Oceanography: Coral reefs

Coral reefs

Corals are tiny animals (polyps) that build skeletons of calcium carbonate. They live in partnership with algae called zooxanthellae inside their tissues: the algae make food by photosynthesis and give the coral its colour; the coral gives them shelter. Over thousands of years, the skeletons pile up into reefs.

Conditions corals need: warm water (about 20 °C or more, ideally 23–29 °C), so they are found mainly between 30° N and 30° S; clear, shallow, sunlit water (usually less than about 50 m); normal salinity; and little mud, which is why there are no reefs at the mouths of big rivers such as the Ganga or the Amazon, and few on the west coasts of continents, where cold currents flow.

India has four main reef areas: the Gulf of Mannar, the Gulf of Kutch, the Andaman and Nicobar Islands, and Lakshadweep. Prelims 2018 tested three statements: Most of the world’s coral reefs are in tropical

waters; more than one-third are in the territories of Australia, Indonesia and Philippines; and Coral reefs host far more number of animal phyla than those hosted by tropical rainforests. All three are correct (option d). Reefs cover well under 1% of the sea floor but shelter about a quarter of all marine species.

When the water stays too warm for weeks, corals expel their algae and turn white: coral bleaching. They can recover if the heat passes; if not, they starve and die. Mains 2019 asked to assess the impact of

global warming on the coral life system with examples: mass bleaching of the Great Barrier Reef (2016, 2017, 2020, 2022, 2024), Lakshadweep and the Gulf of Mannar; acidification weakening skeletons; sea-level rise outpacing growth; stronger storms breaking reefs; and the loss of fisheries, tourism and coastal protection that follows. Prelims 2012 began: The acidification of oceans is increasing. Why is this phenomenon a cause of concern? It harms calcareous phytoplankton, coral reefs, and animals with phytoplanktonic larvae (option a: 1, 2 and 3 only), but not cloud seeding.

Marine resources, the law of the sea and ocean

pollution Mains 2014 asked: Critically evaluate the various resources of the oceans which can be harnessed to meet the resource crisis in the world. Biotic: fish (a key protein source for over three billion people), seaweed and medicines. Mineral: oil and gas on the shelves; polymetallic nodules rich in manganese, nickel, cobalt and copper on the deep floor; sand and placer deposits (monazite on the Kerala coast). Energy: tidal, wave, offshore wind and ocean thermal energy. Water: desalination. The critical part: overfishing, the high cost and ecological damage of deep-sea mining, and unclear rules beyond national waters.

India is one of the pioneer investors with the International Seabed Authority, holding a contract to explore polymetallic nodules over 75,000 sq km in the Central Indian Ocean Basin. The Deep Ocean Mission (approved in 2021) includes Samudrayaan, which aims to take three people to 6,000 m in the submersible Matsya 6000, and work on deep-sea mining and ocean climate services.

The UN Convention on the Law of the Sea (UNCLOS, 1982) sets these zones. Prelims 2022 tested three statements: a coastal state has the right to establish the breadth of its territorial sea up to a limit not exceeding 12 nautical miles; ships of all states enjoy innocent passage through it; and the Exclusive Economic Zone shall not extend beyond 200 nautical miles from the baseline. All three are correct (option d). In the contiguous zone (up to 24 nm) a state can enforce customs, tax, immigration

and sanitary laws; in the EEZ it controls fishing and seabed resources but other ships may pass freely. India’s EEZ is about 2 million sq km.

The ocean also gives us oxygen. Prelims 2025 tested the statement that Marine phytoplankton and photosynthetic bacteria produce about 50% of world’s oxygen: correct, and the only correct one (option b); rainforests do not produce more than the oceans, and surface water holds far less oxygen than air. Prelims 2012 asked what would happen if phytoplankton of an ocean is completely destroyed: the ocean as a carbon sink and its food chains would suffer (option a: 1 and 2 only); the density of water would not change.

Mains 2018 asked: What are the consequences of spreading of ‘Dead Zones’ on marine ecosystem? Mains 2018 Dead zones are areas with too little oxygen (hypoxia) for most marine life, mostly at river mouths where fertiliser and sewage cause algal blooms whose decay uses up the oxygen: the Gulf of Mexico and the Baltic are famous; the Arabian Sea has one of the largest natural low-oxygen zones. Consequences: fish kills or flight, loss of bottom-dwelling life, collapse of fisheries and release of greenhouse gases such as nitrous oxide. Mains 2023 (GS III) asked: What is oil pollution? What are its impacts on the marine ecosystem? Mains 2023 Oil from tanker spills, offshore wells and ship discharge forms a film that blocks light and oxygen, coats birds and mammals, poisons fish larvae and smothers mangroves and corals; it hurts India especially because of its long coastline, fishing communities and heavy tanker traffic.

Mains 2025 asked: How are climate change and the sea level rise affecting the very existence of many island nations? Mains 2025 Global sea level has risen about 20 cm since 1900 and the rise is speeding up. Low atolls such as Tuvalu, Kiribati, the Marshall Islands and the Maldives, mostly only 1–2 m above the sea, face flooding, salty groundwater, coastal erosion and dying reefs. Tuvalu’s 2023 Falepili Union treaty with Australia offers its citizens a path to move; the Maldives is building raised land at Hulhumalé; and Vanuatu led the push for the 2025 International Court of Justice advisory opinion on states’ climate obligations.