Changes in zooxanthellar densities and chlorophyll concentrations in corals during and after a bleaching event

In January 1994 a coral bleaching event occurred on the reefs of Magnetic Island (Australia) immediately after a period of anomalously high air temperatures. Average daily water temperature increased by 2 degrees C over 1 wk, reaching 34 degrees C (reef flat) and 32 degrees C (reef slope). Bleached corals were observed soon afterwards, suggesting the bleaching event was temperature related. Bleached (light tan coloured) and unbleached (dark brown coloured) colonies of staghorn coral Acropora formosa sampled during and after the bleaching event had lower zooxanthellar densities and higher chlorophyll a (chi a) concentrations per zooxanthella than the same colonies sampled 10 mo after the bleaching event. Significant increases in zooxanthellar chi a concentrations were measured in 2 colonies which were sampled and found to be losing zooxanthellae during the event. Intrinsic differences in the densities, chlorophyll concentrations and zooxanthellar cell sizes between neighbouring colonies of the same species resulted in intraspecific variability in coral discolouration during the bleaching event.