Effects of herbivore exclusion and nutrient enrichment on coral reef macroalgae and cyanobacteria

Although phase shifts on coral reefs from coral-dominated to algal-dominate d communities have been attributed to the effects of increased nutrient ava ilability due to eutrophication and reduced herbivore abundance due to over fishing and disease, these factors have rarely been manipulated simultaneou sly. In addition, few studies have considered the effects of these factors on benthic, filamentous cyanobacteria (blue-green algae) as well as macroal gae. We used a combination of herbivore-exclusion cages and nutrient enrich ment to manipulate herbivore abundance and nutrient availability, and measu red the impacts of these treatments on macroalgal and cyanobacterial commun ity structure. In the absence of cages, surface cover of the cyanobacterium Tolypothrix sp. decreased, while surface cover of the cyanobacteria Oscill atoria spp. increased. Cyanobacterial cover decreased in partial cages, and Tolypothrix sp. cover decreased further in full cages. Lower cyanobacteria l cover and biomass were correlated with higher macroalgal cover and biomas s. Dictyota bartayresiana dominated the partial cages, while Padina tenuis and Tolypiocladia glomerulata recruited into the full cages. Palatability a ssays demonstrated that herbivore-exclusion shifted macroalgal species comp osition from relatively unpalatable to relatively palatable species. Nutrie nt enrichment interacted with herbivore exclusion to increase the change in cover of D. bartayresiana in the uncaged and fully caged plots, but did no t affect the final biomass of D. bartayresiana among treatments. Nutrient e nrichment did not significantly affect the cover or biomass of any other ta xa. These results stress the critical role of herbivory in determining cora l reef community structure and suggest that the relative palatabilities of dominant algae, as well as algal growth responses to nutrient enrichment, w ill determine the potential for phase shifts to algal-dominated communities .