Synergism between two anthropic impacts: Caulerpa racemosa var. cylindracea invasion and seawater nutrient enrichment.
Although eutrophication and biological invasions are recognized as important elements of global change, few studies have assessed how their synergism can affect structural and functional traits of marine ecosystems. The present study aimed to evaluate the effect of seawater nutrient enrichment on the spread of the introduced Chlorophyta Caulerpa racemosa var. cylindracea and the interactive effects of nutritional enrichment and C. racemosa invasion on the native macroalgal assemblages of a subtidal rocky bottom. To achieve these objectives, a manipulative field experiment was carried out at 24 m depth in the northwestern Mediterranean Sea by adding C. racemosa and supplying the water column with nutrients. The following treatments were applied: non-manipulated assemblages, nutrient enrichment, addition of C. racemosa, and both nutrient enrichment and addition of C. racemosa. Results showed that C. racemosa spread was enhanced by nutrient enrichment. Moreover, a significant interaction between nutritional increase and C. racemosa invasion was detected: the effects of C. racemosa invasion on native macroalgal assemblages were amplified by nutrient enrichment. Results suggested that synergism between effects of different anthropogenic impacts can have severe consequences on the integrity of marine ecosystems.