Coral Bleaching- Explained Pointwise

Researchers at the ICAR-Central Marine Fisheries Research Institute have found that marine heatwaves since October 2023 have resulted in widespread bleaching of coral reefs in the Lakshadweep Sea. Further, according to the US National Oceanic and Atmospheric Administration (NOAA), the fourth global mass coral bleaching event has been triggered by extraordinary ocean temperatures. The mass coral bleaching could have serious consequences for ocean life and millions of people who rely on reefs for food, jobs, and coastal defence.

What are coral and coral reefs?

Corals- Corals are small (0.25-12 inches), soft-bodied marine invertebrates (not possessing a spine) which belong to the group cnidaria. These are sessile animals which permanently attach themselves to the ocean floor.

Each individual coral animal is known as a polyp and it lives in groups of hundreds to thousands of genetically identical polyps that form a ‘colony’. They use their tiny tentacle-like hands to catch food from the water and sweep into their mouth.

Mutualistic relation between Corals and Algae- These coral polyps have microscopic algae called zooxanthellae living within their tissues. The corals and algae have a mutualistic relationship.
The coral provides the zooxanthellae with the compounds necessary for photosynthesis. In return, the zooxanthellae supply the coral with organic products of photosynthesis, like carbohydrates, which are utilized by the coral polyps for synthesis of their calcium carbonate skeletons. In addition to providing corals with essential nutrients, zooxanthellae are responsible for the unique and beautiful colours of corals.

Types of Corals- There are two types of corals- Hard coral and soft coral.

a. Hard corals- These corals also called hermatypic or ‘reef building’ corals which extract calcium carbonate from the seawater to build hard, white coral exoskeletons.

b. Soft Corals- Soft corals borrow their appearance from plants, and attach themselves to such skeletons and older skeletons built by their ancestors. Soft corals add their own skeletons to the hard structure over the years and these growing multiplying structures gradually form coral reefs. They are the largest living structures on the planet.

Coral Reefs in India- Coral reefs in India are located in seven regions namely Goa coast, Kerala coast, Palk Bay, Gulf of Kutch, Gulf of Mannar, Lakshadweep islands and Andaman and Nicobar islands.

What is the significance of Corals?

1. Rainforests of the sea- Coral reefs support over 25% of marine biodiversity, including fish, turtles and lobsters, while taking up 1% of the seafloor. For ex- The Great Barrier Reef contains over 400 coral species, 1,500 fish species, 4,000 mollusc species and six of the world’s seven sea turtle species.

2. Economic value- Coral structures provide economic goods and services worth about $375 billion each year. More than 500 million people across the world dependent on coral reefs for food and income.

3. Coastal protection- Coral reefs absorb up to 97% of the energy from waves, storms, and floods, which prevents loss of life, property damage, and soil erosion. The absence of coral reefs results in severe ramifications for marine life and humans.

What is coral bleaching? What are the mass coral bleaching events which have taken place?

Coral Bleaching- When corals are stressed by changes in conditions such as temperature, light, or nutrients, they expel the symbiotic algae (zooxanthellae) living in their tissues, causing them to turn completely white. This is called coral bleaching.

Coral Death- Bleached corals can survive depending on the levels of bleaching and the recovery of sea temperatures to normal levels. If heat-pollutions subside in time, over a few weeks, the zooxanthellae can come back to the corals and restart the partnership. But in cases of severe bleaching and prolonged stress in the external environment can lead to coral death.

Mass Coral Bleaching Events

Fourth global bleaching event is currently underway. Nearly 54 countries, territories and local economies- from Florida, the US, Saudi Arabia to Fiji- have confirmed bleaching. Bleaching has been confirmed in the Western Indian Ocean, including Tanzania, Kenya, Mauritius, Seychelles, and off the western coast of Indonesia.

What are the Causes Responsible for Coral Bleaching?

1. Rise in Sea Temperature- The rise in global warming has led to increase in the sea temperatures which has resulted in the destruction of corals.

2. Ocean Acidification- Due to rise in carbon dioxide levels, oceans absorb more carbon dioxide. This increases the acidity of ocean water and inhibits the coral’s ability to create calcareous skeletons, which is essential for their survival.

3. Solar radiation and ultraviolet radiation- Changes in tropical weather patterns result in less cloud cover and more radiations which induce coral bleaching.

4. Infectious Diseases- Penetration of bacterium like vibrio shiloi inhibits photosynthesis of zooxanthellae. These bacteria become more potent with elevated sea temperatures.

5. Chemical Pollution- Increased nutrient concentrations affect corals by promoting phytoplankton growth, which in turn supports increased numbers of organisms that compete with coral for space.

6. Human Induced Threats- Over-fishing, pollution from agricultural and industrial runoff, coral mining, development of industrial areas near coral ecosystems also adversely impact corals.

What are the consequences of coral bleaching?

1. Affecting the Food Chain- Changes in coral communities can affect the species that depend on them, such as the fish and invertebrates that rely on live coral for food, shelter. Loss of such marine animals can disturb the entire food chain.

2. Loss of Biodiversity- Declines in genetic and species diversity occurs when corals die as a result of bleaching.

3. Economic Decline- Bleached and degraded reefs can discourage tourism, which can affect the local economy.

4. Affects Food Availability- Coral bleaching can cause large shifts in fish communities. This can translate into reduced catches for fishers, which in turn impacts food supply and associated economic activities.

5. Impact on Coastal Protection- Coral reefs protect coastlines by absorbing constant wave energy from the ocean, thereby protecting people living near the coast from increased storm damage, erosion and flooding.

What are the Initiatives to Protect Corals?

A number of global initiatives are being taken to address the issues-

What should be the way forward?

1. Halting unplanned development- Halting unplanned coastal development would play a significant role in reversing the decline of reefs in some locations.

2. Promotion of sustainable fishing- Promoting sustainable fishing and providing opportunities for ecotourism can help conserve corals.

3. Minimal use of chemical fertilisers- There is a need to minimise the use of chemically enhanced fertilizers, insecticides, pesticides, and herbicides which are non degradable and harm corals.

4. Measures for prevention of global warming- Taking all possible measures to prevent actions that worsen global warming since Climate change is the greatest global threat to coral reef ecosystems.