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The Effects of Climate Change on Coral Reef Bleaching
by Aaron Royston
-"It's time to buy a mask and snorkel, and see coral while it is still there."
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| This statement from The Economist magazine (2000) reflects the detrimental effects of coral bleaching on reefs worldwide. Since the phenomenon of coral bleaching was first noted over twenty years ago, coral reef bleaching events have increased in frequency and intensity on a global scale (Wellington et al. 2001). All coral species are anthozoans, the most morphologically diverse cnidarian group, and are comprised of an aggregation of tiny polyps that live in association with one another. Each coral polyp builds a skeleton of calcium carbonate that is appropriated from the surrounding ocean. When a polyp dies, its calcium carbonate frame is integrated into the reef structure, allowing corals to achieve incredible sizes (Barnes 1990). However, only the polyps on the surface of the reef framework are actually living, as corals mostly rely on autotrophic dinoflagellates, zooxanthellae, which live within the polyps for nutrition; only the polyps on the surface of the coral structure receive adequate amounts of sunlight to permit zooxanthellae photosynthesis. A number of natural and anthropogenic factors can create conditions where a dissociation occurs between the symbiotic zooxanthellae and coral, which can cause mortality among reefs. Most agree that elevated ocean surface temperatures are the main cause of this coral/algal dissociation on a global scale, with increased solar irradiance, specifically ultra violet radiation, also contributing significantly (Brown 1996). The zooxanthellae give the host coral its coloration, and when the zooxanthellae and coral are separated, the coral loses its pigmentation and turns into a pale, whitish color. Therefore, when the dissociation of the coral and algal symbiosis occurs, it is known as coral bleaching (Barnes 1990). While there are numerous hypotheses describing how coral bleaches, the precise mechanism of coral bleaching is still unknown. |
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