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Shedding More Light on the Causes and Effects of Coral Bleaching

Coral reefs are some of the most diverse habitats in the natural world, and they are deteriorating at an alarming rate. One of the most significant and complex threats to these fascinating organisms is the phenomenon known as coral bleaching. This occurs when the tiny dinoflagellate microalgae that photosynthesize and help to give the corals their vibrant colors, known as zooxanthellae, are ejected from their host. Not only do the corals then lose their color (hence the term “bleaching”), but also, if they do not regain their zooxanthellae quickly, they will die. As a result, marine scientists are expending a lot of time and effort into understanding the various factors that contribute to this global issue.

This article presents the findings of a leading in-depth, longitudinal study that examined a significant bleaching event that occurred in 1998 in Seychelles. They studied data points on 21 different reefs starting in the year 1994, finding that after the bleaching event of 1998 many of the reefs had undergone a significant regime shift in coral to macroalgae, from an average of 31% coral and 3% macroalgae pre-event to post-event levels of 3% coral and 42% macroalgae. These significant shifts directly corresponded to alterations in the functional diversity of these reefs, and of the factors examined in the study, three stood out as the most important: the three-dimensional structural complexity of the reef, water depth, and the abundance of juvenile corals. The researchers found that the combination of higher structural complexity and increased depth could predict whether the reef underwent a regime shift after the bleaching event 98% of the time.  What they also found is that in 12 of the 21 reefs they studied there was no regime shift, meaning the reefs adequately recovered.

The implications of this study are important indeed for the preservation of the world’s reefs. It supports previous works detailing the adaptability of coral reef ecosystems and indicates that perhaps the situation concerning the decline of the reefs is not quite as bad as we believe it to be. But more importantly, it highlights more intelligent methods for the establishment of marine parks. The study found that whether or not the reef fell inside a marine reserve seemed to have no bearing on whether it suffered a regime shift, indicating that current parameters for where to place these marine parks have missed the mark. In order to protect our existing reefs more effectively, factors such as structural complexity and depth should be at the top of the list in determining where these marine reserves should be. 

Posted by Ian Mallor (B)