El Niño-Southern Oscillation mass mortalities of reef corals: A model of high temperature marine extinctions?

Protracted high sea temperature anomalies accompanying El Niño-Southern Oscillation (ENSO) events have caused reef-building coral bleaching (loss of zooxanthellae) and mortality in all major coral reef biogeographic regions during the past two decades. Coral reef degradation in the eastern tropical Pacific has resulted from reductions in live coral cover, declines in coral species population abundances, local to regional scale extinctions, disruption of predator/prey spatial relations and relative abundances, bioerosion of reef frameworks, and low coral recruitment. None of the coral species that have suffered regional extinctions has reappeared after 15 years. Intense external and internal bioerosion by fishes, echinoids, lithophagine bivalves and clionid sponges has occurred on reefs affected by the 1982/83 El Nino coral bleaching event, and 1000-5000 year old reef frame-work accumulations in the Galapagos Islands have been completely eroded and reduced to gravel and sand. Because tropical zooxanthellate reef species are more vulnerable to rising (2-3°C) than falling (8-10°C) temperatures, greenhouse conditions may be more critical in limiting reef growth than icehouse conditions. ENSO warming episodes elicit physiological stress responses resulting in widespread mass coral mortality, leaving scant traces relating to causation. Signals that may help to identify past ENSO disturbances are: (a) temperature-related oxygen isotopic signatures, (b) skeletal stress bands and growth discontinuities, (c) coral debris in beach storm deposits, (d) increases in coral clastics resulting from intensified bioerosion and (e) the skeletal elements of bioeroders. Because this disturbance is the most pronounced and widespread of any known natural perturbation, and may increase markedly in scope with projected global warming predictions, it is considered a likely agent of future and possibly some ancient bioevents.