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Coral bleaching

Coral bleaching refers to the loss of color of corals due to stress-induced expulsion of symbiotic unicellular algae. The corals that form the structure of the great reef ecosystems of tropical seas depend on a symbiotic relationship with photosynthesizing unicellular algae called zooxanthellae that live within their tissues. Zooxanthellae give coral its particular coloration, depending on the clade living within the coral. Under stress, corals may expel their zooxantheallae, which leads to a lighter or completely white appearance, hence the term "bleached".

Coral bleaching is a vivid sign of corals responding to stress, which can be induced by any of:

* increased or reduced water temperatures (often attributed to global warming
* * solar irradiance (photosynthetically active radiation and ultraviolet band light)
* changes in water chemistry (in particular ocean acidification)
* sedimentation (can be contributed to silt runoff)
* pathogen infections
* salinity

High sea surface temperature (SST) coupled with high irradiance is known to be the primary factor in summer coral bleaching. Wind, exposure at low tide, and weather conditions can contribute to coral bleaching. [6] Some of these factors are anthropogenic, while others occur naturally. The US National Oceanic and Atmospheric Administration (NOAA) monitors for bleaching "hot spots," areas where SST rises 1 degree Celsius or more above the long-term monthly average. Some argue this system detected the massive 1998 bleaching event that was worldwide. At the same time, NOAA Coral Bleaching "Hotspot" program uses a 50k satellite resolution at nighttime, which some argue covers too large of a spatial area and does not incorporate the max SSTs occurring usually around height of daytime, noon.

Bleached corals	Healthy corals

 

Once bleaching begins, corals tend to continue to bleach even if the stressor is removed. If the coral colony survives, it often requires weeks to months for the remaining symbiont population to reach a normal density . Following bleaching, corals may be recolonised by the same species of zooxanthellae, or by a different species. Different types of zooxanthellae respond differently to environmental conditions and may be more resistant to coral bleaching than other species. Some corals are known to host multiple clades of zooxanthellae within an individual coral. . Ability to withstand stress and bleaching and ability to recover from a bleaching event varies greatly across coral species. Large massive corals, such as Porites lobata is able to withstand extreme temperature shocks, while fragile branching corals, such as Acropora spp. are far more susceptible to thermal stress following a bleaching event. Recent research has also shown that corals consistently exposed to low levels of stress may in fact be more resistant to bleaching. Factors that protect against mass coral bleaching are bleaching resistance, coral tolerance, reef recovery. Due to the patchy nature of bleaching, local climatic conditions such as shade or a stream of cooler water can reduce the risk of bleaching. Also, the health and genetics of both the coral and its zooxanthellae can influence the risk of bleaching.

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