Coral reefs are found in tropical and subtropical regions worldwide. Sometimes called the rainforests of the sea, they are home to more than a quarter of all known shallow-water marine species.1,2 Coral reefs protect coastlines, support fisheries, foster a lucrative tourism industry, and form countless islands and atolls that are home to millions of people.3-8
Coral reefs are composed of thousands of individual coral polyps that secrete calcium carbonate (CaCO3) skeletons. Coral polyps are naturally transparent; their color comes from microscopic algae that live in the outer layer of the coral’s tissue.9-11 These microscopic algae, known generally as zooxanthellae, provide corals with up to 95% of their food requirements and are essential in the persistence and survival of coral reefs.9,12
Coral bleaching occurs when stressful environmental conditions cause corals to expel the zooxanthellae from their cells, revealing the white calcium carbonate skeleton beneath their tissue.13 Bleaching events that occur over a large geographic area are called “mass bleaching” events because they affect entire reef systems and can result in massive coral mortality.6,9,15,6
Mass bleaching events are closely associated with prolonged elevated sea surface temperatures, particularly those associated with El Niño Southern Oscillation (ENSO) events. 9,13,14,19 ENSO is a climatic phenomenon that has the ability to change global climate dramatically. The largest bleaching event on record occurred in 1998 when reefs in every region of the ocean experienced bleaching.16,20 Current trends in sea surface temperature and bleaching events indicate that by 2030 we may experience El Niño-like water temperatures more frequently and for longer periods. 2,9,13,19
Although not immediately fatal, coral bleaching affects the ability of a coral to grow and reproduce, and may increase susceptibility to disease.9,13,21 Fortunately, if unfavorable environmental conditions abate quickly, corals often can make a complete recovery. Much is still unknown about interactions between corals and their symbiotic zooxanthellae, and recent research has shown that bleached corals have the ability to recapture zooxanthellae that are more tolerant of high temperatures.13 Some healthy reefs in the South Pacific are capable of rebounding rapidly after suffering degradation associated with bleaching.22 Bleaching will become more prevalent with increasing ocean temperatures. By reducing the stressors we can control on a local level, such as sedimentation rates, nutrient pollution, and overfishing, we can reduce chronic stress levels on coral reefs, increasing their resiliency in the face of rising ocean temperatures.