According to the National Oceanic and Atmospheric Association, over 75% of the world's tropical coral reefs encountered heat stress severe enough to cause bleaching between 2014 and 2017. The heat stress was enough to kill coral in 30% of the world's reefs. Coral bleaching occurs when corals become white as a result of environmental stressors such as temperature, light, or nutrient changes. When coral polyps expel the algae (zooxanthellae) that dwell inside their tissue, the coral bleaches, turning white. This can be produced for a variety of reasons, but a rise in sea surface temperature is the most severe and common cause Coral bleaching's effects are becoming more worldwide in scope, as well as more frequent and intense. This article will explain to you about the Coral Bleaching which will be helpful in Environment preparation for the UPSC Civil Service Exam.

Coral Bleaching

• When corals lose their brilliant colors and turn white, this is known as coral bleaching.
• Bleaching or the paling of coral color occurs when
• the density of zooxanthellae decreases and/or
• the concentration of photosynthetic pigments inside the zooxanthellae decreases.
• Coral is brilliant and colorful due to zooxanthellae, a type of microscopic algae.
• The zooxanthellae live in a mutually beneficial relationship within the coral, one assisting the other to survive.
• When coral is stressed by changes in conditions such as temperature, light, or nutrients, they expel the symbiotic algae living in their tissues, causing them to turn completely white.

Features of Bleached Corals

• Corals often lose 60-90 % of their zooxanthellae as they bleach, and each zooxanthellae might lose 50-80% of its photosynthetic pigments.
• The affected corals normally restore their symbiotic algae within a few weeks or months if the stress-causing bleaching is not too severe and subsides over time.
• The coral host dies if zooxanthellae loss is sustained, i.e. if the stress persists and decreased zooxanthellae populations do not rebound.
• High temperature and irradiance stresses have been linked to the breakdown of zooxanthellae enzyme systems that protect them against oxygen toxicity.
• At temperatures exceeding 30 degrees Celsius, photosynthesis pathways in zooxanthellae are hindered, which could lead to the disassociation of the coral/algal symbiosis.
• As a result of cell adhesion malfunction, low- or high-temperature shocks cause zooxanthellae to be low.
• The detachment of cnidarian endodermal cells from their zooxanthellae, as well as the eventual expulsion of both cell types, is involved in this process.

Ecological Causes of Coral Bleaching

• A variety of factors, alone or in combination, can cause it. As a result, identifying the exact source of bleaching outbreaks is challenging.
• Coral bleaching occurrences have been linked to the following factors.

Temperature

• Coral species have a narrow temperature range, and abnormally low or high sea temperatures can cause coral bleaching.
• Bleaching is much more common when the sea water temperature is high.
• Bleaching can also happen when the temperature lowers suddenly, such as during significant upwelling episodes or seasonal cold-air outbreaks.
• Coral polyps expel the zooxanthellae they rely on for food as the temperatures of the world's oceans rise due to global warming.
• When the zooxanthellae die, the coral loses its brilliant color and all that remains is the white exoskeleton which is called coral bleaching.

Solar Irradiance

• Bleaching occurs preferentially in shallow-living corals and on the exposed summits of colonies during the summer months when seasonal temperature and irradiation maxima occur.
• Coral bleaching is thought to be caused by solar radiation.
• Bleaching has been linked to both photosynthetically active radiation (PAR, 400-700nm) and ultraviolet radiation (UVR, 280-400nm).

Subaerial Exposure

• Upper boundary surfaces of sediment or rock that have been exposed to the Earth's surface are called subaerial exposure surfaces.
• Bleaching can occur when they are exposed suddenly to these surfaces, such as during very low tides, ENSO-related sea level dips, or geological uplift.
• Due to the repeated exposure to subareal zones there will be a depletion in the isotopes of oxygen, δ18O and δ13C.

Sedimentation

• Only a few cases of coral bleaching have been connected to sedimentation alone.
• Coastal development, urban stormwater runoff, forestry, and agriculture all contribute to sedimentation.
• Sedimentation has been identified as a major stressor for coral species and their environments' survival and recovery.
• Sediment accumulated on reefs can suffocate corals, preventing them from feeding, growing, and reproducing.

