Coral bleaching reduces reproduction of scleractinian corals and increases susceptibility to future stress

Extensive bleaching of corals occurred at Heron Island Reef during the 1998 mass bleaching event on the Great Barrier Reef, Australia. Two hundred coral colonies were labelled and sampled on the reef flat at Heron Island in March 1998 (when approximately 80% of corals growing in the intertidal reef flat were bleached). These included both bleached and unbleached colonies of many species. Bleached colonies of all sampled species had lower densities of symbiotic dinoflagellates and lower chlorophyll a concentrations per surface area of coral tissue than unbleached colonies. Samples of the colonies were decalcified and polyps dissected to determine fecundity. There were significantly fewer eggs present in the bleached than unbleached colonies in all sampled species. In many species, no eggs were present in the bleached colonies. Eggs that were present were also significantly smaller in the bleached than the unbleached colonies of the majority of species sampled. There were also significantly fewer polyps containing eggs and testes in the bleached than the unbleached colonies. The percentage of tissue made up by lipids in the bleached colonies was significantly lower than that of the unbleached colonies in some species. By July 1998, 23% of the sampled colonies had died and many of the previously bleached colonies had regained their colour, suggesting visually that they had recovered. However, previously bleached colonies in November 1998 still had fewer eggs and reproductive polyps than colonies that had been previously unbleached. In July of the following year, in the middle of the Australian winter, many of the corals that had bleached the year before bleached again and more colonies had died. In contrast, none of the previously unbleached colonies bleached at this time. In November 1999, just prior to the spawning period, there were large areas of coral on the reef slope that were noticeably pale. These pale colonies were sampled along with adjacent normally pigmented corals and the pale colonies were almost entirely devoid of eggs. During the 1998 bleaching event approximately 80% of the reef slope colonies were bleached, so it is likely that these pale colonies were previously bleached colonies. These data suggest that bleaching has adverse and long-lasting effects on coral reproduction and that previously bleached colonies may be more susceptible to future stress.