Coral reefs are underwater ecosystems made up of the skeletons of colonial marine invertebrates known as coral. Coral reefs are one of the world's most biologically diverse marine ecosystems. Coral reefs are colonies of small living animals found in the oceans. They are underwater formations composed of coral polyps bound together by calcium carbonate. Coral reefs serve a vital function in marine ecosystems by providing habitat for marine plants and wildlife. This article will explain to you about the Coral Reefs which will be helpful in Environment preparation for the UPSC Civil Service Exam.

What are Corals?

• Coral is actually a living creature. Coral has a symbiotic relationship with 'zooxanthellae,' microscopic algae that live on coral [i.e., instead of living on the sea floor, the algae lives up on the coral, which is closer to the ocean surface and thus gets enough light].
• Through their photosynthetic activities, zooxanthellae aid the coral in nutrient production.
• These activities provide fixed carbon compounds to the coral for energy, promote calcification, and mediate elemental nutrient flux.
• The coral tissues themselves are not the beautiful colours of the coral reef, but rather clear (white). Corals get their colour from the zooxanthellae that live within their tissues.
• In exchange, the host coral polyp provides its zooxanthellae with a safe environment to live in as well as a steady supply of carbon dioxide for photosynthetic processes.
• Corals are classified into two types: hard corals and soft corals, which include sea fans and gorgonians. Only hard corals can form reefs.
• Coral reefs are built by tiny animals known as polyps.
• As these polyps thrive, grow, and die, their limestone (calcium carbonate) skeletons are left behind.
• New polyps are colonising the limestone. As a result, a coral reef is composed of layers of these skeletons that are eventually covered by living polyps.
• Reef-building corals, also known as hermatypic corals, can take on a variety of shapes.
• Coral reefs can be branched or table-like, or they can resemble massive cups, boulders, or knobs.

Cold Water Corals

• Cold-water corals also called as deep water corals are marine invertebrates distinguished mostly by the presence of harpoon-shaped cells known as cnidocytes, which they utilise to protect and hunt.
• Cold-water corals can be found in the Atlantic, Indian, and Pacific Oceans.
• Cold-water corals live in deep, cold (39-55 degrees Fahrenheit) water. According to the United Nations Environment Programme, cold-water coral reefs outnumber tropical reefs worldwide.
• Deep-water corals are members of the Phylum Cnidaria and are mostly stony corals, but also include black and thorny corals, as well as soft corals.
• Deep-water corals, like tropical corals, provide habitat for other species, but they do not require zooxanthellae to survive.
• There are only about six different coral species associated with the construction of these reefs.
• The Rost Reef off the coast of Norway is the world's largest coldwater coral reef.

*For detailed notes of this topic, check this link Cold Water Corals

Features of Coral Reefs

• They are found in shallow tropical areas with clean, clear, and warm sea water.
• The coral reef cover in Indian waters is estimated to be up to 19,000 square kilometres.
• Coral reefs are among the most productive and complex coastal ecosystems, with a wide range of biological diversity.
• The high productivity is due to a combination of its own primary production and environmental support.
• Reef-building corals are a metaphorical union of polyps (coral animals) and zooxanthellae (microscopic algae).
• Corals are slow growing animal colonies, whereas zooxanthellae are fast growing plants.
• Despite the fact that corals live in nutrient-poor waters, their ability to recycle scarce nutrients (by the entire nutrient community) is enormous.
• Many invertebrates, vertebrates, and plants live in close association with the corals, with tight resource coupling and recycling, allowing coral reefs to have extremely high productivity and biodiversity, earning them the nickname "the Tropical Rainforests of the Oceans."

Formation of Coral Reefs

• Corals are plant and animal superorganisms lying in between population & community in ecological hierarchy on ecological level or organisation.
• Animal component belongs to class anthozoa of the phylum cnidaria whereas plant component is algae called as zooxanthellae that has chlorophyll pigment.
• This algae has photosynthesis abilities that provide carbon compounds to coral polyps, which provide them with energy. In exchange, the polyps protect the Zooxanthellae.
• Corals are calcareous rocks created from the skeletons of tiny sea creatures known as polyps.
• Polyps extract calcium salts from saltwater and use them to build hard skeletons that protect their fragile bodies.
• When free-moving coral larvae attach themselves to sedimentary rocks or hard surfaces near coasts, Coral reefs are formed. The skeletons of dead polyps give rise to new generations. The cycle is repeated, resulting in the formation of coral layers.
• The shallow rock formed during these depositions is known as a reef which eventually becomes islands.
• Corals are cemented calcareous stone masses that grow upwards and outwards as tubular skeletons.
• Coral reefs, depending on the nature of the salts might appear in a variety of shapes and colors.

