Coral Bleaching

Threats to Coral Reefs Across the World


The recently released video that has captured the devastation of the Northern part of the Great barrier reef due to coral bleaching has sent shock waves across the global environmentalists. The fact that this is already the third such mass bleaching event recorded in the past 5 years, underlines the pace at which Coral assets are being eroded. Coral bleaching has become increasingly common in the recent past. It is a global phenomenon jeopardizing the future prospects of Coral eco-systems the world over.

Between 1979 and 1990, nearly 60 major episodes of coral bleaching have been reported. The longest and the most devastating amongst those was the one that occurred between 2014-2017.  During this three-year period, 70% of the coral reefs across the globe have been damaged. As per the United Nation’s reports, 70% of the world’s coral reefs are threatened, and 20% have been destroyed beyond recovery. Also, the report warns that another 24% are on the verge of imminent collapse and 26% are prone to long-term threats.

All the above statistics converge into one major observation that most of the bleaching has happened in the last 40-50 years, tacitly affirming the undeniable link between Coral bleaching on one hand and the growing pollution & Climate change on the other hand. Before decoding the link between them, let us first understand the science behind Corals and their bleaching.

What are Corals?

Coral is a category of animal that belongs to hydroids, jellyfish, and sea anemones. Corals are one of the earliest species that inhabited earth, dating back their origins to circa 200 to 300 million years ago. A coral polyp has a sac kind of body, a mouth with tentacles which are termed as Cnidae. With the aid of these Cnidae, polyps usurp carbonate ions from seawater to build itself a hard skeleton made up of calcium carbonate. This hard external skeleton shields the inner soft and fragile body of the polyps. Fine, we have understood what Coral polyps are. But, what are coral reefs?

Coral Reefs

Coral reefs are built by the strenuous efforts of thousands of coral polyps. Generally, Coral polyps are of two types, i.e., soft type & hard type. Soft corals do not have the external hard calcareous skeleton like their counterparts. Rather they develop wood-like cores for support and fleshy rinds for protection. So, they do not build coral reefs. So henceforth, our discussion would be centred only around hard corals.

Info 1Millions of years of evolution have given coral polyps plenty of time to find its symbiotic partner. And so did the polyps. Hard polyps pair up with their symbiotic partner called Zooxanthellae, which are microscopic algae. As a part of the symbiotic contract, polyps offer algae shelter while the algae return the favour in the form of food that it manufactures through photosynthesis. Also, the algae i.e Zooxanthellae feed upon the waste excreted by the polyps.

Types of Coral Reef Formations

Coral reefs are not homogenous formations. They are several types, of which the below three are most common.

Fringing reefs: These formations grow near the coastline around islands and continental margins. They are usually separated from the shore by narrow, shallow lagoons. These are the most common type across the globe. For example, Sakau Island in New Hebrides.

Info 2Barrier reefs: They have formed off/away from the coast, almost in parallel to the coastline as a broken and irregular ring. These are usually very large and often run more than 100kms in length and several kilometres in width. The iconic Great Barrier Reef of Australia, which is 1200 miles long, is the best example of barrier reefs.

Atolls: They are usually located on mid-oceanic ridgesThey are formed in a circular pattern like a ring, enclosing a lagoon in the midst of seas, far away from the land. They are usually formed when islands surrounded by the fringing reefs sink into the sea and corals keep on developing the reef. Maldives & Fiji have many such atolls.

Geographical Spread

Coral reefs have marked their presence all across the globe in the shallow seas of Tropical and subtropical bands on either hemisphere. The reef building coral usually prefers depths lesser than 30meter and a temperature range Info 3between 16 degrees to 32 degrees, and brightly lit waters. So, naturally, they are found within 30°N and 30°S. The Atlantic Ocean and the Indo-pacific region are home to scores of reefs. Atlantic reefs include Bermuda, The Bahamas, The Caribbean Islands, Belize etc, and the Indo-pacific reefs include Hawaiian Islands, Samoan Islands, Marian Islands etc. They are also found in the Red Sea and the Persian Gulf. Even India has four coral reef areas: Gulf of Mannar, Andaman and Nicobar Islands, Lakshadweep Islands and the Gulf of Kutch.

