Coral Reefs - Introduction
Coral reefs are one of the natural wonders of the ocean. Coral reefs only occupy 0.1% of the area of the ocean but they support 25% of all marine species on the planet. Coral reefs are found in the shallow coastal zones of tropical and subtropical oceans, where light can penetrate, Apart from being aesthetically appealing, they are also among the world’s oldest, most diverse, and most productive ecosystems. Because they are so diverse, coral reefs are often called the rainforests of the oceans.
Corals are formed by huge colonies of tiny organisms called polyps. Individual coral polyps within a reef are typically very small, usually less than an inch. These coral polyps live in a symbiotic relationship with Zooxanthellae algae. Corals are themselves transparent but look colourful because of the algae. The zooxanthellae algae provide much of the green, brown, and reddish colours that coral has. These single-celled algae provide food through photosynthesis to the corals, and in exchange, the coral animal gives nutrients to the algae.
Reef-building corals produce hard skeletons of Calcium Carbonate (limestone) and continue to grow upward layer by layer. It's a protective crust around their soft bodies. When they die, their outer skeletons remain as a platform for others to continue building the coral. These skeletal layers accumulate over a period of time as every generation of the coral colony dies. Over millions of years, a thick sequence of deposits develops called coral reefs.
Coral reefs benefit the environment and people in numerous ways. For example, they
Protect shores from the impact of waves and from storms.
Provide economic benefits to local communities from tourism.
The reefs provide home and shelter to over 25% of fish in the ocean and up to two million marine species. They are also a nursery for the juvenile forms of many marine creatures.
Types of Coral Reefs
The Three main types of coral reefs are -
1. Fringing Reefs - These grow along the edges of continents and around islands, close to shore but sometimes separated from it by a shallow lagoon. Fringing reefs are common in the Gulf of Mannar, Andaman, and Nicobar Islands. Where there is murky water caused by soil run-off, fringing reefs rarely grow to any substantial depth.
2. Barrier Reefs - These develop along the edges of continental shelves or around islands that have become partially submerged and are separated from the mainland or island by a wide, deep lagoon. More corals that are fragile grow on the lagoon side of the barrier than on the open side where they would have to withstand the force of larger and more violent waves. These types of reefs can be found in Andaman & Nicobar Islands. However, the best-known example is the Great Barrier Reef which extends for nearly 2000 km along the east coast of Australia and represents about three per cent of the total of the world’s reefs.
3. Atoll Reefs - These generally begin as fringing reefs around volcanic islands. As the island subsides, because of the sea floor sinking or the sea level rising, the fringing reef forms a circular barrier reef separated from the island by a lagoon. When the island finally disappears, the circle of reefs is left, sometimes capped with small coral islands, enclosing the lagoon. The whole structure is called an atoll. Atoll varies in size from tiny Bitra, the smallest of just 0.10 sq. km, to the largest, Andrott, 4.84 sq. km in Lakshadweep.
Apart from these, there are a few minor reef types as follows:
Platform reef, Patch reef, Coral pinnacle, Reef flat, coralline shelf, Coral heads, and Live coral platform.
Conditions necessary for the coral reef formation
Reef-building corals live in colonies in shallow, warm, tropical seawaters.
Coral reefs are very sensitive ecosystems and can thrive in a very narrow range of environmental conditions:
Temperature - Coral reefs extend over an area of 68 million square miles in tropical and subtropical seas. They are best developed where the mean annual temperature of the water lies within the range of 23°C to 25°C, they do not develop to any significant extent in the regions where temperatures fall below 18° Cand more than 30°C.
Ocean Water Salinity - Most reef-building corals also require saline water ranging from 32 -35 parts per thousand.
Depth of Sea Water - Corals depend upon the photosynthetic zooxanthellae algae for most of their food. As sunlight cannot penetrate oceans beyond the depth of 200 m from sea level, corals cannot survive beyond the photic zone (Photic Zone is the zone of sunlight from sea level up to a depth of 200 m in oceans). They thrive best in shallow water depths of 60 -80 metres below sea level.
Clear Waters - The water must also be clear so that a maximum amount of light penetrates it and photosynthesis goes on. Thus, corals cannot survive in sediments and muddy waters.
Significance of Coral Reefs
Coral reefs are some of the most diverse and valuable ecosystems on Earth. Various ecosystem services provided by coral reefs are:
1. Protect marine creatures - The reefs provide home and shelter to over 25% of fish in the ocean and up to two million marine species. They are also a nursery for the juvenile forms of many marine creatures.
