Monitoring coral bleaching in the Maldives: Lessons to be learned and new hypotheses Le suivi du blanchissement des coraux aux Maldives: Leçons à tirer et nouvelles hypothèses

The 1998 bleaching event, which followed abnormally high sea surface temperatures (up to 34 °C) induced by El Niño, caused widespread mortality in the reefs of the Maldives. In the central atolls (Malé North, Malé South, Felidhoo, Wattaru, Ari, Rasdhoo), mortality rates were highest (approaching 90%) for branching and tabular species of the genus Acropora, for the Pocilloporidae and for the hydrocoral Millepora, particularly in shallow water. Mortality rates were less below 20 m and in general for massive species, whose colonies mostly displayed partial death only. No mortality at all was observed on the octocoral Heliopora coerulea. In 1999, many newly settled small-sized colonies were found. Counts of "recruits" (colonies < 5 cm in diameter) showed a dramatic decrease in their density between 1999 and 2000, followed by stabilisation in the following years. The taxonomic composition of recruits shifted from a dominance of Agariciidae toward a dominance of Acroporidae and Pocilloporidae. Counts of "juveniles" (colony diameter between 5 and 20 cm) suggested high recruit mortality. Juveniles exhibited variable growth. A first inventory of scleractinian species (recruits, juveniles, surviving adults) carried out in 2002 showed that the specific richness of Maldivian corals had not substantially decreased. In contrast, substratum cover was still very low, and the tridimensional structure of the reef was lost due to the destruction of dead colonies, which were reduced to rubble by bioerosion and waves. Contrary to first impressions, Maldivian reefs do not seem to be undergoing a phase-shift toward a dominance of algae or corals other than Acropora and Pocillopora. Rather, reef recovery seems to follow a predictable ecological succession toward the pre-existing situation. The hypothesis of emergency spawning was suggested by the fact that recruitment started soon after the mortality event. It could be a paradoxical strategy to face environmental change and can be compared to hormesis, the phenomenon for which a stress factor produces positive effects at a sublethal level. The present situation in the Maldives, as compared to the 1998 catastrophe, allows cautious optimism about the recovery potential of the reefs. However, the expected increased frequency of thermal anomalies that may lead to bleaching, points to the existence of a major risk for coral reefs. Long term monitoring of the status of Maldivian coral communities remains the only mean to understand their future evolution. © Institut océanographique, Fondation Albert Ier, Prince de Monaco, 2006.