Coral Reefs and Coral Bleaching

• A coral reef is a wave-resistant mass of limestone built by living organisms, chiefly corals and coralline algae, in shallow tropical seas.
• Reefs cover less than 0.1% of the ocean floor but shelter about a quarter of all marine species: the rainforests of the sea.
• Coral bleaching, the loss of the algae that feed corals, is now their greatest threat; four global events have struck since 1998.

Corals and Reef Growth

The coral polyp and its partner alga

• A coral polyp is a tiny cnidarian, a relative of sea anemones: a tube of tissue with a mouth ringed by stinging tentacles.
  • It secretes a limestone cup (corallite), divided by radiating septa, beneath itself.
  • Polyps bud into a colony linked by living tissue. New polyps grow on dead skeletons, so the reef builds upward and outward.
• Hermatypic corals build reefs and carry symbiotic algae. Ahermatypic corals are mostly solitary, lack the algae and range into cold and deep water.
• Zooxanthellae are single-celled dinoflagellate algae (family Symbiodiniaceae) living inside the polyp’s inner tissue.
  • Their photosynthesis supplies up to about 90% of the coral’s energy, speeds up skeleton building and gives corals their colour.
  • The coral gives them shelter, light, carbon dioxide and nutrients. This mutualism lets reefs thrive in clear, nutrient-poor water.
• Calcification: the skeleton is aragonite (calcium carbonate). Massive corals add a few millimetres to about 2 cm a year; branching Acropora can add 10 cm or more.
  • Coralline algae cement the rubble and build the algal ridge that faces the surf.

Conditions for coral growth

• Reefs are scarce on tropical west coasts (Peru, Namibia, California, north-west Africa), where cold currents and upwelling chill the water. Warm currents feed the east-coast reefs of Queensland, East Africa and the Caribbean.
  • Western Australia is the exception: the warm Leeuwin Current sustains Ningaloo Reef.
• Reefs break off at large river mouths, such as the Ganga–Brahmaputra, because of fresh, muddy water.
• Corals grow faster on the seaward side, where surf brings food and oxygen; hence the raised outer rim and slow growth in lagoons.
• Warm currents carry reefs poleward: the Gulf Stream to Bermuda (32°N), the Kuroshio to southern Japan.

Zones of a reef

Types and Distribution of Reefs

Types of coral reefs

• Charles Darwin (1842) divided reefs by their relation to land into fringing reefs, barrier reefs and atolls, the scheme still used.
• Fringing reefs are attached to the shore, or separated only by a narrow boat channel. They are the commonest type.
• Barrier reefs run parallel to the coast but well offshore, beyond a wide, deep lagoon, and are broken by tidal passes.
  • The Great Barrier Reef, the largest reef system on Earth, runs about 2,300 km off Queensland (about 10°S–24°S) as nearly 3,000 separate reefs.
• Atolls are ring-shaped reefs enclosing a lagoon with no central island. About 440 exist, almost all in the Pacific and Indian Oceans.
  • The Maldives also have faros, small ring reefs on the rim of a larger atoll.
• Minor forms: patch reefs (small reefs in lagoons or on shelves), platform (table) reefs (flat-topped shelf reefs with little or no lagoon) and coral banks (shapeless submerged reefs).
• Barrier reefs and atolls rise from depths far below the limit of living coral. Theories of their origin invoke a subsiding island, as Darwin argued, or platforms shaped by Pleistocene sea-level change.

Coral coasts and low-energy coasts

• A coral coast is a biogenic coast built by reef growth; its reef crest takes the surf, sheltering the lagoon and shore.
• A low-energy coast has weak waves (shelter, a wide shallow shelf or short fetch) and is built of fine sediment, as in the Sundarbans mangroves.

World distribution

• Shallow tropical reefs cover roughly 250,000–285,000 km², almost all between 30°N and 30°S.
• The Indo-Pacific holds most of them. The Coral Triangle is the centre of diversity, with about three-quarters of all coral species, and Indonesia has the largest national reef area.
• Atlantic reefs are fewer and less diverse. The eastern Pacific is isolated from the Indo-Pacific larval supply and stressed by El Niño.

Coral reefs of India

• India’s reefs cover about 2,375 km². The Andamans hold the largest share, then the Lakshadweep atolls such as Kavaratti, Agatti and Maliku (Minicoy).
• India has every major type: fringing reefs in the Andamans and the two gulfs, atolls in Lakshadweep, drowned reefs on west-coast banks.
• The Gulf of Kachchh reefs, at about 22.5°N, survive turbid, saline water and a large tidal range: among the hardiest anywhere.

Coral Bleaching

What bleaching is

• Coral bleaching is the whitening of corals when the polyp–zooxanthellae symbiosis breaks down. The coral expels its algae, or the algae lose their pigment, and the white skeleton shows through transparent tissue.
  • A bleached coral is alive but starving.
• Under heat stress, warm water and strong sunlight damage the algae’s photosynthetic system. They release toxic reactive oxygen species, and the coral ejects them.
• Outcomes:
  • Recovery: if the stress ends within weeks, the coral regains symbionts and colour, though growth, reproduction and disease resistance suffer for months.
  • Mortality: if the stress persists, corals die and turf algae cover the skeletons within weeks.
• Mass bleaching of whole regions has been recorded since the 1982–83 El Niño.

