Coral reefs are found in circumtropical shallow tropical waters along the shores of islands and continents. The reef substrate is mainly composed of calcium carbonate from living and dead scleractinian corals. Many other invertebrates, vertebrates, and plants live in close association to the scleractinian corals, with tight resource coupling and recycling, allowing coral reefs to have extremely high productivity and biodiversity, such that they are referred to as ‘the Tropical Rainforests of the Oceans’.
Corals live in very nutrient poor waters and have certain zones of tolerance to water temperature, salinity, UV radiation, opacity, and nutrient quantities.
Coral reef ecosystems world-wide have been subject to unprecedented degradation over the past few decades. Disturbances affecting coral reefs include anthropogenic and natural events. Recent accelerated coral reef decline seems to be related mostly to anthropogenic impacts (overexploitation, overfishing, increased sedimentation and nutrient overloading. Natural disturbances which cause damage to coral reefs include violent storms, flooding, high and low temperature extremes, El Nino Southern Oscillation (ENSO) events, subaerial exposures, predatory outbreaks and epizootics.
Coral reef bleaching is a common stress response of corals to many of the various disturbances mentioned above. Beginning in the 1980s, the frequency and widespread distribution of reported coral reef bleaching events increased. Widespread bleaching, involving major coral reef regions and resulting in mass coral mortality has raised concerns about linkage of the events to global phenomenons including global warming or climate change and increased UV radiation from ozone depletion. This paper examines the causes of coral reef bleaching and addresses the impact of global climate change on coral reefs.
Ecological causes of coral bleaching
As coral reef bleaching is a general response to stress, it can be induced by a variety of factors, alone or in combination. It is therefore difficult to unequivocally identify the causes for bleaching events. The following stressors have been implicated in coral reef bleaching events.
If a global warming trend impacts on shallow tropical and subtropical seas, we may expect an increase in the frequency, severity and scale of coral reef bleaching. Coral mortality could exceed 95% regionally with species extirpation and extinctions. A conservative temperature increase of 1-2 degrees C would cause regions between 20-30 degrees N to experience sustained warming that falls within the lethal limits of most reef-building coral species. In conjunction with sea temperature rise would be a sea level rise, and it has been suggested that sea level rise would suppress coral growth or kill many corals through drowning or lower light levels. Some coral populations and their endosymbiotic zooxanthellae may be able to adapt to the extreme conditions predicted during global climate change. Refuges in benign habitats, such as deep, sunlit reef substrates, oceanic shoals and relatively high latitude locations, might exist, but widespread coral mortality and reef decline would be expected in shallow reef zones in most low latitude. Even if significant sea warming and elevated irradiance levels do not occur, coral reef degradation from anthropogenic pollution and over exploitation will still continue, a result of unrelenting human population growth.
Warmer water temperatures can result in coral bleaching. When water is too warm, corals will expel the algae (zooxanthellae) living in their tissues causing the coral to turn completely white. This is called coral bleaching. When a coral bleaches, it is not dead. Corals can survive a bleaching event, but they are under more stress and are subject to mortality.
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