Coral bleaching is caused largely by excessively warm water. Reef building scleractinean corals have a symbiotic relationship with Zooxanthellae, a single celled dinoflagellate "algae" (actually Protozoan, also called Protist). The Zooxanthellae uses sunlight and carbon dioxide wastes from coral animal metabolism to photosynthesize oxygen and carbon compounds (food) farmed by the coral. It is this relationship that enables a rich coral reef to flourish in otherwise nutrient poor tropical waters. However, if it gets just a little bit too warm--it takes only one degree over the usual annual maximum at Kwajalein--the Zooxanthellae are expelled from the coral, leaving coral colonies pure white. Initially, the coral is still alive, but if the water stays warm too long or the Zooxanthellae fail to recolonize the coral tissue, the coral starves. In addition to warm water, direct sunlight also plays a role. While the Zooxanthellae in coral tissue require sunlight for photosynthesis, when the water is warm, direct sunlight is too much for it to handle. The portions of a coral to bleach first are usually those most exposed to direct overhead sun. Parts in the shade last a bit longer, and are sometimes the areas from which Zooxanthellae may be able to recolonize the rest of the coral after temperatures come down again. In all my years of diving in the Marshalls, starting in the mid 1960s and extending sporatically through parts of the 1970s and early 1980s, then continuously from 1988 through 2016, I never saw coral bleaching until 2009, when the fall temperature peaked at 30°C. Then in happened again in 2013, 2014 and 2016, and I've heard from contacts still out there, again in at least 2017 and 2018. There is no question the water is getting warmer. While some corals recovered from each bleaching, many died, and I understand the 2018 bleaching was especially serious. This page shows some large colonies of Porites lutea and P. lobata bleaching.
Created 20 April 2020