Combined effects of nutrient enrichment, sedimentation and grazer loss on rock pool assemblages

Coastal areas around the world are experiencing increasing disturbance from multiple stressors caused by anthropogenic activities. Although there is good knowledge about the impacts of individual stressors, there is less understanding of the consequences of several stressors acting simultaneously. Eutrophication and sediment deposition are widely recognized as major problems for the functioning of coastal systems, and they are expected to increase during the next decades. In a field experiment, using rock pools as a model system, different levels of nutrients and sedimentation were applied in a factorial experimental design that also accounted for the influence of molluscan grazers. Sedimentation significantly changed assemblage structure, mainly owing to an increase in turfing and filamentous algae and a decrease in crustose algae. Nutrients also caused an increase in the cover of green filamentous algae, which in turn was synergistically magnified by the removal of grazers. Here we showed that these stressors can individually alter the structure of rock pools assemblages; and that in this system top-down control (by grazers) is more important than bottom-up factors (nutrients) in controlling macroalgal assemblage structure. The combined effect of grazers loss and nutrients was larger than the sum of their individual effects. This study enhanced mechanistic understanding of the impacts of multiple stressors on coastal ecosystems, which will help to develop management strategies and conservation of the marine environment.