El Nino-associated disturbance to coral reefs and post disturbance mortality by Acanthaster planci

Abstract

Coral reefs in the tropical eastern Pacific region experienced catastrophic coral mortality during the severe 1982/1983 El Nino event. Pocillopora spp., the dominant scleractinian reef-building corals, were most seriously affected, resulting in large tracts (0.1 to 1 ha) of dead reef surface in Costa Rica, Panama, Colombia, and the Galapagos Islands, Ecuador. A sea star Acanthaster planci is now entering centrally-located reef areas in Panama corals and is feeding on large, massive corals formerly surrounded and protected by live Pocillopora corals and their symbiotic crustacean guards. This note outlines the effects of El Nifio-related differential coral mortality and subsequent mortality resulting from the elimination of a protective biotic barrier The ages of corals killed during the initial physical disturbance, and later by predation, allow an estimate of the period of uninterrupted reef growth, i.e. the minimum number of years since an earlier, major El Nino event: about 190 yr on the basis of present evidence. Warm tropical waters have traditionally been regarded as beneficial to coral reef development and growth (Dana 1843, Wells 1957, Kinsman 1964, Rosen 1971, Stehli & Wells 1971). Occasionally, however, intense natural warming of reef waters has resulted in localized, reef-building coral mortality (Shinn 1966, Jaap 1979, Yamazato 1981). The course of such events has typically involved a stress to the coral and its symbiotic algae (zooxanthellae) with the loss of zooxanthellae ('bleaching'), and subsequent recovery or death of the host coral colony. The El Nilio event of 1982/83, regarded as the strongest this century (Halpern 1983, Kerr 1983), accompanied widespread coral mortality in the equatorial eastern Pacific region in 1983 (Glynn 1983a, Lessios et al. 1983, Glynn 1984a). The abnormally high sea surface temperatures (mean values 30 to 31 'C), their duration (5 to 6 mo, Glynn 1984a), and depth of isotherm penetration (nearly 100 m deeper in some areas than in previous years, Rebert et al. 1983), were probably in large part responsible for the disturbance. Coral mortality was generally highest among species exhibiting the most rapid skeletal growth, e.g. Pocillopora spp. and Millepora spp. (hydrocorals). Several surviving coral species, previously protected from coral-eating sea stars because of their location on the reef, were exposed to predation following the El NiAoassociated catastrophe. These mortality events were observed on coral reefs located off the Pacific coast of Panama in the Gulf of Chiriqui, at Uva Island (7'48' 46 N; 81'45' 35 W), Contreras Islands, and at an