The interactive effect of herbivory, nutrient enrichment and mucilage on shallow rocky macroalgal communities

In this paper the results of a manipulative experiment aimed to evaluate the interactive short- and long-term effect of three different stressors, herbivory, nutrient and mucilage, on a macroalgal assemblage are presented. The experiment was conducted in Tavolara Punta Coda Cavallo Marine Protected Area during a bloom of the benthic mucilage-producing microalga Chrysophaeum taylorii Lewis and Bryan (Pelagophyceae), recently spreading in the Mediterranean Sea. On a rocky substratum, 18 plots 20x20 cm in size were prepared and, according to different treatments, nutrients were added in some of them to simulate eutrophication, macroalgae were removed to simulate clearings produced by grazers and mucilage was manually removed to simulate mucilage-free conditions. Differences in the composition of macroalgal assemblages were found when considering the short term effect of the considered stressors, and also the response of the most abundant taxa (DFA, ECA, Dictyotales, Laurencia spp. and Padina pavonica) varied among treatments, proving that a combined effect of such stressors on the recovery of macroalgae was present. On the contrary, the effect of treatments was neither highlighted on the most abundant algae nor on the whole structure of the macroalgal assemblage.