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Source: Great Barrier Reef Foundation
Context
Hard coral cover dropped almost 70% in some regions of the Great Barrier Reef, its worst loss in 39 years. The fifth global bleaching event since 2016 occurred in 2024 amid record heat stress, storms, and crown-of-thorns starfish infestations. Scientists say frequent and strong bleaching events are leaving the reef with little time to recover, and the window to save it is narrowing.
What are Corals?
- Corals are marine invertebrates composed of genetically identical organisms called polyps.
- These polyps live in colonies and are closely associated with microscopic algae called zooxanthellae that reside within their tissues.
Symbiotic Relationship with Zooxanthellae
- Corals and zooxanthellae exhibit a mutualistic relationship:
- Corals provide shelter and compounds required for photosynthesis.
- Zooxanthellae provide corals with organic products (like carbohydrates) from photosynthesis.
- These nutrients are essential for calcium carbonate skeleton formation in corals.
- Zooxanthellae also contribute to the vibrant colors of corals.
Ecological Significance of Corals
- Corals are often referred to as the “rainforests of the seas” due to their rich biodiversity.
- Coral reefs support a wide variety of marine life and play a crucial role in maintaining oceanic ecological balance.
Types of Corals
- Hard (Stony) Corals
- Also called hermatypic or reef-building corals.
- Found in shallow, sunlit waters.
- Extract calcium carbonate from seawater to build hard, white exoskeletons.
- These exoskeletons accumulate over time to form coral reefs.
- Soft Corals
- Found in deep, cold, and darker ocean waters.
- Resemble plants in appearance.
- Do not build large exoskeletons but attach themselves to hard substrates, including old coral skeletons.
- Add to the structural complexity of coral reefs over generations.
- Coral reefs are the largest living structures on Earth.
- They are vital for marine biodiversity, coastal protection, and livelihoods of millions dependent on fishing and tourism.
What is Coral Bleaching?
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Source: NOAA
On April 15, 2024, NOAA (in partnership with the International Coral Reef Initiative) confirmed the world is in the midst of its 4th global coral bleaching event. From 1 January 2023 to 31 May 2025, bleaching-level heat stress has impacted 83.9% of the world’s coral reef area and mass coral bleaching has been documented in at least 83 countries and territories.
The ongoing global coral bleaching event is the biggest to date. The previous record was during the 3rd global coral bleaching event, which occurred from 2014-2017, when 68.2% of the world's reef area experienced bleaching-level heat stress. The 1st and 2nd global coral bleaching events occurred in 1998 and 2010, respectively.
- Coral bleaching is a phenomenon where corals lose their colour and turn white due to environmental stress.
- Under stress conditions such as increased sea surface temperatures, changes in light, or nutrient levels, corals expel the symbiotic algae called zooxanthellae living within their tissues.
- The white appearance is due to the calcium carbonate skeleton becoming visible after the pigmented zooxanthellae are expelled.
Causes of Coral Bleaching
- Thermal Stress: Sudden rise in sea surface temperature, especially due to climate events like El Niño.
- Light and Nutrient Stress: Changes in solar radiation and ocean nutrient levels.
- Climate Change: Long-term global warming has made oceans warmer than usual.
- Greenhouse Gas Emissions: Increased carbon emissions contribute to ocean warming.
- Marine Pollution: Pollution can exacerbate coral stress and reduce resilience.
Coral Bleaching and Survival
- Bleached corals are not dead immediately and may survive if:
- Sea temperatures return to normal within a few weeks.
- Zooxanthellae are able to repopulate coral tissues.
- Severe or prolonged bleaching often results in coral mortality and degradation of reef ecosystems.
Mass Coral Bleaching Events
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Source: Statista
First Global Event – 1998
- Triggered by El Niño warming in the Pacific Ocean.
- Led to the death of 8% of global coral reefs.
Second Global Event – 2002
- Affected many reef systems globally.
- Indicated increasing frequency of such events.
Third and Most Severe Event – 2014 to 2017
- Reefs in the Pacific, Indian, and Caribbean Oceans were affected.
