The role of surgeonfish (Acanthuridae) in maintaining algal turf biomass on coral reefs

Coral reefs are one of the most highly productive marine ecosystems, with a strong connection between herbivores and the production of benthic algae. The epilithic algal matrix (EAM) is a major source of primary production on coral reefs, and it is one of the dominant benthic microhabitats, covering up to 80% of reef flats and back reefs and up to 70% of reef slopes on outer-shelf sites of the Great Barrier Reef, Australia. Although herbivorous surgeonfish (Acanthuridae) are dominant members of most reef fish assemblages, there is a lack of quantitative information on their grazing impact, particularly those that feed upon the EAM in the Indo-Pacific. Therefore, the aim of this study was to investigate the role of surgeonfish in maintaining algal turf biomass on coral reefs. Spatial patterns of EAM productivity, and herbivorous fish biomass and grazing intensity were quantified at different depths (deep vs. shallow) and exposures (windward vs. leeward) of reef slope environments of Heron Island, southern Great Barrier Reef. Surgeonfish were the numerically-dominant EAM grazing fish family, and to identify the daily impact of surgeonfish species on the EAM, data on their biomass (gm^-2) and grazing intensity (total bitesm^-2day^-1) were combined with EAM productivity estimates (gCm^-2day^-1). EAM productivity was greatest in windward and shallow sites, and herbivorous fish biomass mirrored this pattern. Yet, there was no difference in the EAM standing crop or grazing intensity among habitats. In the most productive habitat (windward-shallow), grazer/detritivore surgeonfish species accounted for 74% of the total herbivore biomass (gm^-2), took 51% of the total bites (m^-2day^-1), and removed an estimated 73% of daily EAM productivity (gCm^-2day^-1). This study quantifies the role of surgeonfish in maintaining EAM biomass, and highlights their possible contribution to preventing shifts from coral- to algal-dominance following disturbance.