Coral Reefs

Notes

Corals are tiny marine invertebrates (polyps, phylum Cnidaria) that secrete calcium carbonate skeletons. Reef-building corals live in a symbiotic relationship with zooxanthellae algae, which give them colour and food through photosynthesis.

Conditions for growth

Warm water, about 20-25°C (not below about 18°C), so reefs lie between about 30°N and 30°S.
Shallow water (less than about 50-70 m) with plenty of sunlight.
Clear, saline water (27-40 per thousand); reefs are absent at river mouths (freshwater, silt).
Moving water for food and oxygen. Reefs are found on the western sides of oceans (warm currents) and are absent on the western coasts of South America and Africa (cold currents).

Types of reefs

Fringing reef: grows attached to the coast, with a shallow lagoon (Red Sea; Andaman).
Barrier reef: separated from the land by a wide, deep lagoon; the Great Barrier Reef (Australia, about 2,300 km, the largest) and the Belize barrier reef.
Atoll: a ring-shaped reef enclosing a lagoon, built on a sinking volcano: Maldives, Lakshadweep, the Marshall Islands (Bikini).

Theories of origin: Darwin's subsidence theory (1842): a fringing reef becomes a barrier reef and then an atoll as the island sinks. Others: Daly's glacial-control theory and Murray's theory.

Importance: home to about a quarter of marine species ("rainforests of the sea"), coastal protection from waves and storms, fisheries, tourism, medicines and carbon cycling.

Threats

Coral bleaching: warm water makes corals expel their zooxanthellae, turning them white. Mass bleaching events occurred in 1998, 2010, 2016-17 and 2023-25 (the fourth global event).
Ocean acidification, pollution and sedimentation, overfishing and destructive fishing (dynamite, cyanide), crown-of-thorns starfish, unsustainable tourism, and cyclones.

India: four main reef areas: the Gulf of Kutch, the Gulf of Mannar (a Marine National Park with 21 islands), the Andaman and Nicobar Islands and Lakshadweep. Protection comes through marine protected areas and the International Coral Reef Initiative (ICRI, 1994). The Great Barrier Reef became a UNESCO World Heritage Site in 1981.

Key Points

Corals need warm (20-25°C), shallow, clear, saline water; between about 30°N and 30°S.
Types: fringing, barrier (Great Barrier Reef), atoll (Lakshadweep, Maldives).
Darwin's subsidence theory explains the sequence.
Bleaching is due to rising sea temperatures.
India's reefs: Kutch, Mannar, Andaman and Nicobar, Lakshadweep.

MCQs

Q1. Coral reefs grow best in water temperatures of about:

(a) 5-10°C
(b) 20-25°C
(c) 35-40°C
(d) Below 0°C
Ans: (b).

Q2. The Lakshadweep islands are examples of:

(a) Barrier reefs
(b) Fringing reefs
(c) Atolls
(d) Volcanic cones
Ans: (c).

Q3. Coral bleaching is mainly caused by:

(a) Cold water
(b) Rising sea temperature
(c) Lack of waves
(d) Freshwater only
Ans: (b).