- Coral reefs in Guam and broader regions experienced extensive heat stress.
- Global temperatures during 2017 were the third highest ever recorded.
- The bleaching-level stress exposed more than three times as many reefs as it did in 1998.
Mass Coral Bleaching Event on the Great Barrier Reef (GBR), 2024–25
- The Australian Institute of Marine Science (AIMS) confirmed that the 2024 mass coral bleaching event had the largest spatial footprint in the 39-year history of its Long-Term Monitoring Program (LTMP).
Survey Coverage:
- Conducted between August 2024 and May 2025.
- Covered 124 reefs across northern, central, and southern GBR.
Key Findings:
- Lowest hard coral cover ever recorded since monitoring began in 1986.
- Hard coral cover decline by more than 70% in some areas.
- Fifth mass bleaching event on GBR since 2016.
- Part of the ongoing fourth global mass bleaching event (began in 2023 in Northern Hemisphere).
Reef Status (2025 Survey Data):
- 48% reefs: Decline in coral cover.
- 42% reefs: No net change.
- 10% reefs: Increase in coral cover.
Aerial Survey (March 2025):
- 162 inshore and mid-shelf reefs assessed.
- 41% reefs showed medium to high bleaching prevalence.
- Bleaching peaked at 60% in severely affected areas.
- Northern and central GBR faced the highest heat stress and damage.
Scientific Findings
- According to the Global Coral Reef Monitoring Network (GCRMN):
- Between 2009 and 2018, the world lost 14% of its coral reefs.
- Thermal stress caused coral bleaching, which accounted for the majority of losses.
- The report highlights the urgency of climate action to prevent further damage.
Factors Contributing to Coral Bleaching in the Great Barrier Reef (GBR)
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Source: Phys.org
Coral bleaching in the Great Barrier Reef is a significant environmental concern, primarily driven by anthropogenic and natural stressors. The key contributing factors are as follows:
Temperature Stress
- Elevated sea surface temperatures lead to corals expelling symbiotic algae (zooxanthellae), resulting in loss of color (bleaching).
- Prolonged periods of above-average temperatures exert intense heat stress, weakening coral resilience.
- Although bleached corals are not immediately dead, they are prone to starvation, reduced growth, and increased disease vulnerability.
- If temperature anomalies persist, it can result in irreversible coral mortality.
Impact of Climate Change
- Global warming has caused a consistent rise in ocean temperatures, increasing the frequency and severity of bleaching events.
- El Niño events further amplify thermal stress, contributing to large-scale coral bleaching across tropical reefs.
- Climate-induced acidification and sea-level rise also undermine reef health and recovery capacity.
Other Environmental Stressors
- Sudden drops in sea temperature, though less common, can also lead to coral bleaching.
- Pollution and sediment runoff from coastal development and agriculture smother corals and degrade water quality.
- Extreme low tides expose corals to air and intense sunlight, causing thermal and desiccation stress.
Breakdown of Coral-Algae Symbiosis
- Corals maintain a mutualistic relationship with zooxanthellae, which perform photosynthesis and provide nutrients.
- Stressful conditions disrupt this symbiosis, depriving corals of essential nutrition and making them vulnerable to infection and disease.
Great Barrier Reef (GBR)
Location and Extent
- The Great Barrier Reef is the world’s largest coral reef system.
- Located in the Coral Sea, off the coast of Queensland, Australia.
- Stretches over 2,300 km, comprising approximately 3,000 individual reefs and 900 islands.
Biodiversity Significance
- Supports a diverse marine ecosystem:
- Around 400 species of coral
- Over 1,500 species of fish
- Habitat for endangered species such as:
- The dugong
- The large green turtle
Global Recognition
- Designated a UNESCO World Heritage Site in 1981.
- Recognized for its ecological significance, marine biodiversity, and natural beauty.
Threats and Conservation Concerns
- Despite its heritage status, the GBR faces serious threats from:
- Climate change
- Ocean warming
- Pollution and runoff
- The UNESCO Heritage Committee in 2023 did not list GBR as "in danger", but emphasized its critical risk status.