Conditions Required for Growth of Coral

• Shallow water: Coral reefs require shallow water to grow. The surface of the Reef should not be more than 80 m above the surface of the water. Zooxanthellae require adequate sunlight to perform photosynthesis.
• Semi-hard or hard surface: The semi-hard or hard surface is required for coral polyp skeleton compaction, cementation, and solidification.
• Low range of temperature: Coral cannot tolerate a high range of temperature and hence require 20-22°C mean annual temperature as they are lying in tropical and subtropical regions (stenothermal).
• Clear water: Beautiful coral polyps are destroyed in sediment-filled waters. They also disappear in shallow water. This is because of the presence of sediments and opaque water that restricts the flow of sunlight to the algae, which sustains their survival.
• Saline waters: The moderate salinity of ocean waters is critical for the development of coral polyps. To protect their bones, the polyps collect calcium from the water. Hence, mild salinity is required for coral reefs to grow (27-30 PPT).
• Rich supply of nutrients: Coral reefs grow in seawater because ocean waves provide a continuous source of rich nutrients. Coral polyps multiply more quickly when nutrients are supplied.

Classification of Coral Reefs

Fringing Reefs (Shore Reefs)

• Fringing reefs form near the continent and stay close to the coastline. Small, shallow lagoons separate these reefs from the coastline.
• They are the most common among all corals.
• Eg: Andaman and Nicobar Island, Gulf of Kutch, Mannar and Palk Bay.

Barrier Reefs

• It is the largest, highest and widest reef among the coral reefs.
• Barrier reefs are linear offshore reef structures running parallel to continental margin and are separated by wide and deep non navigable barriers.
• The water body between reef and the shore is termed as lagoon.
• Barrier reefs extend as a broken, irregular ring around the coast or an island, running almost parallel to it.
• Largest coral of the world is the great barrier reef, present on eastern coast of Queensland of australia.

Patch Reef

• Patch reefs typically form on the island platform or continental shelf between fringing and barrier reefs.
• They are most commonly found in shallow lagoons within larger collective reefs, or atolls.
• Patch reefs are found in waters ranging in depth from 10 to 20 feet and are usually found close to land.
• Examples: the Caribbean and Bahamas, Bermuda and the Pacific Islands.

Atoll Reefs

• Atoll is a ring of narrow growing coral generally in the form of horse shoe pattern or elliptical manner.
• Corals are found on submarine bench or platform and in the middle of coral lagoon is food i.e., no land is found within Atoll.
• A chain of various Atoll collectively are called as Faros.
• Ex: Lakshadweep

Functions of Coral Reefs

• As coral reefs are found in shallow water of tropical or subtropical regions they are characterised by high biomass production and rich floral & faunal activity.
• Corals are a natural protective barrier against erosion.
• Some corals are a source of high quality proteins.
• Coral is a large hot spot of marine biodiversity & fishery resources.
• It is also the largest biogenic calcium carbonate producer.
• They provide substrate for mangroves.
• Coral reefs provide habitat for a large variety of animals & plants including avifauna.
• Coral play a remarkable role in insitu or conservation of marine biodiversity.
• Coral is used as bone substitute in reconstructive bone Surgery as pores & channels are some corals resembling that of human bone.

Threats to Coral Reef

• Natural causes could include the emergence of reef-destroying mechanisms, "bleaching," and the depletion of essential symbiotics.
• Chemical pollution (pesticides, cosmetics, etc.), industrial pollution, mechanical damage, nutrient loading or sediment loading, dredging, shipping, tourism, mining or collection, thermal pollution, intensive fishing, and so on are examples of anthropogenic causes.
• Coral reef ecosystems around the world have suffered unprecedented degradation in recent decades. Coral reef disturbances include both anthropogenic and natural events.
• Natural disturbances that cause coral reef damage include violent storms, flooding, extreme temperature swings, El Nino Southern Oscillation (ENSO) events, subaerial exposures, predatory outbreaks, and epizootics.
• Coral reef bleaching is a common stress response of corals to many of the above-mentioned disturbances.
• Some other threats include:
  • Deforestation
  • Oil and chemical spill (toxicants)
  • Road construction (sedimentation)
  • Agriculture (nutrient & sedimentation)
  • Stormwater runoff (sediment, toxins, nutrients & pathogen)
  • Chemical brom sunscreen (toxicants)
  • Coastal development & Impervious surface (sedimentation & toxicant)
  • Sailed septic supeny (nutrients & pathogens)

