August 11, 2011
In 1997-1998, the marine equivalent of a massive and destructive forest fire overwhelmed numerous coral reefs around the world. Caribbean coral biologists still speak of the bleaching event of 1998 in reverential tones. A powerful El Niño season was a factor in generating extreme tropical sea surface temperatures. As a consequence, over-stressed corals everywhere released their symbiotic algae known as zooxanthellae [zoh-zan-thel-ee]. This phenomenon is known as coral bleaching.
Millions of microscopic zooxanthellae play a critical role in a coral colony’s ability to both metabolize, and process waste. The tiny algae also give corals their color. When zooxanthellae are released into the water column, the remaining coral head appears as white as its underlying calcium carbonate skeleton. A coral colony can survive for a limited time without zooxanthellae, as long as environmental conditions return to normal. Slowly, new zooxanthellae will reappear in the coral tissues, and the corals, although susceptible to disease and algal overgrowth, having a fighting chance of recovery. However, in 1998, conditions did not level out and an estimated 16% of the world’s corals died (Wilkinson 2000). In some regions, the rate of mortality was probably much higher, and the relatively shallow western Caribbean and Bahamas were likely hit especially hard.
A bleached brain coral becoming overgrown with algae. Photo: NOAA
To understand our initial observations taken at eight study sites on Hogsty Reef, thirteen years after the 1998 massive bleaching event, let’s return now to the forest fire analogy. Imagine large, healthy coral heads on a reef as the old growth trees of a forest. When a raging fire burns through a forest, it destroys almost all of the young seedlings and saplings, leaving behind only a few of the strongest old-growth trees. However, any remaining live trees are weakened and damaged by the fire and are susceptible to diseases and parasites. The surviving trees are also responsible for reseeding the forest with the next generation of seedlings.
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