Interaction between nutrients and herbivory in controlling algal communities and coral condition on Glover's Reef, Belize

Abstract

We studied the effects of herbivory, fertilization and their interaction on algal succession on dead coral surfaces and the condition of live coral colonies. We used replicate open, closed, fertil- ized, and unfertilized cages in a 2-factor, 2-level design, sampled 7 times over a 49 d summer period at an offshore reef atoll lagoon in Belize. Herbivory negatively influenced algal biomass, whereas nutri- ents positively influenced wet but not dry or decalcified measures. Total and turf algal cover were pos- itively influenced by nutrients and negatively by herbivory. Biomass was more strongly influenced by herbivory than fertilization, and the opposite was true for cover. Brown frondose algal cover was neg- atively influenced by both herbivory and nutrients, whereas red frondose algal cover was negatively affected by herbivory but unaffected by nutrients. There were more algal taxa and higher dominance in low compared to high herbivory treatments. In contrast to the relative dominance model (RDM), we found that turf algae did best under conditions of low herbivory and high nutrients, and also domi- nated high herbivory and low nutrient conditions, whereas frondose brown algae did best under low herbivory and low nutrient conditions, and appeared to be inhibited by high nutrients. Stony corals did sufficiently well in all conditions such that it was not possible to determine their optimal conditions, but elevated nutrients may provide resistance to end-of-summer bleaching by increasing the standing densities of algal symbionts. There were no detectable changes in symbiont community composition with all symbionts being members of Symbiodinium clade A. Coral mortality and low herbivory are most likely to be responsible for the high levels of brown frondose algae reported on these patch reefs.