2. Fisheries - These provide habitat for about a quarter of marine fisheries and therefore, are the source of livelihood & food security for about 1 billion people around the globe.
3. Coastal protection - Coral reefs protect coastlines from damaging effects of wave action during tropical storms, cyclones etc. These break the power of waves during tropical storms and tsunamis. These prevent coastal erosion, and flooding and therefore save the loss of lives and property.
4. Part of the Carbon Cycle - Coral reefs are complex ecosystems that perform many ecological services. When polyps form their shells, they absorb some carbon dioxide as part of the carbon cycle.
5. Source of medicinal products - A few coral reef organisms are used in the treatment of diseases like cancer and HIV, thus providing valuable medicinal products.
6. Tourism - Sustainably managed coral reef-based tourism can also provide significant alternative or additional sources of income to poorer coastal communities in developing countries. However, unsustainable tourism is lately becoming a major threat to coral reefs.
7. Nutrient Recycling - These also help in nutrient recycling by acting as the source of essential nutrients for marine food chains.
Coral Bleaching
Coral bleaching is the biggest threat faced by all colonies. Whenever any environmental conditions change due to some kind of environmental degradation, corals come under increased stress. Under such stressful conditions, coral polyps expel the zooxanthellae algae from it. This causes the coral polyps to undergo discolouration. This is called coral bleaching.
When a coral bleaches, it is not dead. Corals can survive a bleaching event, but they are under more stress and can eventually die. Thus, coral bleaching is indicative of deteriorating marine environmental conditions.
Causes of Coral Bleaching
In present times of climate change and increased environmental pollution, coral reefs are being subjected to multiple threats. An increase of even one degree in the water temperature can trigger bleaching. Sediment runoff from land that smothers the reef and prevents photosynthesis is another stress. Any pollution and disease could also lead to bleaching.
1. Climate Change - Climate change has become the single biggest cause of large-scale mass bleaching events.
A. Increase in Sea Surface Temperature (SST)-Marine heat waves are becoming more frequent with global warming. These cause mass bleaching events as corals are extremely sensitive to temperature increases beyond 30° C.
B. Ocean acidification (OA)- As coral reefs are calcium carbonate accumulations, an increase in carbon dioxide concentration due to ocean acidification decreases the calcification rates of coral reefs.
C. Increase in destructive tropical cyclones - With warmer oceans, highly destructive tropical cyclones are becoming more common. These physically damage coral reefs.
D. Sea Level Rise (SLR) -As coral reef ecosystems thrive in shallow waters, rising sea levels in future will pose more stress.
As per the IPCC Special Report 1.5°C (2018), coral reefs are likely to decline between 70% and 90%, if the global temperature rise is up to 1.5°C as compared to pre-industrial levels. However, global warming of 2° C, will cause the decline of more than 99% of coral reefs.
2. Marine and Coastal Pollution - Sewage and industrial waste, marine oil spills all poison coral reefs. Agricultural runoff can cause an overgrowth of algae which smothers reefs by cutting off their sunlight.
3. Coral Diseases - With increasing sea temperatures, coral diseases now affect more and more corals. Most diseases occur in response to the onset of bacteria, fungi, and viruses. Human-caused activities like pollution of coastal waters may exacerbate reef-forming coral’s susceptibility to waterborne pathogens.
4. Outbreaks of the Crown of Thorns Starfish (COTS) - COTS is a voracious coral reef predator. Populations of the COTS have increased since the 1970s as its predators have declined either due to pollution or over-harvesting. Large outbreaks of starfish wipe out huge tracks of coral reefs.
5. Overfishing and destructive fishing practices - Overfishing of certain species near coral reefs affects the reef’s ecological balance. For example, overfishing of herbivorous fish causes high levels of algal growth which restricts sunlight penetration into waters. Destructive fishing practices like cyanide fishing, blast or dynamite fishing and bottom trawling physically damage the coral reef structures.
6. Unsustainable tourism - Tourist activities like boating, diving, and snorkelling physically damage coral reefs. Building tourist resorts and other infrastructure on top of coral reefs has also damaged corals.
7. Coral mining - Coral reefs are used as cement material and road fill. Corals are also used as souvenirs and exported. Coral mining physically destroys coral reefs.
Worldwide distribution of coral reefs -
Coral reefs can be found in tropical destinations around the world, mostly in areas around the equator where the water is warmer. Coral reefs cover an area of over 280,300 sq. km. (109,800 sq mi) just under 0.1% of the ocean’s surface area. More than 100 countries have a coral reef within their borders, and over half of the world’s coral reefs are found within six countries: Australia, Indonesia, the Philippines, Papua New Guinea, Fiji, and the Maldives.
