Coral reefs

Q. Give an account of recent observations on Coral bleaching with reference to Clive Wilkinson’s report.(15/14) Q. Compare the subsidence and glacial control theories on the formation of coral reefs.(25/13) Q.”Atolls present the most challenging explanation in the evolution of coral reefs”. Discuss (20/12) Q.Describe the ideal conditions for coral reef formation and discuss the glacial Control theory of coral reef formation.(60/08 ) Q. Write short note:formation of coral reefs. (20/01)

Coral reefs and atolls are submarine features formed due to accumulation and compaction of skeletons of lime secreting organisms known as coral polyps.Coral reefs are also known as rainforest of the oceans as they are more diverse than the tropical rainforests.

CONDITIONS FOR GROWTH

1. Depth: Grow in shallow waters; .cannot grow  deeper than 62-77 m below sea level as they will not get sufficient amount of Sunlight and oxygen.
2. Temperature: They require high average annual temperature (20 °C - 21 °C) and, thus, are found mainly in the tropical Oceans and seas.
3. Clean sediment free water because muddy water clogs the mouth of Coral polyps resulting into their death.
4. Absence of freshwater: Coral avoids fresh water as it is in injurious for their growth.That's, why they live away from coastal lands and river mouths.
5. Moderate salinity:  Salinity ranging between 27 g/kg and 30g/kg is most ideal for the growth of Coral polyps.Higher oceanic salinity is injurious for the growth of Coral polyps because such water contains little amount of calcium carbonate.
6. Ocean currents and waves: are favourable for Coral because they bring necessary food supply for the polyps.So, they grow in open Seas and oceans but they die in lagoons and small enclosed seas because of lack of supply of food.
7. Submarine platform:  must be Shallow and extensive.
8. Absence of human activities like Industrial effluents, urban waste, overfishing, clearance of maritime forest, etc.

TYPES OF CORAL REEFS

Fringing Reef

• These are developed along the continental margins or along the Islands
• Seaward slope is steep and vertical while the landward slope is gentle.
• Upper surface is uneven and corrugated
• Usually attached to the coastal land but sometimes there is gap between them and land in which Lagoon is formed.Such lagoon is known as boat channel.

Barrier Reef

• These are the largest,most extensive, highest and widest reefs of all types of coral reefs.
• There is extensive but Shallow lagoon between the coastal land and Barrier Reef.
• Great Barrier Reef located parallel to the east coast of australia is the largest of all the barrier reefs of the world.

Atoll

• is a ring-shaped coral reef including a coral rim that encircles a lagoon partially or completely.
• Atolls are divided into three types:true atoll,Island atoll and atoll Island

SUBSIDENCE THEORY OF DARWIN

Charles Darwin postulated his subsidence theory first in 1837 and modified it in the year 1842 during his voyage on the Beagle

Postulates

• Coral polyps could grow only in Shallow oceanic waters
• Presence of coral reefs at greater height indicates gradual subsidence of land or Island
• Gradual and slower rate of land subsidence than the rate of upward growth of corals
• Depth of lagoons does not increase in spite of gradual subsidence of the land because there is continuous sedimentation in the lagoon.
• Fringing Reef, Barrier Reef and Atolls are successive stages of the development of coral reefs.
• Following are three stages of formation of coral reefs

1. Fringing reef stage:

• First of all Coral polyps flock together along a suitable submarine platform and grow upward and ultimately reach sea level and fringing Reef is formed.Fringing Reef is formed in the stable condition of the land

2. Barrier reef stage:

• Due to tectonic forces subsidence of land occurs.
• Coral polyps are unable to grow at this depth.So, they grow upward and outward at much faster rate so that they can get food for the survival.
• Growth of polyps is retarded near the Shore but it is very vigorous at the outer edge of the land because of which Lagoon is formed between the coast and fringing Reef
• This results in formation of Barrier Reef

3. Atoll stage

• There is further subsidence of the land and the island is completely submerged under water and a ring of coral reef in the form of a atoll is formed

Evidences

• Shallowness of lagoons indicates gradual subsidence of land if the land is taken to be stable the lagoons would be filled due to deposition of sediments
• Absence of cliffs along the Coral islands validates the idea of subsidence.
• The posts and the islands of the Pacific Ocean having a raised beaches are devoid of barrier and atoll Reefs.
• Islands having atolls are characterized by very steep slopes
• thickness of coral reefs increases downward which indicates coral reefs are formed along subsiding base of submarine platform.