Coral Bleaching Events
- Widespread mass bleaching has occurred repeatedly due to rising sea temperatures:
- Notable years: 1998, 2002, 2016, 2017, 2020, 2022, and most recently in 2024.
- Bleaching reduces coral health and weakens the entire reef ecosystem.
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Implications of Coral Bleaching
Coral bleaching, caused primarily by rising sea temperatures, has far-reaching consequences across ecological, economic, and social dimensions.
Ecological Impact
- Coral reefs, often termed the "Rainforests of the Sea", support nearly 25% of all marine species.
- Bleaching events lead to the weakening and eventual death of corals, resulting in:
- Loss of biodiversity
- Collapse of reef-associated ecosystems
- Disruption of marine food chains
Economic Consequences
- Coral reefs contribute significantly to:
- Tourism (e.g., snorkeling and diving industries)
- Fisheries (which depend on reef ecosystems)
- Coastal protection from storm surges and erosion
- The estimated global value of goods and services provided by coral reefs is around USD 375 billion annually.
- Degradation of reefs due to bleaching adversely affects local and national economies, particularly in island and coastal nations.
Threat to Food Security
- Coral reefs support the livelihoods and nutrition of millions globally, especially in developing nations.
- Bleaching events reduce fish populations and affect marine harvests, leading to:
- Reduced access to affordable protein sources
- Disruption of small-scale and subsistence fishing
Climate Change Indicator
- Coral bleaching is a clear and early biological indicator of climate change.
- It reflects ocean warming, acidification, and broader environmental stress.
Loss of Ecosystem Services
- Coral reefs perform vital ecological functions such as:
- Shoreline protection against waves and storms
- Nutrient cycling and habitat support
- Carbon sequestration in marine systems
- Bleaching undermines these services, making coastal zones more vulnerable to climate risks.
Initiatives to Protect Coral Reefs
International Initiatives
Several global efforts are underway to protect coral ecosystems from climate change and human-induced threats:
- International Coral Reef Initiative (ICRI): A partnership among nations and organizations for the sustainable management of coral reefs globally.
- Global Coral Reef Monitoring Network (GCRMN): A global network focused on long-term monitoring of coral reef health and trends.
- Global Coral Reef Alliance (GCRA): A non-profit organization focused on coral reef preservation, restoration, and sustainable management.
- Global Coral Reef R&D Accelerator Platform: An initiative to accelerate scientific and technological innovations for coral reef protection.
Indian Government Initiatives
India has also undertaken multiple measures to protect coral reefs along its coastline:
- Coastal Zone Studies (CZS): The Ministry of Environment, Forest and Climate Change (MoEF&CC) includes coral studies under CZS to assess and conserve coastal and marine biodiversity.
- Biorock Technology Restoration: The Zoological Survey of India (ZSI), in collaboration with the Gujarat Forest Department, is using mineral accretion (biorock) technology to restore coral reefs.
- National Coastal Mission Programme: A government initiative aimed at conserving and sustainably managing coral reefs along India’s coastline.
Major Coral Reefs
Global Coral Reefs
- Great Barrier Reef (Australia): World's largest reef system; UNESCO World Heritage Site.
- New Caledonia Barrier Reef (South Pacific): Second-longest double barrier reef.
- Belize Barrier Reef (Caribbean): High biodiversity; threatened by tourism and climate events.
- Red Sea Coral Reef (Middle East): Naturally resistant to temperature rise.
- Mesoamerican Reef (Mexico to Honduras): Second-largest reef system globally.
Major Coral Reefs in India
- Gulf of Kachchh (Gujarat): Fringing reef.
- Gulf of Mannar (Tamil Nadu): Fringing reef; declared Biosphere Reserve.
- Lakshadweep Islands: Atoll reef system; vulnerable to sea-level rise.
- Andaman & Nicobar Islands: Fringing and barrier reefs; high biodiversity.
Legal and Policy Measures (India)
Laws and Regulations
- Coastal Regulation Zone (CRZ) Notification:
- Restricts activities that damage coral reefs.