Causes of bleaching

• Heat is the dominant cause. Marine heatwaves, amplified by El Niño, drove every global event. Ocean acidification adds stress by slowing calcification.
• NOAA’s Coral Reef Watch measures heat stress by satellite as degree heating weeks (DHW): the temperature excess above the bleaching threshold, summed over 12 weeks. India’s INCOIS runs a similar coral bleaching alert system.

Recovery and regime shifts

• Reef recovery means regrowth of coral cover by larval recruitment and growth of survivors. Branching corals can restore cover in about 10–15 years; massive corals take decades.
  • It needs time between disturbances: a 2025 study of three Lakshadweep atolls found good recovery only after six or more bleaching-free years.
  • It is fastest where water is clean, herbivorous fish are plentiful and nearby reefs supply larvae.
• A regime (phase) shift is a lasting flip from coral to fleshy macroalgae.
  • Drivers: coral death, overfishing of grazing fish, loss of grazing urchins and nutrient runoff.
  • Feedbacks: algae block larval settlement, shade young corals and trap sediment, so the algal state persists after the stress ends (hysteresis).
  • Jamaica: overfishing, Hurricane Allen (1980) and the die-off of the urchin Diadema (1983) cut coral cover from about half to a few per cent by the early 1990s.
  • Reversal needs restored grazing and cleaner water, the basis of reef-resilience management.

Mass bleaching events

• The fourth global event began in early 2023 and was declared by NOAA on 15 April 2024. It reached 84% of the world’s reef area and at least 83 countries and territories, the most extensive on record.
  • Severe bleaching off Western Australia in early 2025 was its last major episode. In June 2026 NOAA confirmed that it had ended in mid-2025.
  • Scientists now expect near-annual bleaching; the El Niño forecast for late 2026 raises the risk again.
• The Great Barrier Reef has bleached en masse in 1998, 2002, 2016, 2017, 2020, 2022, 2024 and 2025. Its northern and central coral cover was recovering again in 2026.

Clive Wilkinson and the Status of Coral Reefs of the World reports

• Clive Wilkinson, of the Australian Institute of Marine Science, Townsville, coordinated the Global Coral Reef Monitoring Network (GCRMN) under the International Coral Reef Initiative. He edited the Status of Coral Reefs of the World reports of 1998, 2000, 2002, 2004 and 2008.
• His key observations:
  • The 1998 bleaching destroyed about 16% of the world’s reefs in nine months, the largest single cause of reef loss, and perhaps half of these would never recover adequately.
  • The Indian Ocean was worst hit: most shallow corals died in parts of the Maldives, Sri Lanka, Kenya and the Seychelles, with heavy losses in Lakshadweep and the Andamans.
  • Damage was graded catastrophic, severe (about 50–70% mortality), moderate (with quick recovery) and insignificant.
  • Climate change, not local damage, had become the main threat, and bleaching would grow more frequent.

Bleaching in India

• Indian reefs bleached in 1998, 2010, 2016 and 2024, in step with the global events. In 1998, warming of about 2 °C killed many corals in the island groups and the Gulf of Mannar.
• Lakshadweep, 2024: a marine heatwave from late October 2023, more than 1 °C above normal, caused severe bleaching.
  • Live coral cover on three atolls had already fallen from about 37% (1998) to 19% (2022). The 2010 event was the most severe.
• Gulf of Mannar, 2024: reefs were severely bleached, while Palk Bay fared better.
• Local stresses add to heat: siltation in the Gulf of Kachchh, past coral mining in the Gulf of Mannar, sewage, trawling, and the 2004 uplift and subsidence of Andaman and Nicobar reefs.

Importance, Threats and Conservation

Importance of coral reefs

• Biodiversity: the richest marine ecosystem, home to fish, molluscs, turtles and dugongs (Gulf of Mannar, Andamans).
• Food and livelihoods for hundreds of millions of tropical coastal people.
• Coastal protection: a healthy reef absorbs up to about 97% of wave energy.
• Land itself: reefs build atoll nations such as the Maldives and Tuvalu, and islands such as Lakshadweep.
• Tourism, medicines, climate archives in growth bands, and long-term carbon storage in calcareous deposits.

Threats

• Global: warming and marine heatwaves, acidification, stronger cyclones, and sea-level rise faster than reef growth.
• Local: sediment and nutrient runoff, overfishing and dynamite or cyanide fishing, coral mining, marine pollution, crown-of-thorns starfish outbreaks, coral diseases, tourism and coastal construction.
• Stresses compound: a reef weakened by silt and overfishing recovers slowly from bleaching and is more likely to shift to algae.

Conservation

• All reef-building corals are protected under Schedule I of the Wildlife (Protection) Act, 1972; reefs are ecologically sensitive areas under the Coastal Regulation Zone rules.
• Restoration in India:
  • In the Gulf of Mannar, more than 51,000 fragments of 20 native species have been transplanted onto concrete frames since 2002, raising coral cover severalfold.
  • The Zoological Survey of India, with the Gujarat Forest Department, restored branching Acropora in the Gulf of Kachchh (2015) and has trialled biorock since 2020.
  • The National Institute of Oceanography translocated corals from Mumbai’s coastal-road site in 2020.