Why does overfishing of algae-eating fish harm coral reefs?

Algae-eating fish control algal growth. When overfished, algae grow unchecked and can smother corals, blocking light and competing for space. Reef resilience depends on balanced trophic structure; removing herbivores destabilises the ecosystem.

What is the difference between coral bleaching and coral death?

Bleaching occurs when corals expel zooxanthellae (turning white); they are stressed but not immediately dead. If conditions persist, they may die from lack of nutrition. Death is permanent; bleaching can be reversible if stress is removed and zooxanthellae recolonise.

How does blast fishing differ from overfishing in its impact on reefs?

Blast fishing causes immediate physical destruction of reef structure. Overfishing removes algae-eating fish, allowing algae to grow unchecked and smother corals. Both damage reefs but through different mechanisms: direct destruction vs trophic cascade.

What is the difference between local and global threats to coral reefs?

Local threats (pollution, overfishing, coastal development) can be addressed through catchment management, fishing restrictions, and sustainable tourism. Global threats (climate change, ocean acidification) require international action; local management alone cannot reverse them.

What error do students make when they say bleached corals are dead?

Bleached corals are stressed, not necessarily dead. They have expelled zooxanthellae and lack nutrition, but if conditions improve (e.g. temperature drops), zooxanthellae can recolonise and corals may recover. Confusing stress with death underestimates recovery potential.

Why is it wrong to focus only on climate change when discussing reef threats?

Pollution is the most significant human threat in many regions. Overfishing, coastal development, and sediment runoff also cause severe damage. Focusing solely on climate change ignores local threats that can be addressed through management and undervalues conservation efforts.

What mistake occurs if you assume all reef damage is irreversible?

Reefs can recover from natural impacts and some human damage if stresses are removed. Recovery takes time (years to decades). Assuming all damage is irreversible ignores management potential and may lead to undue pessimism about conservation strategies.

What is coral bleaching?

Coral bleaching occurs when stressed corals expel their symbiotic zooxanthellae algae. The coral turns white because the white calcium carbonate skeleton becomes visible. Without zooxanthellae, corals lose up to 90% of their nutrition and may die if stress persists.

What is ocean acidification and how does it affect reefs?

Ocean acidification is the decrease in pH caused by increased dissolved CO2. Corals need alkaline water to build calcium carbonate skeletons. Lower pH weakens skeletons, restricts growth, and makes reefs more vulnerable to erosion and physical damage.

What is thermal stress in the context of coral reefs?

Thermal stress occurs when water temperature exceeds the range corals can tolerate (typically above 29-30°C for warm-water species). It triggers bleaching as polyps expel zooxanthellae. Prolonged thermal stress leads to coral death and reef degradation.

Threats to Coral Reef Ecosystems | AQA A-Level Geography

1. Definition & Core Concepts

Coral bleaching occurs when stressed corals expel their symbiotic zooxanthellae algae, turning white. The algae provide up to 90% of coral nutrition. Bleached corals are not immediately dead but are severely stressed and may die if conditions persist.

Thermal stress from unusually warm water (e.g. El Niño, climate change) is the primary trigger for bleaching. Other triggers include changed salinity, light levels, cold water, and pollution.

Ocean acidification results from increased dissolved CO2. It lowers pH, weakens coral skeletons, and restricts growth rates. Corals need alkaline water to build calcium carbonate structures.

Human-maintained degradation: Unlike natural impacts that allow recovery, sustained human activities can create ongoing stress that prevents reef recovery.

Threats are often cumulative: reefs exposed to multiple stresses (e.g. pollution and warming) have reduced recovery capacity.

Diagram showing healthy coral with zooxanthellae versus bleached coral after stress expulsion

2. Human Activity & Coral Reefs

Pollution is the most significant anthropogenic threat. Sources include land-based runoff, dredging, coastal development, agricultural and deforestation activities, sewage, chemical and heavy metal discharge, and shipping (oil, waste, toxic hull paint).

Overfishing and destructive fishing: Blast fishing destroys reef structure with each blast; cyanide fishing stuns fish but kills corals. Overfishing of algae-eating fish allows algae to smother corals.

Plastic waste and sediment choke reef systems by suffocating corals and blocking light. Increased development in coastal areas worsens runoff.

Unsustainable tourism and coastal development: Touching or standing on reefs, construction near shore, and anchoring on reefs cause physical damage. Reefs thousands of years old can be damaged instantly.

