Coral Mortality and Disturbances to Coral Reefs in the Tropical Eastern Pacific

Abstract

Widespread bleaching and mortality of reef-building corals occurred in the tropical eastern Pacific region during the severe and prolonged El Niño-Southern Oscillation (ENSO) event of 1982-83. At the height of the 10 month sea warming period, Panamanian reefs experienced 2-3 bouts of coral bleaching (loss of symbiotic zooxanthellae), which resulted in coral death 2-4 weeks later. Coral reefs in Costa Rica, Panama and Colombia suffered up to 70-90% coral mortality; in the Galapagos Islands (Ecuador), most coral reefs experienced >95% mortality. The hypothesis that sea warming caused this disturbance is supported by (a) the coincidence between the warming event and stress responses of reef organisms, (b) the correlation between the magnitude of local temperature deviations and the extent of mortality, and (c) the El Niño simulation experiments that resulted in coral bleaching, mortality, and histopathological changes similar to those observed naturally. El Niño warming usually does not extend north of Ecuador where eastern Pacific coral reefs are best developed. A near-decadal (1976-1988) warming trend over much of the tropical and subtropical Pacific Ocean could have affected the susceptibility of reef corals during the short-term 1982-83 warming disturbance. Zooxanthellate corals were affected most severely in nearly all reef areas, however, other organisms – such as benthic algae, non-zooxanthellate scleractinian corals, black corals, molluscs, barnacles, and crustacean symbiotes of corals - often showed local negative responses associated with non-thermal, El Niño-related conditions (e.g., nutrient depletion, low plankton abundance, high sea level, and wave assault). Secondary disturbances included (a) the elimination of coral barriers, allowing the corallivore Acanthaster planci access to formerly protected coral prey, (b) increased external bioerosion of reef surfaces killed in 1983 because of post-El Niño increases in sea urchin densities, and (c) the establishment of damselfish territories on corals that experienced partial mortality in 1983. Such disturbances are currendy causing longer-term changes to their respective local communities. Estimates of the ages of massive corals that were killed or irreparably damaged, and the interruption of reef framework accumulation suggest that a disturbance comparable to that of 1982-83 probably has not occurred in the Galapagos Islands or Panama during the past 200 years. The initial damaging effects of the 1982-83 disturbance to reef coral populations, combined with persistent secondary disturbances and low coral recruitment, could prolong reef recovery for decades or possibly centuries. Periods of intense upwelling cause localized and moderate levels of coral mortality, but persistent ENSO-related sea warming causes widespread and catastrophic coral mortality. El Niño events of extreme severity may limit eastern Pacific reef growth and diversity as much as do distance and isolation of these reefs from the centers of reef development in the western Pacific.