Effects of herbivory and nutrients on the early colonization of crustose coralline and fleshy algae

The persistence of phase shifts from coral-dominated to macroalgae-dominated communities following disturbances has been largely attributed to reduction of herbivores and eutrophication on tropical coral reefs. The influence of reduced herbivory and increased nutrients and their combined interactions on the colonization of crustose coralline algae (CCA) and fleshy algae onto settlement tiles was studied for effects on community structure. Cages were used to manipulate herbivory, and slow-release fertilizer was used for nutrient enrichment. The responses of the following variables were measured at 2 adjacent reefs in Guam, Mariana Islands: percentage cover of CCA, fleshy algal biomass, coral recruitment, and sediment load. Fleshy algal biomass and sediment were greatest on tiles where herbivores were excluded, whereas CCA were most abundant on tiles exposed to herbivores, Nutrients did not have a significant effect on fleshy algae; however, CCA had a lower level of increase in response to increased nutrients at 1 site. Coral recruitment was low; therefore, preferential settlement to substratum type was indeterminable. This study demonstrates that herbivory, in particular, can have a marked effect on the initial composition of the benthic community, even over a relatively short time span, Moreover, the inverse relationship between CCA and both fleshy algae and sediment has implications for the recruitment of organisms with reliance on CCA.