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examBoard: Cambridge
examType: IGCSE
lessonTitle: Threats to Coral Reefs
Coral reefs are among the most diverse and valuable ecosystems on Earth. Often called the "rainforests of the sea," they occupy less than 1% of the ocean floor but support about 25% of all marine species. These complex underwater structures are built by tiny animals called coral polyps that secrete calcium carbonate to form protective skeletons.
Key Definitions:
Fringing reefs: Grow near the coastline around islands and continents.
Barrier reefs: Separated from land by a lagoon; the Great Barrier Reef is the largest example.
Atolls: Circular or oval coral reefs that encircle a lagoon but not an island.
Biodiversity: Home to thousands of marine species.
Coastal protection: Act as natural barriers against waves and storms.
Economic value: Support fishing, tourism and provide medicines.
Cultural significance: Important to many coastal communities worldwide.
Despite their importance, coral reefs face numerous threats that have led to the degradation and loss of reef systems worldwide. The combination of global climate change and local human activities has placed immense pressure on these fragile ecosystems.
Rising sea temperatures cause coral bleaching events.
Ocean acidification makes it harder for corals to build their calcium carbonate skeletons.
Sea level rise affects the amount of light reaching corals.
Stronger storms physically damage reef structures.
Agricultural runoff containing fertilisers causes algal blooms.
Sewage discharge introduces harmful bacteria and nutrients.
Plastic pollution entangles corals and blocks sunlight.
Chemical pollutants from industry disrupt coral reproduction.
Overfishing disrupts reef food webs and ecological balance.
Destructive fishing (dynamite, cyanide) directly damages corals.
Coastal development increases sedimentation and pollution.
Tourism impacts including boat anchors and touching corals.
Coral bleaching is one of the most visible and widespread threats to coral reefs globally. It occurs when corals expel the colourful algae (zooxanthellae) living in their tissues due to stress, causing them to turn completely white. While bleached corals aren't dead, they are under severe stress and at risk of mortality if conditions don't improve.
Corals have a symbiotic relationship with microscopic algae called zooxanthellae. The algae provide the coral with food through photosynthesis and give corals their vibrant colours. When corals experience stress primarily from increased water temperatures this relationship breaks down and the corals expel the algae, revealing their white calcium carbonate skeletons.
1. Stress trigger: Usually a 1-2°C increase in water temperature for several weeks.
2. Symbiosis breakdown: The coral-algae relationship becomes damaged.
3. Expulsion: Corals expel the zooxanthellae from their tissues.
4. Bleaching: Corals turn white as their transparent tissues reveal the skeleton.
5. Recovery or death: If conditions improve quickly, corals can recover; if not, they die.
Loss of biodiversity: As corals die, the habitat for thousands of species disappears.
Reduced coastal protection: Dead reefs provide less protection against waves and storms.
Economic losses: Fishing and tourism industries suffer when reefs degrade.
Food security threats: Many coastal communities depend on reefs for protein.
Australia's Great Barrier Reef has experienced several mass bleaching events in recent years:
These back-to-back bleaching events have given corals little time to recover between episodes, leading to significant declines in coral cover. Scientists estimate that the Great Barrier Reef has lost more than 50% of its coral cover since the 1980s.
As atmospheric CO₂ levels rise, about 30% is absorbed by the oceans, causing a chemical reaction that produces carbonic acid. This process lowers the pH of seawater (making it more acidic) and reduces the availability of carbonate ions that corals need to build their skeletons.
Ocean acidification affects corals in several ways:
Poor land management practices, deforestation and coastal development can lead to increased sediment runoff into coastal waters. This sedimentation has serious consequences for coral reefs:
Smothering: Sediment physically covers corals, preventing photosynthesis.
Reduced light: Murky water blocks sunlight needed by zooxanthellae.
Energy drain: Corals must use energy to remove sediment instead of growing.
Recruitment failure: Sediment prevents coral larvae from settling on suitable surfaces.
Deforestation: Removes vegetation that would otherwise hold soil in place.
Agriculture: Ploughing and irrigation increase soil erosion.
Coastal development: Construction disturbs soil and increases runoff.
Dredging: Directly stirs up seafloor sediments near reefs.
Despite the serious threats facing coral reefs, there are numerous conservation and management strategies being implemented worldwide to protect and restore these valuable ecosystems.
Marine Protected Areas (MPAs) limit or prohibit harmful activities.
Fishing regulations prevent overfishing and destructive practices.
Water quality standards reduce pollution reaching reefs.
Tourism guidelines promote responsible visitor behaviour.
Coral gardening: Growing coral fragments in nurseries then transplanting them to degraded reefs.
Artificial reefs: Creating structures for coral larvae to settle on.
Assisted evolution: Developing more heat-resistant coral strains.
Larval seeding: Collecting and releasing coral larvae in targeted areas.
Climate agreements to reduce greenhouse gas emissions.
International cooperation on reef conservation.
Sustainable Development Goals addressing ocean health.
Research funding for coral reef science and solutions.
The Belize Barrier Reef, the second-largest coral reef system in the world, was added to UNESCO's List of World Heritage in Danger in 2009 due to mangrove cutting, excessive coastal development and the threat of oil exploration.
Through concerted conservation efforts, Belize has achieved significant progress:
As a result of these efforts, in 2018, UNESCO removed the Belize Barrier Reef from the endangered list, making it one of the few marine sites to be removed from this list. This success story demonstrates how coordinated conservation actions can help protect and restore threatened coral reef ecosystems.
The future of coral reefs depends largely on how quickly and effectively we address both global and local threats. While the outlook is concerning, there are reasons for hope:
For coral reefs to survive in the long term, we need a dual approach: addressing climate change at the global level while reducing local stressors like pollution, overfishing and destructive coastal development. Everyone from governments and businesses to local communities and individual tourists has a role to play in protecting these invaluable ecosystems for future generations.
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