12 Coral Reefs and Coral Bleaching
Saleha Jamal
Structure
1.1 Introduction: Coral and Coral Reefs
1.2 Coral Reefs Behaviour, Diet and Range
1.3 Types of Coral Reefs
1.4 Theories Regarding the Formation of Coral Reefs
1.5 Importance of Coral Reefs
1.6 Threats to Coral Reef
1.7 Coral Bleaching
1.8 Causes of Coral Bleaching
1.9 Events of Mass Coral Bleaching
1.10 Coral Reefs in India
1.11 Status of Coral Reef Research in India
1.12 Conservation and Protection of Coral Reefs in India
Objectives:
At the end of this module the student should be able to:
- Explain the definition coral reef
- Recognize different types of coral reefs
- Explain different theories regarding the formation of coral reefs
- Aware of importance of coral reefs and threats to coral reef
- Have clear understanding coral bleaching and its causes
- Describe the distribution, status, conservation and protection of coral reefs in India
1.1 Introduction: Coral and Coral Reefs
Coral is a kind of fleshy anemone which is green, yellow, pink, violet and white in colour. An individual coral is known as a polyp, a very small and simple organism consisting mostly of a stomach topped by a tentacle-bearing mouth. The polyps extend their tentacles at night to sting and ingest tiny organisms called plankton and other small creatures. Each polyp lives in a symbiotic relationship with host Zooxanthellae that gives the coral its color. Zooxanthellae take in carbon dioxide, process it through photosynthesis, and give off oxygen and other important nutrients that are then used by the host polyp. As in all photosynthesizing organisms, this means that corals must be exposed to a sufficient amount of sunlight. This confines most corals to shallow waters that are clean and clear. These coral polyps live in group in the form of colony and forms calcareous shells around them. Coral reefs are formed when the polyps die and their skeleton accumulates over one another along a submarine platform at suitable depth. As the coral die, the shell remain deposited and new corals attach their shells remain deposited and new coral attach their shells to the existing deposits. This cycle of dying and deposition repeated for over millions of years leading to accumulation of layers of corals (shallow rock created by these depositions is called reef). Coral reefs over periods of time transform or evolve into coral island (Lakshadweep).
Coral Reefs are significant submarine features of limestone and dolomite, accumulated by lime secreting organisms known as Coral polyps. Coral reefs are one of the most biologically diverse ecosystems on earth, rivaled only by tropical rain forests. They are made up not only of hard and soft corals, but also sponges, crustaceans, mollusks, fish, sea turtles, sharks, dolphins and much more. The health, abundance and diversity of the organisms that make up a coral reef are directly linked to the surrounding terrestrial and marine environments.
1.2 Coral Reefs Behaviour, Diet and Range
Coral polyps are mostly found in the tropical oceans between Tropic of Cancer and tropic of Capricorn. Their tolerance level to adverse condition is quite low. The average temperature required for their growth is 20°C (18°C to 30°C) and they cannot survive at a depth where light penetration is not enough. Reefs only occur in shallow areas that are reachable by sunlight because of the relationship between coral and algae. If the water is colder the reefs are poorly developed or nonexistent. Various types of microscopic algae, known as Symbiodinium, live inside of the coral, providing them with food and helping them to grow faster. In many ways, reef-building corals are animals that act like plants they stay in one place and get some of their energy from the sun. More precisely, coral reefs are found in shallow areas of depth of less than 150 feet. However some coral reefs extend even deeper upto about 450 feet deep. Both saline and fresh water are injurious to corals thus they grow in areas of moderate salinity away from the mouth of large rivers bringing large amount of fresh water. The water should be clean and sediment free because muddy water or turbid water clogs the mouth of corals polyps resulting into their death. Very high saline water is also injurious for the growth of corals because such water contains only little amount of calcium carbonates whereas lime is an important food for coral polyps. The oceanic salinity ranging between 27°/oo and 30°/oo is most ideal for the growth and development of coral polyps.
1.3 Types of Coral Reefs
Fringing Reef: fringing reefs are reefs that grow directly from a shore. They are located very close to land and often from a shallow lagoon between the beach and the main body of the reef (Fig. 1). A fringing reef runs a narrow belt (1 to 2 km wide).
