Maarg Manthan · Topic 12.2

Conditions for Coral Growth and Distribution of Coral Reefs

Geography › Coral Reefs, Ocean Resources and Law of the Sea · Topic 12.2

Reef-building corals need warm water of about 23 to 29 degrees Celsius, clear, shallow water less than about 50 m deep with plenty of sunlight, normal salinity of about 27 to 40 parts per thousand, little sediment, and a hard base to attach to. These conditions confine reefs to the tropics, mainly between 30 degrees north and south, on the western sides of oceans and away from river mouths.

5 min read · UPSC Prelims · Geography · Coral Reefs, Ocean Resources and Law of the Sea

At a Glance

Temperature About 23 to 29 degrees Celsius
Depth Mostly less than 50 m
Salinity About 27 to 40 parts per thousand
Latitude Mostly 30 degrees north to 30 degrees south
Hotspot Coral Triangle of Southeast Asia
Exam link UPSC Prelims, Geography: Coral Reefs, Ocean Resources and Law of the Sea

Where Does It Fit?

This is the second topic of the category on coral reefs, ocean resources and the law of the sea. The previous topic described the types of reefs, and this one explains the conditions that limit where they can grow. The next topic covers coral bleaching. Previous topic: Coral Reef Types and Their Development. Next topic: Coral Bleaching and Zooxanthellae.

What Conditions Do Corals Need?

Condition	Requirement	Reason
Temperature	About 23 to 29 degrees Celsius; they can tolerate roughly 18 to 34; growth stops below 18	Sea surface temperature is the key control: warm water helps the polyps to build limestone skeletons and keeps the algal partners active; this is why reefs are tropical, as in Temperature Distribution of Oceans
Depth and light	Shallow water, mostly less than 50 m and usually under about 70 m	The algae living in the polyps need sunlight for photosynthesis, as in Coral Bleaching and Zooxanthellae
Salinity	Normal sea water, about 27 to 40 parts per thousand	Fresh water kills corals; very salty water also stresses them, as in Ocean Salinity
Clear water	Low sediment and turbidity	Silt blocks the light and smothers the polyps, so reefs do not grow near the muddy mouths of rivers
Hard base	A rocky or an old reef surface to attach to, such as a volcanic island or continental shelf	Larvae cannot settle on soft mud or sand
Moving, oxygenated water	Waves and currents bring food and oxygen and remove waste	The outer edge of a reef, facing the waves, grows fastest

Cold-water corals are an exception. They live in dark deep water, down to 2,000 m or more, at low temperatures, and feed on plankton, without algal partners. They build reefs off Norway and in the deep Atlantic, but grow slowly.

Where Are Coral Reefs Found, and Where Not?

Between 30 degrees north and south: most reefs lie in the tropical belt, because of the warm surface water.
On the eastern coasts of continents: warm currents, such as the East Australian and Brazil currents, keep the water warm. Bermuda has reefs at 32 degrees north because of the Gulf Stream, and the Leeuwin Current allows coral growth far south on the west coast of Australia, as in Pacific and Atlantic Ocean Currents.
Not on the western coasts of continents in the tropics: the cold Peru, Benguela and Canary currents, with upwelling, lower the water temperature, so there are almost no reefs on the west coasts of South America and Africa.
Not at large river mouths: the Amazon and the Ganga-Brahmaputra deliver fresh water and silt, so reefs are absent off the delta coasts, such as the northeast coast of South America and Bangladesh.
Not in the cold seas: the poles and mid-latitude seas are too cold.

Region	Notes
Indo-Pacific	Contains the great majority of the world’s reefs: the Great Barrier Reef, the Coral Triangle, the Pacific islands, the Maldives and Lakshadweep, the Andaman Sea and East Africa
Coral Triangle	Indonesia, the Philippines, Malaysia, Papua New Guinea, Timor-Leste and the Solomon Islands: the centre of marine biodiversity, with the largest number of coral species
Red Sea	Heat-tolerant corals in a warm, salty sea, now studied as a refuge
Caribbean and Atlantic	Reefs of the Caribbean and Florida, Belize and Bermuda; fewer species than the Indo-Pacific
India	Gulf of Kachchh, Gulf of Mannar and Palk Bay, Andaman and Nicobar Islands, Lakshadweep

What Threatens the Distribution of Reefs Today?

