Revision summary
Corals bleach when heat forces them to expel zooxanthellae. The Great Barrier Reef and many Indian Ocean reefs have had repeated mass bleaching since 1998. Ocean acidification thins aragonite skeletons and raises breakage. Storms, sediment, and disease follow heat and stop recovery. Indian examples include Lakshadweep, Gulf of Mannar, and the Andaman reefs.
Model answer
Introduction
Corals live near an upper heat limit and depend on zooxanthellae algae for food and colour. Global warming raises sea-surface temperature and also shifts chemistry and storms, so the coral life system bleaches, sickens, and can collapse into algae-dominated rubble.
Body
Heat stress and bleaching
- When water stays about 1°C above the local summer maximum for weeks, corals expel zooxanthellae, turn white, and starve if the heat does not break.
- Mass bleaching hit the Great Barrier Reef in 1998, 2002, 2016, 2017, and later years, killing large stretches of shallow branching coral.
- El Niño years raise heat in the central and eastern Pacific and the Indian Ocean, which bleached reefs in the Maldives, Seychelles, Lakshadweep, and parts of Indonesia.
- India’s Gulf of Mannar, Andaman and Nicobar reefs, and Kachchh–Okha patches have recorded bleaching when the summer monsoon heat stays high.
Acidification, storms, and ecology
- Extra carbon dioxide makes seawater more acidic and lowers aragonite saturation, so skeletons of Acropora and other reef builders grow thinner and break more easily.
- Warmer seas feed stronger cyclones and wetter floods, which smash reefs and dump sediment and nutrients that let macroalgae take the space of coral.
- Disease and outbreaks such as crown-of-thorns starfish worsen on stressed reefs, which cuts fish nursery habitat and coastal protein.
- Some deeper or upwelling reefs bleach less, but they cannot replace the lost shallow system that protects islands and feeds artisanal fisheries.
Scale of reef damage
- The impact is not a slow even fade; it is repeated mortality pulses after marine heatwaves, with only partial recovery.
- Local sewage and overfishing make warming deadlier, so the coral life system fails first where heat and human pressure meet.
Flow diagram
flowchart TD W[Warmer seas] --> B[Bleaching starvation] C[CO2 acidification] --> S[Weaker skeletons] B --> L[Lost reef habitat] S --> L E[Examples GBR Maldives Mannar] --> L
Conclusion
Global warming is the main new driver of coral bleaching, weaker skeletons, and lost reef fisheries. Cutting greenhouse gases is the core remedy; local protection of the Gulf of Mannar, Lakshadweep, and similar reefs can only buy time.
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Discuss the causes of depletion of mangroves and explain their importance in maintaining coastal ecology.
Next question in the 2019 paper (Q5). View answer →
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Is bleaching always death?
Not at once. If heat falls quickly, some corals regain algae. Repeated or long heatwaves cause mass death.
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Does only temperature matter?
Heat is the main trigger. Acidification, storms, sewage, and overfishing decide whether the reef can recover.
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