Coral bleaching is a process in which reef corals, soft corals, giant clams, some sponges, etc. lose their colour and become white due to expulsion of algae zooxanthellae from their tissues. This results in the corals turning completely white.
Zooxanthellae are unicellular algae that live in the tissues of corals. They have a symbiotic relationship with the corals, i.e. they live in harmony with coral animals and share their resources. Most importantly, they produce carbohydrates during photosynthesis which they provide to the corals as food. Second to that, they give the corals their normal healthy colour. When the coral loses zooxanthellae, its white skeleton becomes starkly visible through the tissues, which are transparent. The loss of colour is gradual but drastic.
The most common cause for coral bleaching is an increase in the sea temperature. Reef corals are very sensitive to any change in sea temperature. A rise of even 1°C above the average temperature can cause bleaching in many coral species because it causes a breakdown in the symbiotic relation between the algae and the coral. For example, in 2005, warm waters near the Virgin Islands and Puerto Rico spread southwards. The increased thermal stress on the reefs resulted in the death of half of the coral reefs of the United States in the Caribbean in one year. The mass bleaching events that have been reported in the last 5-10 years have all been caused mainly by rises in seawater temperature.
Sometimes, though, bleaching of the reefs can also be caused by a drop in the sea temperature. Water temperatures dropped by 12.06 degrees Fahrenheit below the typical winter sea temperature in the Florida Keys during January 2010. This also resulted in a bleaching event.
Temperature change is not the only cause of coral bleaching. Other factors that could play an important part are changes in the salinity of seawater, high sunlight intensity, or disease. Mankind can also be held responsible for coral bleaching when it is caused by pollution from agricultural run-off or sedimentation from underwater activities such as dredging.
Just because corals are bleached, does not mean they are dead. If the stressful conditions abate and zooxanthellae are reabsorbed by the corals then the corals’ colour and symbiotic relationship with the algae can return to normal. There exist some coral reefs called ‘resilient reefs’ which have the ability to bounce back from bleaching. However, if the stressful conditions persist, then it can result in the death of the corals. So why should this matter to us?
Reefs have their own importance that should not be understated. They protect shorelines and twenty-five percent of marine species. They provide tourist dollars and support fisheries, and their medical properties are still to be fully explored.
That’s why we should care that over 93% of our coral reefs have already been destroyed. Almost all tropical oceans saw bleaching events in 1997/1998, and many isolated events followed in the next few years. Sri Lanka, Maldives, Kenya, Tanzania and Seychelles lost up to 90% of their coral cover during this period. The Great Barrier Reef also experienced a mass bleaching event in 1998, and then another one in 2002, followed by yet one more in 2006. The mass bleaching event occurring in the summer of 2002 in the Great Barrier Reef was the worst that had ever been recorded there. Most of the reefs had relatively lower death rates and survived but some regions, including inshore reefs around Mackay and Bowen, suffered severe damage, as did some of the corals in the Coral Sea. Up to 5% of the reefs in the Great Barrier Reef have been harmed to such an extent that their recovery could very well take decades.
At an individual level anywhere in the world, you can help protect coral reefs. By taking small steps to reduce our carbon footprint, you keep greenhouse gases out of the atmosphere, which helps combat climate change. Simple things you can do are trying to take public transport or walking instead of driving, planting trees, and contacting your local legislation and informing them that you support comprehensive climate legislation. As people concerned about the environment, we should try to stay informed and aware about the situation and show support for organisations like the Marine Aquarium Council that was established to create a set of voluntary standards for those in the harvesting trade. They educate and support the locals who harvest corals. According to their website, they “promote sustainability through the development and deployment of best practices, standards and certification for those engaged in the collection and care of marine ornamentals.”
Efforts are being made to incorporate what is known about reef resilience into the design and management of marine parks in many different countries. Workshops called reef resilience workshops are held that teach participants how to keep their reefs healthy. These workshops also encourage the people to promote reef protection among their communities.