The cover of noncoralline macroalgae increased dramatically on Caribbean reefs during the 1980s and 1990s. A top‐down hypothesis, based largely on observations at Discovery Bay, Jamaica, is that this change was caused by reduced herbivory. Herbivory was reduced by the regional mass mortality of the echinoid Diadema antillarum in 1983‐1984 and by human exploitation of herbivorous fishes. An alternative, bottom‐up explanation is that nutrient concentrations increased past threshold levels for algal blooms. Surveys at Discovery Bay showed that Diadema reappeared on the shallow fore reef after 1996, accompanied by drastically reduced macroalgal cover. There is no evidence to suggest that nutrient levels declined at the same time. These observations corroborate predictions of the top‐down hypothesis, and they confirm the key role of herbivory in structuring shallow reef communities of the Caribbean.