| Macroalgae have become pervasive elements on coral reefs throughout the Caribbean. Many have suggested that the loss of herbivore populations and coastal eutrophication are driving the increases in algal biomass, although competing processes are rarely assessed in the same study. I empirically tested the single and interactive effects of urchin grazing (Diadema antillarum ) and nutrient enrichment on macroalgal community dynamics in the lower Florida Keys, USA. Three levels of urchin density (0,2,5 ind./plot) and two 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-day observation period, algal recruitment remained low and did not appear to have been related to urchin density. However, a significant nutrient effect was observed for several algal groups. I attribute this effect to an unanticipated interaction with fish grazing intensity and suggest that the impacts of fish grazing masked those of Diadema and drove algal succession towards a benthos of algal turfs and crustose coralline algae. I conclude that nutrient enrichment is an unlikely explanation for the algal overgrowth of coral reefs in the Florida Keys. |
