Résumé
Coral reefs, some of the most important marine ecosystems, are mainly structured by interactions between corals and benthic algae. The outcome of these interactions varies depending on the types of corals and algae involved. However, human activities, associated mainly with eutrophication and overfishing of herbivorous organisms, strongly impact the outcome of algae-coral interactions by promoting progressive replacement of corals by algae. The present study aimed to firstly establish the first inventory of algae-coral interactions at the species level on the Bora Bora reefs, and then experimentally determine the role of herbivorous species (sea urchins and Trochus shells) in the regulation of these complex algae-coral interactions. Firstly, visual surveys showed that the outcome of algae-coral interactions varied significantly according to the length of the algae-coral interaction border, and among algae-coral couples. For example, unhealthy coral colonies with large interaction borders were associated with a higher percentage of negative interactions. Then, using experimental enclosures for 70 days, we showed that the sea urchin Tripneustes gratilla controlled the proliferation of algae and thus allowed a better growth of Acropora corals. Overall, the present study suggests that conservation programs should rely on grazer densities as a Nature-Based Solution to attenuate global shifts from coral- to algal-dominance.