THE EFFECTS OF SEDIMENTATION ON ROCKY COAST ASSEMBLAGES

Sedimentation is a widespread and increasing process on most rocky coasts. The literature on its effects is reviewed and support is found for the general conclusion that sedimen- tation is an important ecological factor for hard bottom organisms. Sediments deeply affect the composition, structure and dynamics of rocky coast assemblages, and increased sediment load as a consequence of anthropogenic activities can be a threat to their diversity and functioning. Sediments that accumulate on rocky substrata are important agents of stress and disturbance. They can cause burial, scour and profound modifications to the characteristics of the bottom surface, and interact with other important physical and biological processes. The effects of sedi- mentation are complex, because they involve both direct outcomes on settlement, recruitment, growth or survival of individual species and indirect outcomes through mediation of competitive and/or predator-prey interactions. Not all species and assemblages are equally affected by sedi- mentation and responses vary over space and time, depending on the characteristics of the depo- sitional environment, life histories of species and the stage of development of individuals and assemblages, and in relation to variable physical factors, including hydrodynamics, light intens- ity and bottom topography. Recent studies have much improved our ability to detect and under- stand the effects of sedimentation on rocky coast assemblages. However, little is still known about the underlying mechanisms. Overall, our present ability to make generalisations and pre- dictions is limited by a paucity of quantitative and experimental research, and by the scant atten- tion devoted to measuring the regime of perturbation by sediments and responses of organisms at relevant spatial and temporal scales. Predicting the magnitude of the effects that different sed- imentation regimes have on rocky coast organisms and the critical levels above which detrimen- tal effects become manifest remains a key issue for the ecology of rocky coasts and a challenge for future studies.