Reef-building (zooxanthellate) coral populations and coral reef formations have undergone marked changes in the Galápagos Islands over the past 34 years. These changes, largely negative, occurred predominantly at study sites in the central-southeastern and northern sectors of the archipelago, and were coincident with strong El Niño warming events. Coral communities and reef formations in the far northern islands were affected less during El Niño events. The Galápagos coral fauna is depauperate with 19 known zooxanthellate species. Four species in three genera have constructed small (0.5-1.0 ha) and thin (0.5-5 m) reef formations predominantly at sites sheltered from strong wave assault and from upwelling centers in the western sector of the archipelago. How have recent El Niño events affected coral populations and reef growth in the Galápagos Islands? Specifically, what have been (a) the changes to coral community structure, (b) the effects on coral reef formations, and (c) the causes of these changes? In light of projected global climate change, what is the fate of coral reef growth in the Galápagos Islands? Coral community structure and reef distribution were determined from island-wide surveys in 1975 and 1976. Percent cover of corals and associated benthos were quantified from 1 m2 quadrats, 10 m-long chain transects, and timed swimming surveys over known areas. Sample sites were mapped and established as permanent monitoring sites. Carbonate frameworks were probed with rods to determine thickness. Sclerochronological methods were employed to determine skeletal growth rates, ages, and past sea temperature conditions. Population densities and feeding activities of Eucidaris galapagensis, an omnivorous echinoid that preys on live coral, were determined before and after coral mortality events.
Results/Conclusions
Coral mortality was catastrophic during the strong 1982-83 El Niño event, resulting in 95-99% loss of all reef-building species. Corals that recovered or recruited after the 1982-83 El Niño were also impacted during the severe 1997-98 El Niño, but at a reduced level (~26% overall). Species with massive morphologies (Porites, Pavona) demonstrated higher survival than those with branching morphologies (Pocillopora). All dead reef frameworks experienced extensive bioerosion and disintegration after 1983, resulting in the complete disappearance of most reef formations. Field observations and mesocosm experiments demonstrated that coral bleaching and mortality were caused by elevated sea temperatures that accompanied strong El Niño events. Reef disintegration was primarily a result of intense Eucidaris grazing. Structural coral reefs may not develop with recurring strong El Niño activity and Eucidaris bioerosion.