Coral Bleaching & the Science Behind it

As we have discussed above, Coral polyps have microscopic marine algae (plants called zooxanthellae) living inside their tissue under symbiotic arrangement. They cater polyps with color and food and Polyps, in return, provide shelter to the algae. Info 4But when the coral polyps are subjected to stress, they can no more harbor their symbiotic partner i.e., algae and consequently start expelling them. This would cut down on the food supplies of the polyps and hence their colored external skeletal structure turns into bright white in color. This is what we call as Coral bleaching. Coral bleaching does not imply their death. Nevertheless, when the stress on the corals persists inordinately, it may result in the eventual death of the corals.


As we have understood that stress causes the expulsion of Zooxanthellae by the polyps, let us understand what are the different stress that are constantly destabilising the coral reefs. And in order to understand different stresses, we need to know why a slight change in the atmosphere plays a stressful impact up on the corals. It is because of the susceptibility of the Corals, which can operate only within the narrow range of atmospheric parameters, which are as follows.

Temperature:  Corals thrive in sea temperatures ranging narrowly between 25° to 29°C ( as per IUCN). So, naturally, tropical seas are conducive for corals where both the diurnal and annual temperature ranges are narrow.

Depth: Coral can survive only in brightly lit sea waters. They cannot survive either too much light or the total absence of light. This narrows their thriving range to 45 m to 55 m below the sea surface.

Salinity: Can survive between 32%–40% of Salinity, as per various studies.

Abundancy of Plankton: Corals feed on microscopic Phyto plankton and hence their growth prospects are largely dependent on the availability of plankton. As more plankton are available on the sea-ward side, Reefs keep on expanding towards the sea-ward side.

Pollution: Corals are highly susceptible to pollution, and even a slim increase in marine pollution can be fatal.

So, any of the anthropogenic activities or natural causes that disturb these ideal conditions would ultimately culminate in catastrophic changes in Coral Reefs. Some of the factors that trigger a change in ideal conditions for corals are discussed below.

Ecological factors

Climate change: Climate change is widely believed to be the prime reason behind coral bleaching.

Corals are sensitive to Temperature and thus they can only thrive within the narrow band of the temperature range. Reef-building corals cannot tolerate water temperatures below 18° Celsius. Many grow optimally in water temperatures between 23°–29°Celsius, but some can tolerate temperatures as high as 40° Celsius for short periods. So, a change in global temperatures, induced due to climate change are proving to be lethal for coral reefs.

Fluctuations in sea- levels: 

  • A rise in sea-level would submerge reefs, thereby depriving them of adequate sunlight for its symbiotic algae to photosynthesize.
  • A drop in sea-level, owing to reasons like low-tide period, tectonic uplifts etc, (which are not related to climate change), would expose the reefs to open sunlight, thereby resulting in bleaching due to over exposure to insolation, more than their tolerable limits.

Fluctuations in Sea temperatures:

  • Rise in temperatures may seriously harm the zooxanthella that Coral polyps depend upon. A steady exposure to aggravated temperatures would result in depletion of Zooxanthellae, and coral reefs lose their colours and turn while. The strongest sea surface warming event that happened in 1998 was followed by devastation of 46% of coral reefs in the western Indian ocean. Also, the mass bleaching in the Caribbean islands was preceded by the warmest sea temperatures that have been recorded in the past 100 years in the Caribbean region.
  • Even colder temperatures are equally harmful as evident from the fact thatcorals are absent on the western coasts of tropical temperate continents owing to the presence of the cold currents. In January 2010, Colder Ocean water triggered mass bleaching in Florida. 

Ocean acidification: It is a phenomenon in which there is a decrease in PH values of oceans owing to the increased uptake of atmospheric carbon dioxide. It has been studied that increased acidity in waters would deprive corals of their reef-building nutrient i.e., calcium. The more acidic seawater becomes, the less calcium carbonate it contains. Most marine species, especially coral, depend on calcium carbonate to build their protective shells. Gradually, shells become weak due to the lack of additional absorption of calcium and would ultimately lead to coral bleaching.

Fresh Water Dilution: As was mentioned above, Corals can survive the salinity only within the range of 32%–40%. So, any addition of fresh water would be detrimental to the health of corals as freshwater dilutes the salinity of ocean water. Increased discharge from rivers, over precipitation due to cyclonic activities, etc would often result in dilution of sea waters. However, such bleaching events are rare and localised within a relatively small areas, near by the shores.

