Causes of Coral Bleaching

Coral bleaching refers to a condition that affects corals due to changing conditions in its environment. Coral bleaching is usually caused by climatic stressors stemming from anthropogenic or natural factors in the reef ecosystem, such as changing water temperatures, variations in sunlight, changes in ocean water chemistry, sedimentation and addition of nutrients, among others. Coral bleaching has increased in both extent and frequency in the last 20 years around the world and it is predicted that the trend will continue in the following years with the destruction of the reef ecosystem and the extinction of many coral species.

Coral bleaching results when a coral loses its symbiont algae, zooxanthellae. Corals receive their varied colors from zooxanthellae through photosynthetic pigmentation. The hability of zooxanthellae to carry on photosynthetic pigmentation is impaired at temperatures above 30 ºC. When the coral reef ecosystem is subjected to environmental stressors, it may be forced to expel their symbiont algae, leading to a lighter or whitish color. When the stressor is not too severe, corals usually recover their symbiont algae within weeks or months, but if the stress is prolonged, corals may eventually die.

Causes of coral beaching

Corals thrive in poor-nutrient waters and their tolerance to water temperatures have certain limits. coral bleaching occurs when certain conditions needed to maintain zooxanthellae are affected. These conditions most commonly stem from anthropogenic or natural causes, such as variations in water temperatures, solar irradiance, subaerial exposure during low tides, sedimentation, water dilution, acidification, bacterial infections, inorganic nutrients, overfishing and high sea levels due to global warming. The whitish appearance of corals during coral bleaching is due to the depletion of their calcerous skeleton due to a decline of photosynthetic pigments within zooxanthellae.

Global climate

Most instances of coral bleaching occur due to global warming. Increases in ocean water temperature and solar radiation are thought to play an important role in coral bleaching. Instances of coral bleaching have been reported during times when the sky is clear and the oceans show low turbidity and calm sea waters, conditions that improve heating and the penetration of ultraviolet radiation. When corals come close to their limits in thermal tolerance (23-29 ºC), slight increases in water temperature over a number of days can cause bouts of coral bleaching.

Coral bleaching events

There have been reports on coral mortality in many important coral reef ecosystems during the past two centuries; however, the instances of coral bleaching have increased since the 1970s. Nearly all major coral reefs in the world experienced some levels of coral bleaching and mortality throughout the 1980’s. Before the 1980s most damage to coral reefs was associated to climatic disturbances, including aerial exposures and storms. Coral bleaching events prior to the 1980s occurred in isolated reef regions. On the other part, coral bleaching instances during the 80’s affected extended geographic regions.

The coral reef ecosystem has been subjected to severe degradation world-wide over the last decades. Coral reefs have been affected by natural and anthropogenic threats. Natural threats include changes in water temperatures, nutrient variations and violent storms, among others. The anthropogenic effects include overfishing, nutrient overloading and sedimentation. According to the IPCC, coral reefs are expected to experience more bleaching events in the next decades.