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.

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“…Such a threat was realised following the 1982-83 El Niño Southern Oscillation event (ENSO). This resulted in mass coral bleaching and a 50 % reduction of live coral cover on Uva Reef (Glynn 1985a), which was followed by a dramatic increase in the population size of D. mexicanum from 2-5 ind m -2 pre 1983 to 60-100 ind m -2 post 1982-83 ENSO (Glynn 1988(Glynn , 1990Eakin 1991Eakin , 1992. Such large numbers of D. mexicanum resulted in high rates of reef framework erosion, shifting the reef environment into a state of near accretionary stasis (Eakin 1992;Glynn 1997).…”

“…Glynn 1973Glynn , 1974Glynn , 1976Glynn , 1977Glynn , 1981Glynn -1985aGlynn ,b, 1990. Glynn (1973) assessed the possible effect of A. planci on the coral species on Uva Reef in the Gulf of Chiriquí.…”

“…This occurs due to the displacement of the surface water by northern winds and can result in a surface water temperature decrease of 12°C within 24 h (Glynn 1972). The Gulf of Panama is also subjected to the effects of the El Niño-Southern Oscillation (ENSO) every two to seven years (Guzman and Cortes 2007) resulting in the warming of the water up to 31°C (Glynn 1985a). In contrast, the Gulf of Chiriquí has a narrower continental shelf and less freshwater input from continental runoff.…”

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“…Coral mortality resulting from ENS O warming also alters the relative abundances of coral prey and their spatial relationships relative to corallivore feeding activities (Glynn 1985b). Surviving coral populations may be subject to increased corallivory during and following such disturbances.…”

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“…Mechanically induced mortalities of corals and reef associates by storms and increased wave assault are more likely during ENSO activity (Robinson, 1985;Lirman et al, 2001). The immediate effects of El Niiio warming (i.e., the loss of zooxanthellae leading to coral bleaching and mortality) may be followed by several longer-term secondary effects such as: (a) increased emigration and mortality of obligate coral crustacean symbionts ; (b) the disruption of live-coral bamers that prevent Acanthaster from entering and feeding on patches of preferred prey (Glynn, 1985); (c) predator concentration on surviving corals (Glynn, 1990;Guzmin and Cortes, 1992); (d) interference with coral reproduction and reduced recruitment ; and (e) the invasion of dead coral patches by echinoids and damselfishes that interfere with coral regeneration and perhaps larval settlement (Wellington, 1982;Glynn, 1990). Other effects that can interfere with corals are related to La Niiia or the ENSO cool phase, such as (a) dinoflagellate blooms (Guzmin et al, 1990); (b) extreme low tidal exposures (Eakin et al, 1989;; (c) stressful upwelling events (Glynn and D'Croz, 1990); and (d) the proliferation of macroalgae that compete with, and sometimes overgrow, corals (Glynn and Mate, 1997).…”