Marine ecosystems — AQA A-Level Geography
Test yourself on Marine ecosystems with AQA A-Level practice questions.
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Marine ecosystems explained
Coral reefs are highly biodiverse marine ecosystems primarily distributed between 30°N and 30°S of the equator, notably in the Indo-Pacific and Caribbean.
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Their main characteristics include complex calcium carbonate structures built by polyps in a symbiotic relationship with zooxanthellae algae, supporting immense marine life. Reef development requires specific environmental conditions: shallow water (typically less than 25 m deep) for light penetration essential for photosynthesis, warm water temperatures (optimally 23 °C to 29 °C), high salinity (32 to 42 parts per thousand), and clear, well-oxygenated water free from excessive sediment which can smother the polyps.
The following aspects should be examined with reference to a named, located coral reef: Factors in the health and survival of reefs: Natural: Water temperature, acidity, salinity, algal blooms.; Human activity and its impact: Major drainage basin schemes, onshore development, desalination, pollution, tourism, fishing.; Future prospects for coral reefs.
Using a named example like the Great Barrier Reef, students must evaluate factors affecting reef health. Natural threats include rising water temperatures causing bleaching, ocean acidification reducing calcium carbonate availability, salinity changes, and toxic algal blooms. Human impacts are severe: major drainage basin schemes and onshore development increase sediment runoff; desalination plants discharge hypersaline brine; pollution introduces toxins; tourism causes physical damage; and overfishing disrupts food webs. The future prospects of reefs depend on mitigating these local stressors alongside global climate action, as combined pressures threaten their long-term survival and biodiversity.
Your focus
- Map the global distribution of coral reef ecosystems.
- Describe the main biological and physical characteristics of coral reefs.
- Explain the specific environmental conditions required for coral reef development.
Show all 6 objectives
- Analyse the natural factors influencing the health and survival of a named coral reef.
- Evaluate the varied impacts of human activities, from onshore development to fishing, on reef ecosystems.
- Assess the future prospects of coral reefs in the context of combined natural and anthropogenic stressors.
Marine ecosystems exam tips
Marking Points
- Identify the global distribution of coral reefs, focusing on tropical and subtropical shallow waters between 30°N and 30°S.
- Describe the biological characteristics of reefs, including the symbiotic relationship between coral polyps and zooxanthellae.
- Explain the physical characteristics, such as the secretion of calcium carbonate to form complex, three-dimensional reef structures.
- Detail the environmental prerequisites for reef formation: temperatures of 23 °C to 29 °C, depths under 25 m, and salinities of 32 to 42 parts per thousand.
- Explain why clear, sediment-free water is necessary to allow sunlight penetration for algal photosynthesis.
- Apply knowledge of natural and human factors to a specific, named coral reef, such as the Great Barrier Reef or Andros Barrier Reef.
- Explain how natural factors like increased water temperature and acidity compromise coral health and calcification.
- Assess the impact of human activities, detailing how onshore development and drainage schemes increase sedimentation and nutrient runoff.
- Evaluate the localized effects of desalination brine, marine pollution, destructive fishing practices, and unmanaged tourism.
- Discuss the future prospects of the named reef, weighing current conservation strategies against ongoing global climate threats.
Examiner Tips
- 💡Use specific data when describing environmental conditions, such as the optimal temperature range of 23 °C to 29 °C.
- 💡Link the physical conditions directly to biological processes, for example, explaining how shallow water facilitates photosynthesis for zooxanthellae.
- 💡Be prepared to annotate a world map showing the distribution of major coral reef systems like the Great Barrier Reef.
- 💡Create a detailed case study fact file for your chosen reef, ensuring you have specific evidence for every human and natural factor listed in the specification.
- 💡When discussing future prospects, use a balanced approach that considers both pessimistic scenarios, such as continued warming, and optimistic interventions, such as marine protected areas.
- 💡Clearly distinguish between the impacts of major drainage basin schemes, which alter freshwater and sediment flow, and direct onshore development, such as coastal construction.
Common Mistakes
- Stating that corals are plants; correction: corals are marine animals (polyps) that host photosynthetic algae (zooxanthellae).
- Claiming reefs can grow at any depth; correction: reef-building corals require shallow water (usually under 25 m) because their symbiotic algae need sunlight.
- Confusing coral bleaching with coral death; correction: bleaching is the expulsion of zooxanthellae due to stress, which can lead to death if conditions do not improve, but the coral is not immediately dead.
- Discussing human impacts generally without linking them to the specific named reef; correction: always integrate specific case study details, such as agricultural runoff from the Fitzroy River affecting the Great Barrier Reef.
- Confusing ocean acidification with global warming; correction: acidification is caused by the ocean absorbing CO₂, lowering its pH, which impairs calcification, whereas warming causes thermal stress and bleaching.
- Overlooking the impact of desalination; correction: explicitly mention that desalination plants release hypersaline brine that sinks and creates toxic, oxygen-depleted zones on the reef.