Context

• The Great Barrier Reef is in the grip of a mass coral bleaching event driven by rising global temperatures. 

About Coral reefs

• Coral reefs are large underwater structures composed of the skeletons of colonial marine invertebrates called coral. 
• The coral species that build reefs are known as hermatypic, or “hard,” corals because they extract calcium carbonate (CaCO3) from seawater to create a hard, durable exoskeleton that protects their soft, sac-like bodies. Other species of corals that are not involved in reef building are known as “soft” corals. 

• Most reef-building corals contain photosynthetic algae, called zooxanthellae, that live in their tissues. The corals and algae have a symbiotic relationship. 
• The coral provides the algae with a protected environment they need for photosynthesis. In return, the algae produce oxygen and provide coloured pigments to corals,

Findings

• Covering less than one percent of the ocean floor, coral reefs support an estimated 25 percent of all known marine species.
• Between 2009 and 2018, some 14 percent of the world’s hard coral population was lost. 
• On  the other hand, between 2011 and 2019, algal cover in coral reef systems saw a 20 percent increase. 

About Coral Bleaching

• When corals are stressed by changes in conditions such as temperature, light, or nutrients, they expel the symbiotic algae living in their tissues, causing them 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.

Causes for coral Bleaching

• Change in ocean temperature, such as high temperatures caused by climate change is the leading cause of coral bleaching. 
• Various pollutants, including agricultural runoff, sewage discharge, and industrial waste, can harm coral reefs and trigger bleaching.
• Overexposure to sunlight can contribute to bleaching in shallow-water corals. 
• Exposure to the air during extreme low tides can cause bleaching in shallow corals.’

Implications of coral bleaching events

• • Healthy corals have a symbiotic relationship with microscopic algae called zooxanthellae. During bleaching, stressed corals expel these algae, losing their primary source of energy.

• Bleached corals are weakened and more susceptible to diseases.

• Coral reefs are regarded as foundational species and support food chains in the marine ecosystem. Coral bleaching events can disrupt existing food chains in oceans.

• Bleached and degraded reefs increase the vulnerability of coastlines to storms and floods unlike healthy coral reefs which act as natural barriers in protecting coastlines from erosion.

Conclusion

• Thus there is an urgent need to limit the impacts of climate change and reduce greenhouse gas emissions and strengthen monitoring of potential coral bleaching events.