Cloudy Weather May Have Saved Society Island Reef Corals During the 1998 ENSO Event

During the 1998 El Niño-Southern Oscillation (ENSO) event, mass coral bleaching in French Polynesia was patchy at a scale of 100s of km. Bleaching was extensive in parts of the Tuamotu archipelago (creating up to 99% coral mortality) but extremely mild in the Society Islands (Tahiti, Moorea), ca 350 km to the south-west, despite sea surface temperature (SST) anomalies being of similar magnitude to previous years in which mass bleaching occurred. We examine whether environmental variables account for this unexpected paucity of bleaching using a 50 yr record of SST, a 17 yr record of daily wind and cloud cover, and a 17 yr record of monthly sun hours. Records from Tahiti reveal that exceptionally high cloud cover significantly reduced the number of sun hours during the summer of 1998. Quadratic discriminant analyses of annual bleaching occurrence based on up to 3 predictors (cumulative degree heating months, wind speed, and cloud cover during periods of elevated summer SST) only predicted the correct bleaching scenario for 1998 when cloud cover was added to the function. The results demonstrate that the interactive effect of cloud cover can reverse the bleaching predictions of such statistical models. We suggest that reduced radiative stress, resulting from high cloud cover, may have prevented large-scale coral bleaching in 1998.

INTRODUCTION

Coral bleaching refers to the paling of coral tissue brought about by loss of symbiotic dinoflagellate algae (zooxanthellae) and/or their pigments (Glynn 1993, Brown 1997). Although bleaching is a generalised response that may result from a variety of environmental stresses (e.g. low salinity), the term ‘mass coral bleaching’ has been used to describe large-scale events in which intense bleaching occurs throughout entire reef systems and geographic realms (Glynn 1993, Hoegh-Guldberg 1999). Mass bleaching may result in mass coral mortality (Glynn 1993, Wilkinson 2000). Based on laboratory studies, several authors have suggested that heat and radiative stress interfere with zooxanthellar photosystems (Lesser et al. 1990, Fitt & Warner 1995, Lesser 1996, Warner et al. 1996, Brown 1997, Iglesias-Prieto 1997, Jones et al. 1998, Hoegh-Guldberg 1999). Reduced efficiency of utilising photosynthetic energy (Osmond 1994) occurs when the zooxanthellar photosystem is saturated with photosynthetically active radiation (PAR). Unless excess excitation energy is dissipated (e.g. by the xanthophyll cycle, Brown et al. 1999), photo energy is directed to the creation of harmful…