Effects of nutrient enrichment and grazers on coral reefs: an experimental assessment

Abstract

Macroalgae have become pervasive elements on coral reefs throughout the Caribbean. Many studies have suggested that the loss of herbivore populations and coastal eutrophication are driving this increase in algal biomass, although competing processes have rarely been assessed concurrently. We tested the single and interactive effects of urchin Diadema antillarum grazing and nutrient enrichment on macroalgal community dynamics in the lower Florida Keys, USA. Three levels of urchin density (0, 2, 5 ind. plot–1) and 2 levels of nutrient enrichment were controlled in a fully orthogonal field experiment using calcium carbonate tiles as experimental substrata. Changes in the percent cover of macroalgal functional groups were estimated using weekly photoquadrats. After a 78 d observation period, algal recruitment remained low and did not appear to have been related to urchin density. However, we observed a significant nutrient effect on the relative abundance of algal functional groups. We attribute this effect to an unanticipated interaction with fish grazing intensity and suggest that the impacts of fish grazing masked those of D. antillarum and drove algal succession towards a benthos of algal turfs and crustose coralline algae. We conclude that nutrient enrichment is an unlikely explanation for the algal overgrowth of coral reefs in the Florida Keys.