Limits to grazing by herbivorous fishes and the impact of low coral cover on macroalgal abundance on a coral reef in Belize

Abstract

Widespread rises in the abundance of fleshy macroalgae on Caribbean reefs within the last 20 yr have variously been attributed to eutrophication, overfishing, or the 1983-4 mass mortality of the grazing sea urchin Diadema antillarum. However, none of those factors can satisfactorily explain why macroalgae are abundant today even on lightly fished mid-depth reefs in sparsely populated areas. Here we explore another explanation for rises in macroalgal cover on such reefs, namely that they are a side effect of declines in coral cover. We suggest that grazing by herbivorous fishes can exclude macroalgae from mid-depth reefs with high cover of hard corals, but that on low-cover reefs, the amount of space occupied by algae overwhelms the ability of grazing fishes to crop it down. We simulated the effect of 10% and 25% rises in coral cover by attaching 'pseudo-corals' (PVC tiles covered in a non-toxic anti-fouling coating) to reef substratum in 5 × 5 m plots (3 groups: control, 10%T and 25%T) on a 12 m deep forereef site in front of Ambergris Caye, Belize. Within 3 mo of attaching tiles, macroalgae had declined by approximately 10% in 10%T plots and 25% in 25%T plots compared with control plots, and these new states persisted for the duration of the experiment (macro-algae averaging 38.0% in control plots, 28.7% in 10%T plots, and 13.6% in 25%T plots). Dramatic reductions in macroalgae may have made the 25%T plots more attractive to herbivorous fishes, as, in the later stages of the experiment, herbivorous fish biomass and acanthurid feeding rate were higher in those plots than in 10%T and control plots.