This type of reef grows from the deep sea bottom with the seaward side sloping into the deep sea. This reef is the most common reef among all the reefs.
Fig. 1 Fringing Reef
Barrier Reef: the barrier reefs are like fringing reefs but they are situated in the sea nearly 1 km to 15 km away from the shore (Fig.2). Therefore, navigable channel called lagoon separates these reefs from the shore. The lagoon may be 20 to 40 fathoms deep, hence, it becomes navigable. The Great Barrier Reef on the north-east coast of Australia is 1,200 miles long, about 20-70 miles wide and situated nearly 90 miles away from the shore.
Images originally from USGS (http://pubs.usgs.gov/fs/2002/fs025-02/)
Fig. 2
Atolls: atolls are rings of reef, often encircling an island (sand and coral rubble). They typically have a shallow, sandy, sheltered lagoon in the middle. Access to the open sea beyond is through a number of channels. These provide fresh and colder water for the lagoons. Corals atolls are on the top of submarine mountains. These mountains are remnants of volcanoes. Once there were fringing reefs around the volcano. As it slowly submerged the corals continued to grow up to the surface of the water. What remained after the volcano became invisible is a ring of coral reefs surrounded by deep ocean.
Images originally from USGS (http://pubs.usgs.gov/fs/2002/fs025-02/)
Fig. 3
1.4 Theories Regarding the Formation of Coral Reefs
Reefs grow seaward but the vertical growth is limited and depends upon the availability of light and the depth of the water. Some samples collected during the modern time shows that the thickness of the coral reef is from 6 meters to more than 1000 meters. The lower parts of many platforms are located below the photic zone and this can be explained by various theories.
Darwin’s Subsidence Theory: Darwin was of the opinion that on the platform on which the coral grows was unstable and gradually sinking and subsiding down. As the platform goes downward, coral finds it tough to be in deeper water thus they started to grow upward and outward balancing the subsidence of the land. As the land subsides the fringing reef would grow upward and outward resulting into the formation of a shallow lagoon. Further subsidence leads to the formation of barrier reef and as the rapid outward growth of reefs and coral debris are deposited, so width of the reef is increased. Finally there is a complete disappearance of land by subsidence and the barrier reef transform into a atoll (see figure 4).
Fig.4
Daly’s Glacial-Control Theory: This theory states that during the last glacial period the formation of ice caps lowered the ocean level by 60 to 70 metres below the present surface. Waves cut the shores to make flat platforms suitable for growth of corals. As the ice caps melted and temperature rose, corals began to grow on these platforms and rose upwards with rising ocean level, and all types of reefs were formed on the pre-existing platforms. There is evidence that coral reefs are growing today on submerged land and the foundations of reefs are now at a much greater depth than they were when corals first began to grow.
Plate Tectonics and Coral Reef Formation: the subsidence theory given by Darwin is supported by modern evidences based on the concept of plate tectonics. The corals grow upward with a rate of 1.8 meter/1000 years easily keeping pace with sea level changes. The subsidence theory also deals well with the modern idea of sea floor spreading and subsidence away from the oceanic rises. As a seamount is formed and moves away from hot spot with the plate and subsides down. The wave action converts the sea mount into a flat topped guyot. Thus reef building starts upward on the guyot which keeps pace with the subsidence. Thus gradually fringing reefs are converted to barrier reef and atoll with complete subsidence of the island.
1.5 Importance of Coral Reefs
In addition to being home to countless marine animals, coral reefs are very important ecosystems for coastal peoples. They are often the first line of defense against strong tropical storms for coastal communities, and at least 400 million people rely on coral reef fisheries for income and food. Coral reefs are vulnerable ecosystems that harbour 25% of all marine species and provide goods and services worth $375 billion annually. Furthermore, high value tourism in many places relies on healthy, intact coral reefs to attract visitors to remote parts of the world. These services, and others, combine to make coral reefs extremely valuable to nearby communities. Unfortunately, coral reefs face numerous threats to their continued survival. Destructive fishing practices, pollution, and invasive species threaten local coral reefs in populated areas. Climate change and ocean acidification threaten all coral reefs around the world. Without careful management of human activities and an active reversal of global threats, entire coral reefs may be lost. That loss would risk the million species and hundreds of millions of people that rely on coral reefs’ existence for their survival.
