of rocky shores

Abstract

Climatic models predict an increase in temporal variance and intensity of extreme events such as storms and rainfall for the near future. These events are likely to modify current pat- terns of sediment delivery on rocky shores. We used a factorial experiment to test the hypothesis that changes in intensity and temporal variance of disturbance due to scouring of sediment affected the establishment of algae and invertebrates on a rocky shore in the northwest Mediterranean. The experimental design included replicated sequences of disturbance events within the high level of temporal variance, so that the effect of variance could be separated from effects associated with any particular sequence of events. Multivariate analyses showed a trend towards an interaction between intensity and temporal variance of disturbance, with assemblages exposed to a combination of low intensity and high temporal variance of disturbance being distinguished from the other assemblages. Similar patterns were displayed by filamentous and coarsely branched algae. Temporal variance enhanced their cover under low intensity of disturbance, whilst the opposite occurred under high intensity. Increasing intensity of disturbance reduced the mean number of taxa in experimental areas regardless of levels of temporal variance. Sequence of disturbance was also significant for all these response variables. These results support the proposition that changes in intensity and temporal vari- ance of disturbance by sediment may have reverberating effects on the structure and diversity of rocky shores assemblages, contributing to explain regime shifts in marine coastal habitats such as the replacement of canopy algae by turf-forming species.