Fresh Water Dilution

• Coral reef bleaching has been linked to the rapid dilution of reef waters caused by storm-generated precipitation and runoff.
• In general, bleaching events are uncommon and limited to tiny, nearshore areas

Inorganic Nutrients

• Increasing ambient elemental nutrient concentrations (e.g., ammonia and nitrate) does not cause coral reef bleaching.
• Instead, it increases zooxanthellae numbers by 2-3 times.
• Although eutrophication is not a direct cause of zooxanthellae loss, it can have unintended consequences.
• They include weakening coral resistance and increased disease susceptibility.

Xenobiotics

• A xenobiotic is a chemical compound identified within an organism that was not created naturally or that was not expected to be present.
• Zooxanthellae loss happens when coral is exposed to high quantities of various chemical pollutants such as copper, pesticides, and oil.
• Because high xenobiotic concentrations are necessary to produce zooxanthellae loss, bleaching from such sources is typically confined and/or transient.

*To know more about the topic, click this link Xenobiotics

Epizootics

• An epizootic denotes a disease outbreak caused by pathogens.
• Bleaching caused by pathogens is distinct from other types of bleaching.
• The majority of coral illnesses result in patchy or complete colony death as well as the sloughing of soft tissues.
• This also leaves a white skeleton (not to be confused with bleached corals).
• A protozoan has typically been identified as one of the infections that causes translucent white tissues.

Other Causes

• Ocean Acidification: The increasing acidification of the ocean occurs when oceans absorb massive amounts of carbon dioxide released into the atmosphere by the combustion of fossil fuels.
• Rise in carbon dioxide levels increases the acidity of ocean water and inhibits the ability of corals to create calcareous skeletons.
• High amounts of solar radiation lead to the decline of coral due to global warming which eventually results in coral bleaching.
• Water Pollution: Water pollution caused by Agricultural pesticides and fertilizers, oil and gasoline, sewage discharge, and sediment from eroded landscapes make it difficult for coral to thrive.
• It harms the complex relationships that exist between the plants, coral, and other animals that make up the reef ecosystem.
• Bacterial Infection: The prevalence of coral disease appears to have increased considerably during the last ten years, contributing to the global decline of coral reef communities.
• The majority of diseases are caused by bacteria, fungi, and viruses.
• Because coral disease has been proven to proliferate after coral bleaching events, the evidence for a link between warmer-than-normal water and coral disease is growing.

Impact of Coral Bleaching

• Ecological Impacts: Ecological Impacts of bleaching include dwindling of marine species diversity, immediate exposure of waves to land masses posing a risk of erosion and alterations in in coral communities that have an impact on the organisms that rely on them.
• Socioeconomic Impacts: Coral reefs that have been degraded are unable to offer the ecosystem services that local human groups rely on.
• Coral bleached reefs can swiftly lose many of the traits that contribute to the aesthetic appeal that is so vital to reef tourism. As a result, there is a loss in tourism revenue.
• It has the potential to cause major changes in fish populations. Fishers targeting reef fish species catch less fish as a result, affecting food availability and accompanying economic activity.
• Pharmaceutical substances are abundant in coral reefs. Reefs that are degraded or dead are less likely to provide vital therapeutic resources.

Ideal Conditions for Coral Growth

• Shallow water: Coral reefs must grow in shallow areas of water. The reef's surface should not be more than 80m above the water's surface.
• Sunlight: Zooxanthellae require adequate sunlight to perform photosynthesis.
• The semi-hard or hard surface is required for coral polyp skeleton compaction, cementation, and solidification.
• Beautiful coral polyps die in areas with sediment-filled waters. This is due to the presence of sediments and opaque water, which restricts the passage of sunlight to the algae, which sustains their life.
• Temperature: Coral reefs are in direct contact with warm oceanic waters. The water temperature should be around 20 degrees. As a result, coral reefs predominate on the eastern coasts of continents.
• Salinity: The slight salt in ocean waters is extremely important for the development of coral polyps. The polyps extract calcium from the waters to protect their skeletons. Hence, mild salinity is a necessity for coral reefs to flourish.
• Nutrients: Coral reefs flourish in sea waters because ocean waves support the constant supply of rich nutrients. Coral polyps multiply faster when supplied with nutrients.

Conclusion

Apart from potentially hosting the world's most diversified ecosystems, coral reefs are essential for a variety of reasons. They protect coastlines from wave action and tropical storms, provide habitat and refuge for many marine animals, are a source of nitrogen and other vital nutrients for marine food chains, aid in carbon and nitrogen fixation, and aid in nutrient recycling. However, coral bleaching has been documented in various sites around the world this year. If sea surface temperatures continue to rise, coral bleaching will become more common and severe, reducing coral reefs' ability to adapt and supply many of the services that people rely on.