Anthropogenic Causes

Irresponsible Tourism: Physical damage to the coral reefs can occur through contact from careless swimmers, divers, and poorly placed boat anchors. Hotels and resorts may also discharge untreated sewage and wastewater into the ocean, polluting the water and encouraging the growth of algae, which competes with corals for space on the reef.

Coastal Development: The haphazard proliferation of coastal settlements poses a range of threats to nearby coral reefs. Due to the want of space for urban developments like Airports &other infrastructure, constructions works are undertaken upon the lands reclaimed from the sea. In the process, the fragile reef eco-systems are being eroded due to dredging activities to undertake constructions. These construction activities may also result in release of additional fresh water, nutrients and sediment causing further devastation to the reefs. Suspended silt, generated from discharged sediments, can smother corals when particles settle out on the reefs, reducing the availability of sunlight & thereby potentially preventing coral from photosynthesising its food.

Pollution: Coral reefs prosper in clean water. But oceans, off late have been becoming the sink for global discharge of waste from various sources. From plastics, Industrial wastes, Soil runoff to Oil spills, etc, are having a catastrophic impact on the reefs. Increased industrial runoff elevates the nutrients in the oceans, thereby transpiring in mushrooming of algal blooms, which obstruct sunlight to Reefs.

Info 5Destructive fishing methods: To meet the ever-increasing demand for marine food, the fishing industry has resorted to using dynamites, cyanide and other perilous methods. These methods leave debilitating effect on the fragile reefs. Blast fishing can destroy an entire reef in one go. Also, Boat anchors are often dropped directly onto reefs, causing significant damage to reefs.

Overfishing: Overfishing in the oceans would reduce their population to below critical levels that are required to keep the coral eco-system healthy. A drastic fall in numbers is reported by various studies to negatively impact the reef’s ecological balance and biodiversity. For instance, overfishing of herbivorous fish can also lead to high levels of algal growth which in turn result in coral bleaching.

Coral mining: Coral mining is rampant in south and southeast Asia, for its limestone and other elements used as construction materials. The reef is blasted using explosives to remove the coral, causing immediate destruction, not only to the reef but also to the scores of species that are dwelling in the reef. In 1995, it was estimated that nearly 20,000 cubic meters of coral per year were mined in the Maldives alone for construction materials.

Apart from the threats mentioned above, there are a few more like Oil spills, Sedimentation, Chemical pollution, trinket & aquarium trade, diseases, invasion by alien species etc, that have cumulatively turned the future of reefs bleak.

The Growing Imperative to Conserve Coral Reefs

Coral reefs are some of the most diverse and valuable ecosystems on Earth. They are museums of Bio-diversity and are often regarded as Rain-forests of the ocean. In order to understand the imperative to conserve them, we need a thorough understanding of the services that the reefs render to nature and humankind.


Coral reefs, that are found in more than 100 countries host a wide range of marine species. Corals that build reefs, themselves are of more than 800 species. These reefs provide rich habitat for nearly 32% of all marine species. 37% of species of fish are found to have taken refuge in coral reefs. The bio-diversity of this scale sounds astonishing considering the fact that reefs cover about only 0.1% of the ocean surface. This magnanimous diversity, would in turn provide a range of services like maintaining ecological balance, nutrient recycling, medicinal services, absorption of toxicity present in the seas, etc. Recent estimates affirm that more than 90% of the coral reef species are yet to be identified and named.

 Coastal Protection

 Coral reefs act as a natural barrier shielding the coastal landscapes, urban dwellings, and communities from the action of waves. Around 200 million people across the world depend on the reefs to protect them from storm surges, tidal ingressions and wave advances. They are of great worth especially during the cyclones and storms, which are a frequent phenomenon in the tropical belts.


It is estimated that nearly half a billion people across the world are reliant on reefs for their daily subsistence. A 2014 assessment published in the journal Global Environmental Change had calculated the monetary value of social, cultural and economic services of coral reefs to be US$1 trillion. Info 6A 2015 study by WWF estimates that the climate-related loss to reef ecosystem services will amount to US$500 billion per year by 2100. Coral reefs often form the backbone of the economies of the local communities. Tourists flocking to reefs for diving would trigger a domino effect on the local economy as scores of economic entities like restaurants, hotels, guides, entertainment facilities etc., would spring up to cater to the needs of tourists. Bonaire, a small Caribbean Island is a classic example of this case. Bonaire earns about $23 million (USD) annually from activities centred around the reefs by just expending a meagre sum of less than $1 million per year on maintenance of the reefs.


