Marine ecology
Use ecological terms and sampling methods to interpret open-ocean, shore, mangrove and coral-reef ecosystems and organism adaptations.
Section Topic 5 · Marine ecology
Exam scope and assessment notes
This original lesson maps Cambridge IGCSE Marine Science 0697 syllabus sections 5.1, 5.2, 5.3, 5.4, 5.5, 5.6, 5.7 for examinations in 2024–2029. These are original StudyForA explanations and practice, not Cambridge questions, mark schemes or official grades. Paper 2 assesses practical skills in both familiar and unfamiliar contexts. This course can rehearse planning, measurement, data handling and evaluation, but it cannot provide real laboratory or coastal-field experience, verify equipment use, certify safety, or award a practical mark. Follow teacher supervision, local safety rules and environmental permissions for all practical work.
Before you begin
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Before you start: Which method is best for estimating abundance in a habitat area?
What you will learn
- Define ecosystem, population, community, habitat and related ecological terms
- Use random quadrats and systematic transects appropriately
- Compare open-ocean zones and rocky or sedimentary shore conditions
- Explain mangrove and coral-reef adaptations and the factors affecting distribution
Ecosystem components and sampling
An ecosystem includes organisms and their physical environment. Population is one species in an area; a community is interacting populations; a habitat is where an organism lives. Population growth and distribution are affected by biotic and abiotic factors.
Random quadrats estimate abundance and species richness; line or belt transects examine distribution across a gradient. Measure shore profile, sediment particle size and moisture with suitable methods. Use adequate repeats and consistent placement; do not disturb protected organisms or infer an entire coastline from one small sample.
Open ocean and shore ecosystems
The pelagic zone is the open-water column; benthic is the seabed. The sunlight zone is about 0–200 m, twilight 200–1000 m and midnight below 1000 m. Light, oxygen, biomass, pressure and temperature differ with depth. Plankton drift in currents; phytoplankton are producers and zooplankton consumers.
Rocky shores have zones shaped by exposure, desiccation, wave action and competition. Sandy and muddy shores differ in sediment size, stability, slope, water movement and oxygen availability; organisms show adaptations such as burrowing. Explain distribution using named conditions and organism features.
Mangroves and coral reefs
Mangroves are often intertidal and estuarine. Across a tidal cycle, salinity, temperature and dissolved oxygen change. Pneumatophores aid gas exchange in waterlogged, low-oxygen soil; prop roots provide support. Other organisms have adaptations to tidal and saline conditions.
Coral polyps form colonies and may live with photosynthetic zooxanthellae in a mutualistic relationship. Temperature, light, clear water and hard substrate affect coral distribution. Reef water can be nutrient-poor because nutrients are retained in organisms. The syllabus includes reef biodiversity, organism adaptations and the Darwin–Dana–Daly model of atoll formation.
Sources for this lesson
Pause and explain
What does a transect add to repeated quadrats across a shore?
Worked example
A transect runs from the upper shore to the water. A barnacle species is common at the lower points and absent at the highest points. Suggest a measured abiotic factor and a sampling check.
Show the worked solution
- Measure exposure time or moisture at each point using a consistent method.
- Repeat quadrats at set intervals along the transect.
- Compare the pattern while noting other factors such as wave action and competition.
Answer Moisture or exposure time may help explain the pattern, but one factor may not be the only cause. Repeated quadrats at consistent intervals provide a clearer distribution pattern; other correlated factors should be considered.
Try a topic test
Choose Easy, Normal or Difficult. Save your answers and open worked review after finishing.
Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
A student wants to compare species richness between two rocky-shore heights. Design a fair quadrat sampling plan.
Give me a hint
Use the same quadrat area, effort and placement rule at both heights.
Compare my reasoning
- Define the upper and lower shore zones and use the same quadrat size.
- Place multiple quadrats randomly within each zone using a repeatable method.
- Record species present, calculate richness per quadrat and compare means or distributions.
Use equal-sized quadrats, random repeated placements and the same identification method at each height. Compare richness per quadrat and report sample size and variation; a small sample may miss rare species.
Look for these in your work
- Controls quadrat size and effort
- Uses random repeats
- Reports variation and sampling limits
A depth profile shows decreasing light and increasing pressure. Predict one likely change in photosynthetic production and explain why.
Give me a hint
Relate light availability to energy input for photosynthesis.
Compare my reasoning
- Identify the trend in light with depth.
- Connect light energy to photosynthesis by marine producers.
- Qualify the prediction because species, nutrients and water clarity also matter.
Photosynthetic production is likely to decrease as available light falls, especially below the sunlight zone. The exact pattern depends on species, nutrients and water clarity; a depth profile is needed to test it locally.
Look for these in your work
- Uses the light trend
- Links light to photosynthesis
- Recognises other factors
A proposed mangrove project measures tree height only. Suggest two additional measurements that better connect tidal conditions to organism distribution.
Give me a hint
Consider the changing water and soil conditions across a tidal cycle.
Compare my reasoning
- Measure salinity at defined times or tidal stages.
- Measure dissolved oxygen or soil moisture at matched locations.
- Record organism distribution using consistent quadrats or transects and note timing.
Salinity and dissolved oxygen or soil moisture can show the changing conditions mangroves experience. Pair these with repeated organism observations at known tidal stages; tree height alone cannot explain distribution.
Look for these in your work
- Measures relevant abiotic factors
- Controls location or timing
- Links conditions to organism observations
Common mistakes
- Using random quadrats to infer a gradient without a transect
- Treating correlation as proof of one environmental cause
- Calling every planktonic organism a producer
- Ignoring sampling area and repeat count
What can you explain now?
What does a transect add to repeated quadrats across a shore?
Compare with the explanation
It records how distribution changes along an environmental gradient
Use the scientific relationship, relevant evidence and correct units. State the limit of any conclusion when the evidence does not support a universal claim.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
Why should a survey combine an appropriate sampling method with repeated measurements?
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.