*For detailed notes of this topic, check this link Threats to Coral Reefs

Causes of Decline in Coral Reefs

Temperature

• Coral Species live within a relatively narrow temperature merging and anomalously low & high sea temperature can induce coral bleaching.
• Bleaching is much more frequently reported from elevated seawater temperatures.
• Bleaching event also occurs during sudden temperature drops accompanying intense upwelling episodes, seasonal cold air outbreaks

Solar Irradiance

• Bleaching during the summer months, during seasonal temperature & irradiance maxima open occurs disproportionately in shallow living corals & on the exposed summits of colonies.
• Solar radiation has been suspected to play a role in coral bleaching.
• Both photosynthetically active radiation (PAR, 400-700mm) & ultra violet radiation (UVR, 280-400-700mm) have been implicated in bleaching.

Subaerial exposure

• Sudden exposure of reef flat corals to the atmosphere during events such as extreme low tides.
• ENSO related Sea level drops or tectonics uplift can potentially induce bleaching.

Sedimentation

• Relatively few instances of coral bleaching have been linked solely to sediment.
• It is possible but not been demonstrated, that sediment loading could make Zooxanthellae species more likely to bleach.
• Epizooties, a pathogen induces bleaching like white band disease, black band disease.

Conservation of Corals

• Environmental protection act, 1986 prohibits the use of corals and sands from the beaches & coastal water for construction & other purposes.
• India is participatory to green Coast project.
• Coastal regulation zone rules (CRZ) bans the collection and destruction of corals along with dredging and under water blasting in and around coral formation.
• Concepts of marine protected areas & marine national parks help in coral conservation.
  • For example: Flist national marine park gulf of Mannar (Gujrat), Great nicobar Biosphere Reserve, Mahatma Gandhi Marine National park (Andaman) are playing an important role in Conservation of corals.
• India has ratified the Paris agreement and became the 62nd country of the world to do so, a step toward Climate change mitigation.
• India has ratified the second commitment period of Kyoto protocol (2013-20).

Coral Bleaching

• Bleaching, or coral colour paling, occurs when the density of zooxanthellae decreases and/or the concentration of photosynthetic pigments within the zooxanthellae decreases.
• Corals commonly lose 60-90 percent of their zooxanthellae when they bleach, and each zooxanthella may lose 50-80 percent of its photosynthetic pigments.
• If the stress-induced bleaching is not too severe and fades over time, the affected corals usually regain their symbiotic algae within a few weeks or months.
• If zooxanthellae loss continues, i.e. if the stress persists and depleted zooxanthellae populations do not recover, the coral host will eventually die.
• High temperature and irradiance stressors have been linked to the disruption of enzyme systems in zooxanthellae that provide oxygen toxicity protection.
• At temperatures above 30 degrees Celsius, photosynthesis pathways in zooxanthallae are impaired, which could lead to the disassociation of the coral/algal symbiosis.
• Low or high temperature shocks cause zooxanthellae to die due to cell adhesion dysfunction.
• This involves the separation of cnidarian endodermal cells from their zooxanthellae, followed by the expulsion of both cell types.

*For detailed notes of this topic, check this link Coral Bleaching

Conclusion

For over a million years, coral reefs have supported life. Its rich environment keeps marine life flourishing. Interfering with the coral reefs is not in our best interests. To safeguard these beautiful polyps, conservation measures should be implemented. Laws should be strictly enforced, and if they are not followed, penalties should be levied as early as possible.

FAQs

MCQs

1. Which of the following is NOT a type of coral reef?

A) Fringing reef
B) Barrier reef
C) Atoll
D) Island reef

2. Coral bleaching is caused by which of the following factors?

A) Rising sea temperatures
B) Pollution
C) Overfishing
D) Coastal development

3. Which type of coral reef is formed around a submerged volcanic island?

A) Fringing reef
B) Barrier reef
C) Atoll
D) Lagoon reef

4. What role do coral reefs play in coastal protection?

A) They absorb pollutants
B) They prevent hurricanes
C) They act as natural barriers, reducing wave impact
D) They increase coastal erosion

5. Which of the following human activities poses a significant threat to coral reefs?

A) Sustainable fishing
B) Overfishing
C) Marine protected areas
D) Coral farming

GS Mains Questions and Answers

Q1: Discuss the ecological and economic significance of coral reefs and the major threats they face today.