1.6 Threats to Coral Reef
Coral reef ecosystems world-wide have been subject to unprecedented degradation over the past few decades. Despite the high natural, cultural and economic value of these ecosystems, human activities are a major threat to the persistence of coral reefs. Anthropogenic pressures, such as climate change, tourism, fishing, coastal development, eutrophication and sedimentation, are increasingly linked to the severe degradation of coral reefs worldwide as they can significantly affect the health of the coral animal and its microbial symbionts. Natural disturbances which cause damage to coral reefs include violent storms, flooding, high and low temperature extremes, El Nino Southern Oscillation (ENSO) events, sub-aerial exposures and predatory outbreaks.
Climate Change: climate change leads to global warming and global warming leads to coral bleaching and it has been predicted that the intensity and severity of coral bleaching will increase in coming future.
Fishing: fishing processes like cyanide fishing, blast or dynamite fishing, bottom trawling and muroami (banging on the reef with sticks).
Careless tourism: Careless boating, diving, snorkeling, and fishing happens around the world, with people touching reefs, stirring up sediment, collecting coral, and dropping anchors on reefs. Some tourist resorts and infrastructure have been built directly on top of reefs, and some resorts empty their sewage or other wastes directly into water surrounding coral reefs.
Pollution: in the contemporary times pollution is on the rise, urban and industrial waste, sewage, agrochemicals, and oil pollution are poisoning reefs. These toxins are dumped directly into the ocean or carried by river systems from sources upstream. Some pollutants, such as sewage and runoff from farming, increase the level of nitrogen in seawater, causing an overgrowth of algae, which ‘smothers’ reefs by cutting off their sunlight.
Sedimentation: erosion caused by construction (both along coasts and inland), mining, logging, and farming is leading to increased sediment in rivers. This ends up in the ocean, where it can ‘smother’ corals by depriving them of the light needed to survive. The destruction of mangrove forests which normally trap large amounts of sediment is exacerbating the problem.
Coral Mining: Live coral is removed from reefs for use as bricks, road-fill, or cement for new buildings. Corals are also sold as souvenirs to tourists and to exporters who don’t know or don’t care about the longer term damage done, and harvested for the live rock trade.
1.7 Coral Bleaching
Coral bleaching is a common stress response of corals to many of the various disturbances mentioned above. Coral bleaching occurs when coral polyps lose their symbiotic algae, the Zooxanthellae. When Zooxanthellae, the living tissues are nearly transparent and you can see nearly right through to the stony skeleton which is white hence the phenomena is called as coral bleaching. In simple words it understood as Zooxanthellae algae are photosynthetic phytoplanktons which prepare food through photosynthesis and provide atleast 60 per cent of the food requirement of coral organism. Thus, coral organism depend on their symbiotic partner Zooxanthellae algae for their survival. Thus coral bleaching may be defined as a process which causes loss of different colours from coral organisms and turns them white. The bleached coral organisms lose their food supplies due to removal or expulsion of their symbiotic partner Zooxanthellae algae from their bodies and ultimate they die of starvation. Beginning in the 1980s, the frequency and widespread distribution of reported coral reef bleaching events increased. Widespread bleaching, involving major coral reef regions and resulting in mass coral mortality has raised concerns about linkage of the events to global phenomenon including global warming or climate change and increased UV radiation from ozone depletion.
1.8 Causes of Coral Bleaching
As it is a known fact now that the coral polyps thrive in shallow water where sunlight is available. Coral reefs live within a relatively narrow temperature margin and very high and very low temperature of the water leads to coral bleaching. This type of bleaching occurs when the temperature drops suddenly due to intense upwelling of water from -3°C to -5°C for 5 to 10 days. Bleaching also takes place during the summer months, during seasonal temperature when solar radiation is maximum it leads to bleaching because of photosyntheticaly active radiation (PAR 400-700 nm) and ultraviolet radiation (UVR 280-400 nm). Coral bleaching also takes place when the corals comes over the surface water either due to extremely low tides, ENSO related sea level drops or tectonic uplift can potentially induce bleaching. Sometimes due to storm generated precipitation and addition of water due to surface run-off leads to dilution of reef water and this dilution of water leads to bleaching but this type of bleaching is rare and area specific.
