Nutrient and herbivory effects on benthic marine algal biomass and community structure on the Kona Coast of Hawai'i

Coral reef ecologists have documented shifts from coral-dominant to algal-dominant reefs due to increased nutrient inputs (bottom-up) and reduced herbivory (top-down). This study investigated the combined effects of herbivory and nutrients on benthic algal biomass and composition at two sites (Kaloko and Kīholo) with documented submarine groundwater discharge (SGD) on the leeward coast of the Island of Hawai'i. Experiments were conducted using settling tiles placed both within and outside of herbivore exclusion cages and nutrient-enriched SGD plumes. Algal biomass was significantly greater within herbivore exclusion cages, which was dominated by turf and cyanobacteria with some macroalgae present. No significant differences in algal biomass or composition were found between stations located in SGD plumes with stations outside of plumes, indicating current nutrient concentrations in SGD plumes have little effect on benthic algal growth or changing algal functional group dominance. In addition, no significant relationships were found between herbivore surveys or nutrient concentrations, suggesting intense herbivory throughout the study area along the narrow nutrient gradients at the benthos. This research strongly supports the importance of herbivory in controlling phase-shifts, as it was the primary control of algal biomass and composition at both sites. Nutrient concentrations were high enough to support algal growth throughout all sites. Increases of nutrients have been shown in numerous studies to exacerbate or even cause phase-shifts, and thus regulations prohibiting excessive nutrient input to the marine environment are important to prevent reef degradation. This research highlights that both top-down and bottom-up controls are essential to protect benthic biodiversity.