Nutrients and herbivory, but not sediments, have opposite and independent effects on the tropical macroalga, Padina boryana

Multiple anthropogenic stressors including overfishing, nutrient loading, and sediments are major drivers that may support the proliferation of macroalgae on coral reefs; however, few studies have investigated all three simultaneously. We evaluated the effects of all three stressors (nutrient enrichment, small and large sediment grain sizes, and herbivory) on biomass accumulation of the dominant coral reef macroalga Padina boryana in a multifactorial field experiment on a fringing reef in Mo'orea, French Polynesia. Biomass accumulation of P. boryana was controlled by the separate, but not interactive, effects of herbivory and nutrients; although we did not detect effects of sediments, these may have been obscured by high variability leading to low statistical power. With herbivores, biomass remained the same or decreased up to 50% in 12 days compared to ~50-100% increases in biomass without herbivores. Overall, enriched P. boryana grew ~ 4-fold greater compared to ambient treatments, both with and without herbivory. Our results suggest that any release from bottom-up and/or top-down controls that may occur simultaneously on these nearshore reefs may facilitate proliferation of this tropical macroalga, and, in contrast to the prevailing paradigm, we found these effects can function independently.