1.9 Events of Mass Coral Bleaching
There were two main events of mass coral bleaching that have been recorded in the past.
The 1882-83 Coral Bleaching: the strong El-Nino-Southern Oscillation (ENSO) event during 1882-83 caused mass coral bleaching resulting into death of 70 per cent of coral organisms off the coast of Central America in the Pacific Ocean. The strong ENSO event raised the sea temperature above normal and this raised the temperature of seas water and when the temperature of sea water increases even by 1°C or 2°C the symbiotic Zooxanthellae algae which are embedded in the tissue of the coral organisms are expelled by corals and hence living coral lose colour.
The 1997-98 Coral Bleaching: this coral bleaching was one of the most catastrophic bleaching events which accounted for large scale death of corals in the tropical oceans of 60 countries and island nations. This event of coral bleaching accounted for 70 per cent death of coral off the coasts of Kenya, Maldives, Andaman and Lakshadweep islands in the Indian Ocean and 75 per cent death in the Seychelles Marine Park System and the Mafia Marine Park off Tanzania was reported by Global Coral Reef Monitoring Network (GCRMN)
1.10 Coral Reefs in India
Inspite of having a coastline of about 7500 kms and subtropical climatic condition India have a very few coral reef areas. The place which deserves to have coral reef is the Bay of Bengal but unfortunately coral reefs are absent because of immense quantity of fresh water and silt brought by rivers. Fresh water is harmful for the growth of coral polyps and sediments brought by the rivers block the mouth of coral polyps. Another reason for the absence of coral reef in Bay of Bengal is the heavy monsoonal rainfall and high human presence on the coastline. The mainland coast of India has two widely separated area’s containing reefs: Coral reefs in India are mostly found in the Andaman and Nicobar Islands, the Gulf of Mannar, the Gulf of Kutch and the Lakshadweep Islands. Recently, the Zoological Survey of India reported finding three pristine reefs off the coast of Sindhudurg in Maharashtra. Coral reefs are also found at Goa coast, Kerala Coast and Palk Bay.
Andaman and Nicobar Group of Island: The Andaman and Nicobar group of Islands are located in the SE of the Bay of Bengal, between 6°-14° N lat and 91 °-94° E longitude. Almost all the islands of the Andaman and Nicobar groups exhibit narrow, linear and extensively well developed fringing reefs. The reef-flat occupies an area of 795.7 sq km. Coral heads and coraline shelf occupies 17.5 and 45 sq km respectively. 8.4 sq km is occupied by mud over reef. According to a study conducted by the Society of Andaman and Nicobar Ecology (SANE) based at Port Blair there has been mass coral bleaching in 1998 around the Andaman reef and 30 to 70 per cent bleaching around the Nicobar reefs. This bleaching was related to 2°C rise in temperature from the normal temperature in the Andaman Sea
The Gulf of Mannar: The Gulf of Mannar reefs on the other hand are developed around a chain of 21 islands that lie along the 140 km stretch between Tuticorin and Rameswaram. These islands are located between latitude 8°47′ N and 9° 15′ N and longitude 78° 12′ E and 79° 14’E. The islands lie at an average of about 8 km from the main land. They are a part of the Mannar Barrier reef which is about 140 km long and 25 km wide between Pamban and Tuticorin. Different types of reef forms such as shore platform, patch, coral pinnacles and atoll type are also observed in the Gulf of Mannar. The islands have fringing coral reefs and patch reefs around them. Narrow fringing reefs are located mostly at a distance of 50 to 100 m from the islands. On the other hand patch reefs arise from depths of 2 to 9 mt and extend to 1 to 2 km in length with width as much as 50 meters.
