Stability of Benthic Coral Reef Communities: Top-down Herbivory Control versus Bottom-up Eutrophication

Anthropogenic activities, including pollution and overfishing, threaten the equilibrium of the coral reef ecosystems, which often lead to a shift from coral dominated to macroalgae dominated benthic communities. Several reef sites of Guadeloupe Island (FWI) have been studied in order to estimate the effects of the algal growth boosted by nutrients in eutrophic waters (bottom-up) and of the control exercised by herbivorous fishes (top-down) on the stability of coral communities. Herbivorous fishes were counted on seven reef sites differing by their exposure to coastal sources of nutrients and by their protection status. Sixteen environmental variables were measured. The coverage rate of the bottom by benthic organisms was established and the algal consumption by herbivorous fishes was evaluated. Analysis of the results showed that reef zones subjected to anthropogenic influence were characterized by high levels of nitrates, phosphates and matters in suspension. In these areas, algal productivity was high and benthic communities were dominated by macroalgae. An exception was constituted by the reef flat of the barrier reef of the Grand Cul-de-Sac Marin, located in a marine protected area (MPA): macroalgae occupied less than 3 %of the bottom, although the nutrient level was important there. In this nonprotected zone, herbivores were numerically abundant with a high biomass. Results concerning the herbivory pressure showed that in the MPA the fishes were able to consume the whole algal production. In fished areas, herbivorous fishes were of small size and could not regulate the algal growth enhanced by the nutrient enrichment of the waters.