Coral reefs are a vital source of food for the local communities. Reef fish and other critters that the reefs harbour is a great source of protein for nearly a billion people. In developing countries, it is estimated that reefs contribute to one-quarter of the total fish catch, thereby feeding millions of people.


Many of the medicines that are presently in wide use, have been sourced from the reefs. Also, it is estimated that there are many medicinal sources in the reefs that are yet to be discovered. Info 7A number of reefs bound organisms produce chemical compounds that have found their way in human medical applications. They have already been employed in treating various illnesses such as cardiovascular diseases, skin cancers, and leukaemia. Also, they are used for suppressing inflammation, to fight viruses and as muscle relaxants. Some species such as sea horses and pipe fish are being widely used in traditional Chinese Medicine. Not only the coral polyps are of medicinal value, but even their unique skeletal formations are also of great value in bone-grafting applications.

The Global Efforts

The International Coral Reef Initiative (ICRI) is an informal partnership between nations, international organisations and NGOs to conserve the coral reefs globally. It is the only global entity solely devoted to coral reefs. It’s stated objectives are in line with Chapter 17 of Agenda 21, Aichi Target 10 of the Convention on Biological Diversity’s 10-year Strategic Plan. Its functions are:

  • Generating adequate global awareness on the state of coral reefs across the world.
  • Promoting and sharing best practices in coral reef management and build a strong network of reef managers across the globe.

In May 2020, ICRI members adopted a recommendation for the inclusion of coral reefs within the Convention on Biological Diversity’s Post-2020 Global Biodiversity Framework (GBF). It includes a bunch of targets like maintaining health, integrity, stability of the reefs and also of the marine organism dependent on them. Parties to the CBD are requested to prioritise coral reefs in their national plans.

  • As of today, there is no dedicated global financial mechanism for coral reef protection. To address this lacuna, “Global Fund for Coral Reefs”, a first of its kind, funding mechanism was instituted in 2020 by the UN General Assembly.
  • Global Initiative to reduce Land Degradation and Coral Reef program has been launched at G20, in which environment ministers of respective countries come together and collectively work for the conservation of reefs.
  • Coming to India, several NGOs like Reef Watch Marine Conservation, Terra Conscious, Society for Marine Research and Conservation, etc, have been working individually towards the betterment of the health of Reefs. Moreover, manifold species of corals have been covered under the radar of the Wild life protection act, 1972.

However, the problem so far has been that Coral conservation hasn’t gained adequate traction among the policymakers of National governments. Despite of invaluable services that Coral Reefs offer to humankind and the impending crises that reefs across the globe are staring at, there hasn’t been a requisite number of Global institutions working to set the clock back on coral bleaching. Most of the efforts have been fragmented and with scant financial resources. Estimates indicate the global investment required for coral reefs is at least five times greater than current levels.

What can be done?

Our oceans have no borders. This is a global crisis. So, every stake holder i.e., National Governments, Local authorities, Communities, Civic associations, etc., should come forward to put up a strong united front to fight the impending crisis.

  • Confining global average temperature to below pre-industrial levels i.e., less than 2 degrees, in spirit with the Paris Agreement on climate change.
  • Cutting down on Carbon emission by shifting to Eco-friendly energy sources & a strict prohibition on untreated industrial discharges into the oceans.
  • Equipping the local communities with awareness, knowledge and resources to safeguard the reefs upon which their livelihoods are dependent. Local communities are the best persons to bank upon to do the job.
  • Regulating marine fishing & prohibiting unscientific and irresponsible exploitation with international collaboration.
  • Regulating the proliferation of urban-dwelling nearby the coastlines through strict implementation of Coastal regulation zones (CRZ) rules.
  • The national governments should find a way to increase their expenditure on reef conservation which hitherto had been so meagre & utterly insufficient. Governments should realise that every penny spent on reefs is worth its weight in gold.

These measures are easier said than done. However, we have no other option than to collectively wake up before it’s too late. The devastation has not been beyond recovery as of yet. It is still within our hands to pass on these priceless natural assets to posterity.


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