Bleaching poses a major threat to corals the world over. Caused by stress inducing factors like temperature rise, coral bleaching results in the symbiotic algae being expelled by corals. This results in nutrition deficiency for corals and also their bleached appearance.
According to figures released by the Global Coral Reef Monitoring Network (GCRMN), between 1980 and 1997, approximately 370 observations of coral bleaching were reported globally. Whereas between 1998 and 2010, the number of reported observations increased to more than 3,700!
Imagine jumping into the ocean, armed with your mask, fins and snorkel, to watch fascinating, bright corals and coming face to face with dead white coral skeletons. This is not a nightmare, but unfortunate reality. Coral bleaching due to a various factors, primarily climate change and human activity, has caused destruction of these underwater creatures.
About Coral Reefs
When you think of coral reefs the image that springs to mind is that of brightly-colored and often weird looking underwater plants. Coral reefs or rather groups of coral polyps however, are not plants but translucent animals that are known for living in vast colonies.
They are often found in warm, clear waters of tropical and subtropical countries. Corals are soft-bodied, tiny organisms that host tiny algae that live in their tissues.
These microalgae, also called zooxanthellae, provide food to corals through carbohydrates and energy produced during photosynthesis, while coral polyps provide algae with a protected environment, close to the ocean surface where they can receive a lot of sunlight. These algae are also responsible for imparting the bright beautiful color to the coral reef.
What is Coral Bleaching?
● Coral bleaching occurs as a result of a stress response. This can cause corals to shed the algae, leading to the white, skeleton-like appearance of the reef. This is vastly different from their usual brown and green appearance.
● Sometimes, rather than turning bright white, corals turn a shade of pastel yellow, pink or blue.
● Apart from pale coloration, a bleached coral can be identified by transparent tissue with white skeleton showing through. Some may have a fluorescent appearance.
● Despite its sturdy appearance, corals are extremely sensitive to stressful environmental conditions like ocean acidification, changes in salinity, high UV radiation levels, and cooling or warming of the waters in which corals reside.
● One common cause of coral bleaching is temperature rise due to global climate change. This is because most corals live close to their maximum thermal limits. When temperature of water increases even by 1°C and thermal limits extend, photosynthetic process of the algae is altered. This produces toxins. This is the reason that the algae needs to be expelled.
● Human induced factors can also lead to coral bleaching. A report from the World Resources Institute (WRI) found that around 60% of coral reefs are threatened by human activities like overfishing, destructive fishing, marine-based pollution and damage, coastal development, watershed-based pollution.
● If the stressful conditions prevail for a prolonged period and corals live without the algae, then they become susceptible to diseases, starvation and reduced growth rates.
● In case the stress causing factors are temporary and decrease with time, most resilient corals have the ability to recover from bleaching by regaining the symbiotic microalgae.
● Not all corals are susceptible to bleaching at the same rate. Compared to slow-growing corals, faster growing corals like Pocilloporids are more likely to suffer from coral bleaching. Some corals have fluorescent pigments that act as a shield against sunlight, changing the harsh UV rays into lower energy wavelengths.
About Mass Coral Bleaching
● When coral bleaching affects reefs globally, it is mass coral bleaching. It has been observed and reported since 1870s. The first mass coral bleaching was seen after 1982-83 El Nino event on Pacific coast of Panama. Such events have increased in the past 20 years.
● According to the Great Barrier Reef Marine Park Authority (GBRMPA), the two most significant mass bleaching events occurred in 1998 and 2002, in the Great Barrier Reef in Australia.
● Aerial surveys by scientists at Great Barrier Reef Marine Park Authority (GBRMPA) found that mass coral bleaching of 2002 affected around 54% of 641 reefs observed, and 41% offshore and 72% inshore reefs experienced moderate to high bleaching.
● According to Seaweb, out of 109 nations where coral reefs are found, reef degradation was in 93 nations.
● The severely damaged reefs from each of the mass bleaching events will take decades to recover. The recovery depends on young coral recruits growing on the reef and this can be quite a time-consuming process.
● While recovering, if the reefs are hit with another mass bleaching event then recovery may be prevented or slowed down further.
Mistaken Coral Bleaching
● Some corals, like the Fire Coral, have white tips. This is not be confused with bleaching where the entire coral branch is white or pastel colored. Some corals have white tips while they grow out.
● Sometimes coral diseases like White Syndrome can cause a part of the coral to appear white. Unlike bleached corals however, the diseased corals will have bands or white strips.
● The reef eaten by the Crown-of-Thorn starfish (COTS) has a bleached appearance, but on a close look, absence of transparent tissue and tentacles covering skeleton can signify COTS feeding. When COTS eat the leaves they leave an irregular patch of white, unlike uniform bone-white appearance.
A lack of awareness and steps in conservation of reefs may increase the frequency and severity of reef bleaching. Scientists estimate that we have already lost around 10% of all reefs, and in the next 30 to 50 years will lose many more.
Why is this significant? Although they cover only 1% of the world’s surface, coral reefs are host to 25% of all marine life, which includes more than 1000 fish species. The degradation of coral reefs due to bleaching can impact tourism, fisheries and directly or indirectly affect a lot of people who derive food and income from them.
In fact, according to the World Resources institute, destruction of 1 km. of coral can lead to an estimated loss of US$137,000-1,200,000 over a 25-year period.