Answer: Coral reefs are one of the most diverse ecosystems on Earth, supporting around 25% of all marine species. Ecologically, they provide habitat, shelter, and food for a variety of marine organisms, making them crucial to marine biodiversity. Economically, coral reefs are valuable for coastal protection, fisheries, and tourism. They act as natural barriers, reducing coastal erosion and protecting communities from storm surges. Coral reefs also support the livelihoods of millions of people worldwide through fishing and tourism.

However, coral reefs face numerous threats, including climate change, which causes coral bleaching due to rising sea temperatures. Ocean acidification, pollution, and destructive fishing practices also contribute to the degradation of coral reefs. Overfishing, coastal development, and unsustainable tourism put additional pressure on these delicate ecosystems. Protecting coral reefs requires global action to address climate change and the implementation of conservation strategies to reduce human impacts.

Q2: Analyze the impact of coral bleaching on marine ecosystems and discuss measures that can be taken to protect coral reefs from further degradation.

Answer: Coral bleaching occurs when corals expel the symbiotic algae (zooxanthellae) living in their tissues, which are essential for their survival. This leads to the loss of color and the death of coral polyps if the stress persists. Bleaching events are triggered primarily by rising sea temperatures due to climate change. Coral bleaching significantly impacts marine ecosystems by reducing biodiversity, as many species rely on healthy reefs for food and shelter.

To protect coral reefs from further degradation, measures such as reducing greenhouse gas emissions to mitigate climate change, establishing marine protected areas (MPAs), and promoting sustainable tourism practices are crucial. Additionally, coral restoration projects, such as coral farming and transplantation, can help rehabilitate degraded reef areas. Raising awareness about the importance of coral reefs and encouraging responsible fishing practices are also essential for their conservation.

Q3: Evaluate the role of international agreements and conventions in the protection of coral reefs.

Answer: International agreements and conventions play a vital role in the protection of coral reefs by fostering cooperation among countries to address global environmental challenges. The United Nations Convention on the Law of the Sea (UNCLOS), for example, provides a framework for the conservation of marine ecosystems, including coral reefs. The Convention on Biological Diversity (CBD) promotes the protection of biodiversity, including coral reefs, and encourages the establishment of marine protected areas (MPAs).

The Paris Agreement on climate change is also significant, as it aims to limit global temperature rise, which directly impacts coral reefs through coral bleaching. Additionally, regional agreements like the Coral Triangle Initiative (CTI) focus on conserving coral reef ecosystems in specific regions. However, the success of these agreements depends on their effective implementation and the commitment of nations to reduce their environmental impact and protect marine ecosystems.

Previous Year Questions on Coral Reefs

1. UPSC CSE Prelims 2020:

Question: Coral reefs are primarily found in which of the following regions?

A) Polar regions
B) Tropical and subtropical regions
C) Deep-sea trenches
D) High-altitude lakes

Answer: B

Explanation: Coral reefs thrive in warm, shallow waters and are primarily found in tropical and subtropical regions where conditions are ideal for coral growth and survival.

2. UPSC CSE Mains 2019 (GS Paper 1):

Question: "Coral reefs are vital to the marine ecosystem and the economy, but they are increasingly under threat." Discuss the challenges faced by coral reefs and the measures needed to conserve them.

Answer: Coral reefs are essential to marine ecosystems, providing habitat for numerous species and supporting biodiversity. Economically, they protect coastlines, support tourism, and provide food security through fisheries. However, coral reefs are under severe threat from climate change, particularly rising sea temperatures that cause coral bleaching. Ocean acidification, overfishing, coastal pollution, and unsustainable tourism further exacerbate the destruction of coral reefs.

Conservation measures include reducing carbon emissions to mitigate climate change, creating marine protected areas (MPAs), promoting sustainable fishing practices, and raising awareness about the importance of coral reefs. Coral restoration projects, such as coral farming and transplantation, can also aid in the recovery of degraded reefs. International cooperation through conventions like the Paris Agreement and the Convention on Biological Diversity (CBD) is critical in addressing the global challenges facing coral reefs.