Additional info from the National Oceanic and Atmospheric Administration
Healthy tissue of most stony corals ranges from yellow to brownish in color, a function of the photosynthetic pigments of their symbiotic “zooxanthellae” (internal algae that supply nutrients to the corals). When corals are inordinately stressed, they often expel their zooxanthellae. This response is known as bleaching. During a bleaching event, a coral’s coloration disappears or becomes pale, and the white of the coral skeleton shows through the translucent coral tissue. In some species, such as the massive starlet coral Siderastrea sidereal, the tissue can appear pinkish or bluish, due to pigments within the animal tissue. Localized bleaching has been observed since at least the beginning of the 20th century. However, beginning in the 1980s, regional and global bleaching affecting numerous species has occurred on reefs worldwide. Bleaching usually is not uniform over single coral colonies within coral communities or across reef zones, and some species are more susceptible to bleaching than others under the same conditions. In some instances, only the upper surface or lower surface of the colony is affected. In others, bleached tissue appears as a circular patch or in the shape of a ring or wedge.
Localized bleaching has been attributed to exposure to high light levels, increased ultraviolet radiation, temperature or salinity extremes, high turbidity and sedimentation resulting in reduced light levels, and other environmental factors. In addition, bleaching in some species has occurred in response to a bacterial infection. However, the seven major episodes of bleaching that have occurred since 1979 have been primarily attributed to increased sea water temperatures associated with global climate change and El Niño/La Niña events, with a possible synergistic effect of elevated ultraviolet and visible light.
Debilitating effects of bleaching include reduced skeletal growth and reproductive activity, and a lowered capacity to shed sediments and resist invasion of competing species and diseases. Prolonged bleaching can cause partial to total colony death. If the bleaching is not too severe, and the stressful conditions decrease after a short time, affected colonies can regain their symbiotic algae within several weeks or months.