The world’s reefs cover about a quarter of a million square kilometres, and their condition is deteriorating. Warming oceans cause mass bleaching, the acidification of the sea reduces the ability of corals to build skeletons, and coastal development, dredging, overfishing, pollution and runoff damage them directly. Warmer water means that the suitable zone for corals may move toward the poles, but the speed of change is far greater than the growth of corals, and ocean acidity and light limits will also apply at higher latitudes. As an analytical point, the requirements show why reefs are uniquely fragile: they live near the upper limit of their temperature range, so a rise of only one or two degrees in the sea temperature can push them beyond their tolerance, which is the focus of the next topic.

Why Is This Topic Important?

Concepts Conditions asked in Prelims
Map-based Where reefs are found
Threat Narrow tolerance

What Came Next?

The next topic explains coral bleaching and the role of the algae that live in corals. Next topic: Coral Bleaching and Zooxanthellae.

Key People and Terms

Turbidity Cloudiness of water from suspended particles
Hard substrate Solid surface for attachment
Coral Triangle Region of greatest coral diversity
Cold-water coral Deep coral without algal partners
Ocean acidification Fall in pH of sea water
Indo-Pacific Region with most of the world’s reefs

Exam Corner

Points to Remember

Temperature should be 23 to 29 degrees Celsius.
Depth is under about 50 m.
No reefs at river mouths.
No reefs on west coasts with cold currents.
The Coral Triangle has the most species.

Do Not Confuse With

Warm-water and cold-water corals Light and algae versus dark and plankton
Eastern and western coasts Warm current versus cold current
Salinity and turbidity Salt content versus cloudiness

Memory Hook

Warm, shallow, clear, salty and hard: that is a reef; river mouths and cold currents say no.

Mains Angle

Questions ask why reefs are found in some places only. These points add depth.

Conditions Narrow tolerance
Pattern East coasts and Indo-Pacific
Threat Warming and acidification

A Question You May Face

An original practice question, not a past paper question.

“Explain the conditions necessary for the growth of coral reefs and account for their absence along the west coasts of Africa and South America.”

How to Answer

Conditions Temperature, depth, salinity.
Pattern Tropical east coasts.
Absence Cold currents and rivers.
Threat Warming.

GS Relevance

Prelims: conditions for coral growth, where reefs are absent and why, Coral Triangle. GS Paper 1: oceanography and distribution of reefs.

Frequently Asked Questions

What conditions are needed for coral growth?

Reef-building corals need warm water of about 23 to 29 degrees Celsius, shallow depth of less than about 50 metres for sunlight, normal salinity, clear water with little silt, a hard surface to attach to, and moving water that brings oxygen and food.

Why are coral reefs found only in the tropics?

Reef-building corals need warm sea water, usually above 18 degrees Celsius and best between 23 and 29. Only the tropical seas between about 30 degrees north and south stay this warm throughout the year, so reefs are mostly confined to this belt, with exceptions where warm currents extend the range.

Why are there no coral reefs at the mouths of large rivers?

River mouths bring fresh water and large amounts of silt into the sea. Fresh water lowers the salinity below what corals can tolerate, and silt makes the water cloudy, blocking sunlight needed by the algae in corals, so reefs are absent near rivers such as the Amazon and the Ganga-Brahmaputra.

Why are there few coral reefs on the western coasts of continents?

The western coasts of the tropical continents have cold currents such as the Peru, Benguela and Canary currents, together with upwelling of cold water. The low sea temperature is below what reef-building corals need, so reefs are mostly found on warmer eastern coasts.

What is the Coral Triangle?

The Coral Triangle is the marine region around Indonesia, the Philippines, Malaysia, Papua New Guinea, Timor-Leste and the Solomon Islands. It has the highest diversity of corals and reef fish in the world, and supports the livelihoods of millions of people.

PYQ Practice — Statement Analysis

1 Coral reefs grow best in water deeper than 200 m.

False

They need shallow, sunlit water.

2 Reef-building corals need warm water of about 23 to 29 degrees Celsius.

True

They grow slowly below 18 degrees.

3 Coral reefs are abundant at the mouth of the Amazon.