Responses of two common coral reef macroalgae to nutrient addition, sediment addition, and mechanical damage

Abstract

Shifts from coral-dominated to macroalgal-dominated states on coral reefs are typically attributed to overfishing, often in combination with natural or other anthropogenic disturbances. However, a recent study attributed increased macroalgal cover to enhanced nutrient supplies alone without associated reductions in herbivorous fishes. This highlights the need to understand macroalgal responses to terrestrial runoff and how this can contribute to their proliferation on coral reefs. We explored the responses of two common brown macroalgae, Sargassum mangarevense (Grunow) Setchell and Padina boryana Thivy to nutrient addition, sediment addition, and mechanical damage in 3 field experiments. In the first experiment, we exposed all experimental units to herbivores and found that nutrient addition ameliorated biomass loss of both S. mangarevense and P. boryana by 79% and 47%, respectively, while sediment addition had no effect on either species. Exploring the underlying causes of these findings, we determined nutrient addition caused a 5% increase in toughness of S. mangarevense and a 51% increase in calcification of P. boryana. In a final experiment, both mechanical damage and nutrient addition independently increased calcification of P. boryana, while an antagonistic interaction between these factors for S. mangarevense suggests an upper limit to thallus toughness. Overall, we demonstrate that nutrients from anthropogenic terrestrial runoff may support macroalgal proliferation on coral reefs by ameliorating short-term biomass losses in the presence of herbivores. Further, we suggest the mechanism behind this amelioration may be changes in algal physical characteristics, specifically thallus toughness and calcification, in response to anthropogenic or natural disturbances.