An experimental analysis of the effects of herbivory and nutrient enrichment on benthic community dynamics on a Hawaiian reef

Reduced herbivory and enhanced nutrient concentrations have both been suggested as probable mechanisms driving phase shifts from coral to algal dominance on tropical reefs. While there is considerable information regarding the effects of herbivores on reefs, there is little experimental evidence of nutrient effects or the interactive effects of both of these factors. This study experimentally tested the role of these factors on benthic community structure on a coral-dominated reef in Hawaii. A randomized factorial block design was used to simultaneously investigate the effects of nutrient enrichment and herbivore exclusion on the development of benthic algal communities on artificial surfaces over a 6-month time period. Total algal biomass was greatest on settlement surfaces exposed to both nutrient enrichment and herbivore exclusion simultaneously. Fleshy algal biomass was greatest on surfaces removed from grazing whereas calcareous biomass was greatest on surfaces exposed to nutrient enrichment. Control surfaces exhibited consistently less total, fleshy and calcareous algal biomass than that on any of the experimental surfaces. Microinvertebrates were most abundant on surfaces within herbivore exclusion treatments but increased in number on all settlement surfaces over time. Sediment accumulation was positively correlated with fleshy algal biomass and was most abundant on surfaces within herbivore exclusion treatments: there was no pattern in sediment accumulation over time. This research demonstrates that on short time scales (less than 6 months), nutrient enrichment and herbivore exclusion can independently and interactively support shifts in benthic algal community structure on a Hawaiian reef.