FAQs

MCQs

1. What is the primary cause of coral bleaching?

A) Overfishing
B) High water temperatures
C) Ocean acidification
D) Pollution from plastics

2. How can coral reefs recover from bleaching?

A) By increasing water temperatures
B) By reducing the amount of sunlight
C) By returning to normal water temperatures and improved water quality
D) By introducing invasive species

3. Which of the following is NOT a factor contributing to coral bleaching?

A) High water temperatures
B) Increased sunlight exposure
C) Ocean acidification
D) Ocean salinity

4. What role does carbon dioxide play in coral bleaching?

A) It helps corals absorb nutrients
B) It raises ocean temperatures
C) It lowers the pH of oceans, increasing acidification
D) It accelerates coral growth

5. Which of the following measures can help mitigate coral bleaching?

A) Reducing plastic usage
B) Increasing fish populations
C) Reducing greenhouse gas emissions
D) Increasing water salinity

GS Mains Questions and Model Answers

Q1: Discuss the causes and impacts of coral bleaching on marine biodiversity and ecosystems.

Answer: Coral bleaching occurs when corals expel the algae that live within their tissues due to stress, often caused by rising ocean temperatures. This leads to the loss of color in corals and weakens their structure, making them more vulnerable to disease and death. The primary causes of coral bleaching are increased sea temperatures, ocean acidification, and pollution. The impacts on marine biodiversity are severe, as coral reefs are home to over 25% of all marine species. The loss of healthy coral reefs results in a loss of habitat for marine organisms, disrupts food chains, and reduces biodiversity. The decline of coral reefs also impacts the livelihoods of communities dependent on them for food and tourism, resulting in economic losses and affecting food security for coastal populations.

Q2: What are the global efforts to address coral bleaching and protect coral reefs?

Answer: Global efforts to address coral bleaching include reducing greenhouse gas emissions to mitigate climate change, which is the main driver of rising ocean temperatures. Additionally, marine protected areas (MPAs) are established to safeguard coral reefs from human-induced stressors such as overfishing and pollution. International initiatives like the Coral Triangle Initiative aim to protect coral reefs through collaboration among countries. Furthermore, coral restoration projects, such as coral gardening and transplanting resilient species, are being pursued to restore damaged reefs. Research on coral resilience and the development of coral species that can withstand higher temperatures are also underway. These efforts are crucial for the long-term survival of coral reefs and the ecosystems they support.

Q3: Evaluate the role of individuals in preventing coral bleaching and conserving coral reefs.

Answer: Individuals can contribute to preventing coral bleaching and conserving coral reefs by adopting sustainable practices. Reducing carbon emissions through energy conservation, using less fossil fuel, and supporting clean energy initiatives helps mitigate climate change and the warming of oceans. Individuals can also reduce plastic and chemical pollution by avoiding single-use plastics and using eco-friendly products, thereby reducing pollution that harms coral reefs. Participating in responsible tourism practices, such as avoiding touching coral reefs and using reef-safe sunscreen, also helps reduce the human impact on coral ecosystems. Supporting coral conservation organizations and donating to reef restoration projects further contributes to the preservation of coral reefs.

Previous Year Questions on Coral Bleaching

1. UPSC CSE Prelims 2020:

Question: What is the primary cause of coral bleaching?

A) Ocean acidification
B) Increased water temperatures
C) Overfishing
D) Increased UV radiation

Answer: (B)

Explanation: The primary cause of coral bleaching is elevated water temperatures, which stress corals and lead them to expel their symbiotic algae.

2. UPSC CSE Mains 2019 (GS Paper 3):

Question: Discuss the impact of coral bleaching on marine ecosystems and global biodiversity.

Answer: Coral bleaching has a devastating impact on marine ecosystems as corals provide essential habitat for a large number of marine species. The loss of corals disrupts food chains and leads to reduced biodiversity. Coral reefs are considered the "rainforests of the sea" due to their rich biodiversity, and their decline threatens numerous marine organisms. The economic and ecological consequences of coral bleaching affect coastal communities, particularly those dependent on fishing and tourism. Efforts to mitigate coral bleaching and promote the conservation of coral reefs are essential for sustaining marine biodiversity and ecosystem services.