Desalination: Plants discharge brine and chemical waste, altering salinity and water chemistry. Many reef regions depend on desalination for freshwater, creating localised stress.

3. Drainage Basin & Onshore Development

Drainage basin changes affect runoff into coral reefs. Forest clearance increases sediment, freshwater runoff, and land-based pollutants. These can cause reef die-off in downstream areas.

Dams alter the amount and timing of freshwater flow, disrupting ecosystems that rely on seasonal freshwater inputs for their life cycle.

Onshore development affects reefs directly (dredging for ship channels) and indirectly (mangrove clearance increases sediment and nutrients).

Too much sediment limits light for photosynthesis. Increased nutrients can be toxic and cause algal blooms that smother corals.

Key principle: Land-use changes in drainage basins have downstream marine impacts. Integrated management of catchment and coast is essential.

4. Natural Impacts & Climate Change

Natural impacts: Extreme rain (freshwater dilution), storm waves (physical damage), volcanic activity (ash smothering, magma), tidal changes (low tide exposes coral to UV and drying; prolonged high tide restricts light).

Marine predation: Crown-of-thorns starfish and other predators can damage reefs when populations explode. Natural impacts rarely destroy entire reefs; recovery usually occurs over time.

Climate change: Warming oceans cause thermal stress and bleaching. Rising sea levels increase sedimentation. Storm patterns change (stronger, more frequent). Altered currents affect food supply and larval dispersal.

Ocean acidification: Increased CO2 lowers pH, restricting growth and structural integrity of coral skeletons.

Cumulative stress: When reefs face continuous stress (natural or human), recovery capacity is overwhelmed and polyps die. Mass bleaching events are becoming more frequent, giving less time for recovery.

5. Key Distinctions

Threat Type	Examples	Mechanism
Direct physical	Anchoring, touching, blast fishing	Immediate structural damage
Water quality	Pollution, sediment, nutrients	Light blocked, toxicity, algal blooms
Climate	Warming, acidification	Bleaching, weakened skeletons
Biological	Overfishing algae-eaters	Algae smother corals

Local vs global threats: Pollution (local) can be addressed by catchment management; climate change (global) requires international action. Both contribute to reef decline.

Acute vs chronic: Blast fishing causes acute damage; chronic pollution and warming cause gradual decline. Combined effects are often synergistic.

6. Management & Future Prospects

Restriction of fishing: Limit catches, especially protected species. Commercial fishing fees can fund research, education, and reef management.

Tourism: Ecotourism can support conservation; reef monitoring and reporting by tourists can aid management. Sustainable practices (no touching, no anchoring on reef) reduce damage.

Water quality: Governments, farmers, and industry working together to limit pollutants from land. Reef trusts can improve water quality and habitats.

Education: Local communities monitoring and protecting reefs while maintaining livelihoods. Reporting changes enables early intervention.

Limitation: Local management is effective but does not tackle global climate change and sea-level rise. Evaluation should include both positive conservation strategies and ongoing challenges.

Exam tip: When evaluating future prospects, include both threats and management strategies. Do not focus only on negatives.

7. Exam Strategy & Tips

Always distinguish threat types: Physical (anchoring, blast), chemical (pollution, acidification), biological (overfishing, predation).
Link causes to mechanisms: e.g. warming → thermal stress → bleaching → zooxanthellae expelled → coral death.
Include management when asked to evaluate: restriction of fishing, pollution control, education, reef trusts.
Case study: Great Barrier Reef for bleaching events and catchment management (e.g. Queensland runoff).
Balance: Acknowledge that local conservation strategies can be effective but do not address global climate change.

8. Connections & Extensions

Links to Coral Reef Ecosystems (anatomy, zooxanthellae, environmental conditions) and climate change (ocean warming, acidification, storm patterns).

Drainage basin management connects to fluvial geomorphology and land-use planning.

Threats to Coral Reef Ecosystems

Summary

1. Definition & Core Concepts

2. Human Activity & Coral Reefs

3. Drainage Basin & Onshore Development

4. Natural Impacts & Climate Change

5. Key Distinctions

6. Management & Future Prospects

7. Exam Strategy & Tips

8. Connections & Extensions

Threats to Coral Reef Ecosystems

Summary

1. Definition & Core Concepts

2. Human Activity & Coral Reefs

3. Drainage Basin & Onshore Development

4. Natural Impacts & Climate Change

5. Key Distinctions

6. Management & Future Prospects

7. Exam Strategy & Tips

8. Connections & Extensions