Coral bleaching is a natural stress phenomenon characterized by the expulsion of zooxanthellae from the animal host. Without the zooxanthellae, the transparent tissue and white skeleton are revealed.
Image Credit: Bleached Coral at Heron Island, Great Barrier Reef J. Roff
The increased frequency and severity of environmental stressors have led to a rapid degradation of coral reefs worldwide. Coral bleaching is regarded as the major threat to the survival of coral reefs globally.
The main contributor to coral bleaching is higher than usual oceanic temperatures related to greenhouse gas emissions and climate change.
Ocean warming is a well-studied phenomenon caused by an increase in emission of man-made greenhouse gases due to burning fossil fuels like coal, oil and gas.
Warming temperature stresses the coral, causing the coral to expel its zooxanthellae. Runoff from storm generated precipitation can also carry pollutants in to the water and bleach near-shore corals. Mechanistically an overexposure to sunlight, high temperature and high solar irradiance also contributes to bleaching in shallow corals.
Most recently, in 2016 the Great Barrier Reef experienced a massive die off of over 50% of corals in the northern region.
It is estimated that about 25% of the atmospheric carbon dioxide is then dissolved in the ocean, which leads to another threat known as ocean acidification.
Over the past 250 years, acidity has risen over 30%. More acidic waters will break down the coral’s delicate calcium carbonate structure and make the coral more susceptible to other environmental stressors, together leading to a higher chance of coral bleaching and subsequent coral death.
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