- Designates development zones to protect sensitive areas.
- Wildlife Protection Act, 1972:
- Coral reefs protected under schedules covering marine biodiversity.
- Environment Protection Act, 1986:
- Empowers central government to protect ecologically sensitive zones.
- Marine Protected Areas (MPAs):
- Special areas for coral reef conservation (e.g., Gulf of Mannar Marine National Park).
- National Biodiversity Action Plan:
- Coral reef protection included as part of biodiversity conservation strategies.
- Integrated Coastal Zone Management (ICZM):
- Holistic management of coastal and marine ecosystems.
Important Marine Protected Areas
- Gulf of Mannar Marine National Park (Tamil Nadu)
- Mahatma Gandhi Marine National Park (Andaman & Nicobar)
- Rani Jhansi Marine National Park (Andaman & Nicobar)
- Malvan Marine Sanctuary (Maharashtra)
Barriers in Coral Reef Management
Systemic and Policy-Level Challenges
- Fragmented management across multiple government departments.
- Weak enforcement of CRZ and marine conservation laws.
- Lack of real-time monitoring, research, and data on coral health.
- Insufficient funding and capacity building for reef conservation.
Local-Level Issues
- Destructive fishing methods (e.g., blast and cyanide fishing).
- Unregulated tourism leading to reef trampling and boat anchor damage.
- Pollution from untreated sewage and industrial discharge.
- Coastal infrastructure and sand mining disrupting reef ecosystems.
- Climate change impacts: marine heatwaves, coral bleaching, ocean acidification.
- Weak community participation and limited awareness at the village level.
- Dependence on reef resources for livelihoods, leading to overexploitation.
Global Best Practices
Australia (Great Barrier Reef)
- Reef 2050 Long-Term Sustainability Plan.
- Coral restoration through nurseries and larval reseeding.
- Reef credit programs for carbon sequestration and conservation.
USA (Florida Keys)
- Coral gardening and reef restoration by NGOs and scientists.
- Strong regulatory oversight and community awareness programs.
Palau
- Marine spatial planning and total ban on harmful sunscreens.
- Community-based marine conservation and eco-tourism incentives.
Indonesia (Raja Ampat)
- Well-managed MPAs with support from local communities.
- Diversification of livelihoods to reduce reef pressure.
Philippines
- No-take zones and community-led monitoring.
- Legal backing for reef protection and local empowerment.
Way Forward
Ecological and Environmental Measures
- Prevent unplanned coastal development, which leads to habitat degradation.
- Promote sustainable fishing practices to reduce damage to reef ecosystems.
- Encourage ecotourism as an alternative livelihood to support conservation efforts.
Pollution Control Measures
- Minimize the use of non-degradable chemicals like synthetic fertilizers, insecticides, and pesticides which adversely affect marine life.
- Ensure proper treatment of industrial waste before discharge into water bodies.
- Avoid dumping oils and chemicals in oceans and rivers to reduce water pollution.
Climate Action Measures
- Implement and support policies aimed at mitigating climate change, as global warming is the most significant threat to coral reefs.
- Raise awareness and encourage international cooperation to limit carbon emissions and adopt climate-resilient conservation strategies.
Source: Down to Earth
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Practice Question
Q. Assess the impact of global warming on the coral life system with examples.
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Frequently Asked Questions (FAQs)
Coral bleaching occurs when corals expel the symbiotic algae (zooxanthellae) living in their tissues due to stress (mainly from rising sea temperatures), causing them to turn white and vulnerable.
- Rising sea surface temperatures (global warming)
- Ocean acidification
- Pollution and sedimentation
- Overexposure to sunlight (UV radiation)
- Unsustainable coastal development
Bleached corals are weak and more prone to disease. Prolonged bleaching can lead to coral death, impacting marine biodiversity and livelihoods dependent on reef ecosystems.
Coral bleaching has been reported in Gulf of Mannar, Lakshadweep, Andaman and Nicobar Islands, and Gulf of Kutch—all home to major coral reef systems in India.
