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Many marine scientists have concluded that coral reefs are moving toward or are locked into a seaweed-dominated state. However, because there have been no regional- or global-scale analyses of such coral reef 'phase shifts,' the magnitude of this phenomenon was unknown. We analyzed 3581 quantitative surveys of 1851 reefs performed between 1996 and 2006 to determine the frequency, geographical extent, and degree of macroalgal dominance of coral reefs and of coral to macroalgal phase shifts around the world. Our results indicate that the replacement of corals by macroalgae as the dominant benthic functional group is less common and less geographically extensive than assumed. Although we found evidence of moderate local increases in macroalgal cover, particularly in the Caribbean, only 4% of reefs were dominated by macroalgae (i.e., >50% cover). Across the Indo-Pacific, where regional averages of macroalgal cover were 9-12%, macroalgae only dominated 1% of the surveyed reefs. Between 1996 and 2006, phase shift severity decreased in the Caribbean, did not change in the Florida Keys and Indo-Pacific, and increased slightly on the Great Barrier Reef due to moderate coral loss. Coral reef ecosystems appear to be more resistant to macroalgal blooms than assumed, which has important implications for reef management. Key words: Caribbean; coral reefs; Florida Keys; Great Barrier Reef; Indo-Pacific; macroalgae; phase shifts; reef management.

Paleontological data suggest that coral populations in the Caribbean were consistently healthy for the past 2-3 k.y. Beginning in the 1980s, however, disease outbreaks, bleaching episodes, hurricanes, and other perturbations caused catastrophic coral mortality and regional turnover. An alternative hypothesis, based on historical sources, posits that coral populations were already declining more than a century ago. These hypotheses can be distinguished only if turnover events are reliably preserved in the Holocene record of coral reefs. Push-cores extracted from uncemented lagoonal reefs in Belize showed that a direct hit by Hurricane Iris in 2001 did not disrupt the signature of the recent turnover event, which entered the subfossil record essentially intact. Cores from lagoonal systems in several areas of the Caribbean do not support the hypothesis that corals declined before the 1980s. Keywords: Caribbean, coral reef, event layer, hurricane, preservation potential, taphonomy.

Coral reefs are changing dramatically, particularly those in the Florida Keys and the Caribbean, due to many types of disturbance. These include hurricanes, diseases, over-fishing, sea urchins and pollution. The most damaging is caused by ship groundings, leading to the direct loss of corals, fracturing and crushing. However the damage is avoidable, and can be repaired. Most reef restoration is based on legal precendents, rather than science, but some advanced restoration methods have been developed within the Florida Keys National Marine Sanctuary.

ALTHOUGH WE AGREE WITH J. M. PANDOLFI et al.'s ('Are U.S. coral reefs on the slippery slope to slime?', Policy Forum, 18 Mar., p. 1725) vision of reversing coral reef [...]

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.

Author(s): Richard B. Aronson (corresponding author) [1]; William F. Precht [2]; Ian G. Macintyre [3]; Thaddeus J. T. Murdoch [1] The highest sea surface temperatures ever recorded, related both to [...]