Interplay of experimental harvesting and climate-related disturbance on benthic assemblages of rocky seashores

Assessing responses of assemblages to compounded perturbations is a crucial goal of research on ecological impacts of current and predicted environmental changes. We examined the interactive effects of changes in temporal variance of 'storm-like' mechanical disturbance and in harvesting intensity of mussels (Mytilus galloprovincialis) on sessile assemblages associated with mussel beds on rocky shores in north Portugal. Response variables were measures of temporal variance in the abundance of individual taxa and in the structure of assemblages. Events of disturbance regularly distributed over the period of the experiment determined a lower temporal heterogeneity in assemblage structure than events characterized by larger temporal variance, but only in the presence of an intact mussel bed. The opposite pattern was documented in plots where mussels were completely removed. Univariate results indicated taxon-specific responses, with macroalgae, such as Corallina spp. and Porphyra linearis, showing patterns analogous to the multivariate one and grazing invertebrates displaying an opposite trend. This is one of the first manipulative examples aimed at examining effects of temporal variability of climate-related disturbance on systems increasingly depleted by concomitant stressors. The main finding was that effects of temporal variability of climate-related events of mechanical disturbance are mediated by the presence of intact stands of a foundation species. This may contribute to understanding and predicting responses of populations and assemblages under scenarios of concomitant climatic and non climatic changes.