Nutrient versus herbivore control of macroalgal community development and coral growth on a Caribbean reef

Coral reefs are in global decline, with seaweeds replacing corals as spatial dominants. Overfishing of herbivores, anthropogenic eutrophication, and interactions between these factors have been postulated as causes, but long-term tests of these factors are uncommon. We factorially manipulated herbivorous fishes and nutrients at a depth of 16 to 18 m for 7 to 10 mo in 2 experiments over 2 yr. Herbivore exclusion increased algal cover by about 4 to 10x, algal biomass by 4 to 6x, and suppressed cover of crustose coralline algae by 80 to 100%. Nutrient enrichment had no effect on the cover or mass of upright algae, but altered species composition by suppressing cyanobacteria and facilitating red macroalgae in the absence of herbivores. Nutrient addition increased macroalgal species richness in the absence, but not the presence of herbivores. Feeding by herbivorous fishes increased by 3 to 13x on nutrient-enriched vs. control plots. This herbivory facilitated cover of crustose coralline algae, demonstrated that herbivores selectively target more nutritional prey, and suggested that herbivores could suppress macrophyte accumulation at sites with increased nutrient availability. Effects of fishes and nutrients on corals varied as a function of coral species. For the branching coral Porites porites, 56% of individuals exposed to fishes were completely consumed; however, individuals that survived grew 60 to 80% more in the presence of fishes. For Porites astreoides, exposure to fishes did not affect mortality, but increased net growth by 3 to 4 times. For this coral, nutrient addition decreased growth when exposed to fishes but not when protected from fishes, suggesting that fishes may have fed more on nutrient-enriched corals.