Herbivory and algal dynamics on the coral reef at Discovery Bay, Jamaica

The cover of noncoralline macroalgae increased dramatically on Caribbean re efs during the 1980s and 1990s. A top-down hypothesis, based largely on obs ervations at Discovery Bay, Jamaica, is that this change was caused by redu ced herbivory. Herbivory was reduced by the regional mass mortality of the echinoid Diadema antillarum in 1983-1984 and by human exploitation of herbi vorous fishes. An alternative, bottom-up explanation is that nun-lent conce ntrations increased past threshold levels for algal blooms. Surveys at Disc overy Bay showed that Diadema reappeared on the shallow fore reef after 199 6, accompanied by drastically reduced macroalgal cover. There is no evidenc e to suggest that nutrient levels declined at the same time. These observat ions corroborate predictions of the top-down hypothesis, and they confirm t he key role of herbivory in structuring shallow reef communities of the Car ibbean.