Coral Reefs in India
In India, coral reefs are found in Lakshadweep, Andaman and Nicobar Islands, Gulf of Kutch, Gulf of Khambhat, Gulf of Mannar, Malwan Reef and Angria Bank (Maharashtra) and Netrani Islands (Karnataka). Coral reefs of India are also experiencing massive bleaching and mortality mainly due to warmer waters and other anthropogenic pressures.
Global Initiatives to Protect Coral Reefs -
Reefs are at the nexus of our triple planetary crises -biodiversity breakdown, a climate emergency and pollution resulting from decades of unsustainable production and consumption. Scientists predict that even if global warming is maintained at 1.5 C up to 90% of coral reefs might disappear by 2050 due to ocean heatwaves.
There aren’t any conventions or agreements between nations for coral reef conservation. The global initiatives include many organizations dedicated to coral reef conservation
The International Coral Reef Initiative (ICRI): ICRI is an informal partnership between Nations and organizations that strives to preserve coral reefs and related ecosystems around the world. It was conceived at the UN Conference on the Sustainable Development of Small Island Developing States in Barbados in 1994 in recognition of the problems facing coral reefs.
The Coral Triangle Initiative: It was signed in 2006 by the Governments of Indonesia, Philippines, Malaysia, Papua New Guinea, the Solomans and Timor Leste to conserve coral reefs with the highest biodiversity. It is a type of Marine Protected Area network to regulate human activities in coral-rich areas.
50 Reefs: 50 Reefs is a global plan to save coral reefs from climate change, pollution, and poor fishing practices. It was launched in 2017 in Bali. It brings together leading ocean, climate and marine scientists around the world to develop a list of the 59 most critical coral reefs to protect.
World’s largest marine reserve: -
The Coral Sea is set to become the site of the world’s largest marine reserve. East of the Great Barrier Reef, the proposed Coral Sea Commonwealth Marine Reserve would cover about 385,000 square miles.
The park would encompass remote coral reefs, ancient sponge gardens, deep-sea canyons, and submerged volcanoes in the Coral Sea, among the last places where ocean giants like sharks, and billfishes can be seen in large numbers.
The Coral Sea harbours high biodiversity and relatively healthy ecosystems. This makes the Coral Sea a unique large ecosystem with an irreplaceable value globally.
Frequently Asked Questions (FAQs)
Question: What is Coral Reef?
Answer - A particular class of animal belonging to phylum coelenterate is known as coral. It is a soft-bodied radially symmetrical marine invertebrate that secretes a calcareous skeleton. The reef is formed by the cementing together of millions of these calcareous skeletons over a long period of time.
Question: What is a coral reef made of?
Answer - A coral reef is made of thin layers of calcium carbonate. Reef-building corals live in colonies in shallow, warm, tropical seawaters. These corals produce hard skeletons of limestone (Calcium Carbonate) and continue to grow upward layer by layer.
Question: Why are coral reefs important?
Answer - Coral Reefs help protect the coastal zone from the impact of waves and storms. They are the nurseries for hundreds of marine organisms. On the economic side, they provide fish, shellfish, building materials, medicines, and employment to people.
In their biodiversity and intricacy of relationships, they can be called the rain forests of the ocean.
Question: What is Coal bleaching?
Answer - Coral bleaching is the biggest threat faced by all colonies. When a reef becomes stressed, it expels the zooxanthellae, loses its colour and food, and ultimately dies. This causes the coral polyps to undergo discolouration. This is called coral bleaching.
Any pollution and disease could lead to bleaching. An increase of even one degree in the water temperature can trigger bleaching.
Question: What are the Three causes of coral bleaching?
Answer - Like many other parts of nature, coral reefs are also in trouble. They are very vulnerable to damage because they grow very slowly, get disrupted easily, and are very sensitive to variations in temperature and salinity.
Coral reefs are being subjected to multiple threats.
Climate Change - Climate change has become the single biggest cause of large-scale mass bleaching events. An increase in Sea Surface Temperatures and ocean acidification can trigger bleaching.
Marine and Coastal Pollution - Sewage and industrial waste, marine oil spills all poison coral reefs.
Overfishing - Overfishing of certain species near coral reefs affects the reef’s ecological balance.
Question: How much of the world is covered by coral reefs?
Answer - Coral reefs only occupy 0.1% of the area of the ocean but they support 25% of all marine species on the planet.
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