Criticism

• Fringing Reef and Barrier Reef have been found on idle side of the same Island at the same level which define Darwin's idea of stages
• Its subsidence theory is accepted, then most of the islands of the Pacific Ocean would be submerged
• evidences of existence of coral reefs associated with the emergent island

STAND STILL THEORY OF MURRAY

Murray propounded his theory in 1880 on the basis of the information received during the Challenger expedition.

Postulates

• No subsidence.Sea level and submarine platforms are stable.
• Coral polyps cannot survive below 30 fathoms
• Several submarine platforms volcanic islands are present below sea level
• Ideal depth of submarine platform for coral growth is reached either through wave erosion or through deposition of marine sediments.
• Once ideal height is attained, coral polyps starts to grow upward to form fringing reef.
• Coral polyps after some time also grow outward on the foundation of their own debris. This continuously outward growing fringing Reef is transformed into Barrier Reef.
• Lagoon is formed between land and Barrier Reef because of the solution of dead corals.
• Atolls are formed due to outward growth of corals in all direction.Lagoon ward side of coral is characterised by dead corals, while the seaward side has living coral.
• Lagoon also becomes shallower because of the deposition of dissolved dead corals.

Criticism

• Murray theory required the existence of numerous suitable submarine platform at the depth of 180 feet which is not possible.
• Murray describe two contradictory views of marine erosion and deposition at the depth of 180 feet at the same time.
• Mechanism of Lagoon formation due to dissolution of dead corals is doubtful as pelagic deposits do not get dissolved.
• Due to stable submarine platforms lagoon would be completely filled up with marine sediments.
• If submarine platforms are stable than lagoon would be completely filled up with marine sediments.
• Contrary to Murray's assumption coral reef has been found below 180 feet.

GLACIAL CONTROL THEORY OF DALY

Daly propounded his theory of coral formation in the year 1915 after he was convinced that coral reefs were formed after pleistocene Ice Age.

Postulates

• Sea level fell by 33 to 38 fathoms due to glaciation during pleistocene Ice Age.
• Existing corals died due to lowering of temperature of marine water.
• Wave cut platform were formed along Continental coast and islands due to wave erosion.
• After the end of Ice Age the sea level again rose and the wave cut platform got submerged under seawater up to depth of 33 to 38 fathoms.
• Coral which could survive during glacial period and new Coral police begin to grow and established their colonies on the seaward edges of submerged platforms.
• Thus, fringing reefs were formed on narrow wave cut platform while barrier reefs were formed on Broad platforms Atolls was formed around isolated wave eroded Island peaks.
• Lagoons of uniform depth was formed between the reef and the land because of uniform lowering of sea level due to glaciation.

Criticism

• According to W M Davis depth of different Lagoon varies considerably.
• Rare presence of cliffs found by wave erosion.
• Presence of islands between coast and coral reefs

CONCEPT OF W.M.DAVIES

Davies revived subsidence theory in early 20th century

Some Physiographic evidences

• presence of indented and embayed coastlines found in coral seas.
• flatness of the bottoms of lagoons and the uniform depth are not due to uniform lowering of sea level and these are not true bottom.They are due to deposition of marine sediments and without subsidence sediments will fill the lagoon.
• Rock arrangement in the Lagoon suggested the existence of a peak.
• He suggested isostatic adjustment as mechanism for subsidence.

Coral bleaching

It refers to loss of algae from the corals resulting into the white colour which is indicative of death of corals.

CLIVE WILKINSON REPORT(1998)

• Coral bleaching resulted in  70% death of Corals off the coast of Kenya,Maldives, Andaman and Lakshadweep Islands in the Indian Ocean and 75% death off the seychelles and Tanzania coasts.
• Clive Wilkinson identified for overlapping levels of Coral bleaching:

1. catastrophic bleaching: adversely affecting 95% of corals like in Maldives,Tanzania
2. Severe bleaching:  accounting for 50-70% death of corals like in Kenya,Seychelles.
3. Moderate bleaching :resulting into 20-50% Coral mortality but quick recovery.
4. Insignificant bleaching or no bleaching.

CAUSE

• Global warming

• El-Nino Phenomenon resulting in warming of ocean
• Outbreak of Coral diseases like black band disease, Coral plague, aspergillosis etc.
• Increasing siltation of seawater due to mass flux of sediments and nutrients brought by river streams consequent upon land use changes.
• pollution of seawater caused by Industrial effluents, urban sewage and oil slick
• destructive fishing practices and Overfishing
• clearing of marine forest around coral reefs
• mining of Coral rocks for building materials
• collection of rare Coral species