The Gulf of Kutch: The Gulf of Kutch located at 22°15′-23°40′ N Latitude and 68°20′-70°40′ East Longitude. The coral formations of the Gulf of Kutch represent one of the extreme northern limits of corals in the Indian ocean. The approach to the corals is difficult due to the existence of vast intertidal mud flats which are difficult to negotiate by foot at low tide. The sudden influx of tidal waters also renders it risky to work on the exposed bank. These reefs are mostly of fringing type along with offshore platform reefs, patch reefs and coral pinnacles. There are some 40 islands with patchy coral formation of which the largest is Pirotan Island. The coral reefs are in a highly degraded condition. The major source of degradation has been mud deposits on various coral reefs for example. Bural Chank, Kalubhar, Munde ka bet and Jindra reef. Living coral area rarely exceed 20-30 per cent thus the preventive measures taken in the marine national park has resulted in the restoration of the area under reef significantly.
The Lakshadweep Islands: The Lakshadweep islands lie scattered in the Arabian sea about 225 to 450 km from the Kerala coast. Geographically, the islands lie between 8°N – 12°3’N latitudes and 71 °E- 74°E longitudes. The islands consist of coral formations built up on the Laccadive-Chagos submarine ridge rising steeply from a depth of about 1500 m to 4000 m off the “west coast of India. There are 36 tiny islands, 12 atolls, 3 reefs and 5 submerged banks, covering an area of 32 km2 with lagoons occupying about 4200 km2. Coral reefs of the islands are mainly atoll except one platform reef at Androth. Almost all the atolls have an orientation of NE-SW with the low lying island on the east, a broad well developed reef on the west, with a lagoon in between, connected to the open ocean by one or more channels. According to the study by National Institute of Oceanography (NIO) the coral reefs of the Kavaratti and Kadamat Islands in Lakshadweep have suffered great damage from coral bleaching due to bacterial diseases and warmer sea temperature.
Fig. 5 Major Coral Zones of India
Source: D. O. D. and S. A. C. 1997(Prepared by Author)
1.11 Status of Coral Reef Research in India
The major institutions involved in research of Coral reefs and management are: Department of Ocean Development, The Space Applications Centre, Ahmadabad, The Zoological survey of India, Central Marine Fisheries Research Institute, Madurai-Kamaraj University, CAS, at Parangipettai, Annamalai University, Centre for Earth Studies, Trivandrum, Institute for ocean Management, Madras, National Institute of Oceanography, Goa and the World Wide fund for Nature-India. The Space Applications Centre at Ahmadabad has used remote sensing data to assess the area under coral reefs and prepare a coral reef atlas of India. The Department of Ocean Development has recently received a grant from the World Bank to prepare a CIS based information system for critical habitats for coastal ecosystems. This will include all the coral reefs ecosystems in India including the patches and submerged banks found along the West coast of India.
1.12 Conservation and Protection of Coral Reefs in India
There is much that we can do locally to protect coral reefs, by making sure there is a healthy fish community and that the water surrounding the reefs is clean. Well-protected reefs today typically have much healthier coral populations, and are more resilient (better able to recover from natural disasters such as typhoons and hurricanes). The protection of coral reef has been stressed under Wildlife Protection act, 1972 and Environmental Protection Act, 1986 and Coastal Regulation Zone Notification (CRZN) of 1991 coming under it. Other acts like Indian Forests Act,1927, Forest Conservation Act, 1980 and Indian Fisheries Act also offer a sort of relief in the conservation of Coral reefs of India. But there is no separate legal status for coral conservation even under Wildlife Protection Act. The State forest department, fisheries departments and recently the state coastal management authority at the state level are taking up the responsibilities for coral reef conservations in India. Wildlife Protection Act include the protection of major ecosystems, there is no direct stress on coral reef conservations. So the recommendation for law reform and policy making for coral reef conservation and management in the country essentially concern amending the Wildlife (Protection) Act 1972 to include species of coral in the schedules and specifically state that the extraction of coral is prohibited under the provisions of Chapter V-A of Wildlife Protection Act.
Fig. 6 Coral Island
Fig. 7 Great Barrier Reef
Courtesy: Mike McCoy, Australian Geographic, 2011
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