Human impacts and marine conservation
Trace pollution and climate processes through ecosystems, then evaluate conservation choices against evidence and feasibility.
Section Topic 9 · Human impacts and marine conservation
Exam scope and assessment notes
This lesson maps Cambridge International AS & A Level Marine Science 9693 sections 9.1, 9.2, 9.3, 9.4 for examinations in 2025–2030. AS candidates study Topics 1–5; A Level candidates study all nine topics and are expected to know the AS content. These are original StudyForA explanations and practice, not Cambridge questions, mark schemes or grades. The syllabus requires core practical activities and scientific-method experience. Online explanation and simulated data questions can rehearse methods but do not count as completing practical work or demonstrate safe, competent use of equipment. Learners need suitable teacher-supervised laboratory and field opportunities.
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 process describes increasing toxin concentration at higher trophic levels?
What you will learn
- Explain impacts of industries, agriculture, waste, fishing, desalination and renewable energy
- Trace toxin bioaccumulation, biomagnification and plastic pathways
- Explain global warming, ocean circulation, acidification and effects on calcifying organisms
- Evaluate local, regional and global conservation strategies and constraints
Pollution and ecological impacts
Oil, agricultural run-off, renewable installations, sewage, refuse, desalination brine and fishing methods can affect water quality, habitat, organisms and food webs. Bioaccumulation is toxin build-up within organisms; biomagnification is increasing concentration across trophic levels. Microplastics are under 5 mm and may be primary or formed from larger non-biodegradable plastics broken by UV, wind and waves. They can be eaten by plankton, transfer through food chains, carry toxins and cause ingestion or entanglement. Prevention, waste control and product redesign address sources differently from clean-ups.
Warming and ocean acidification
Warming can raise sea level through thermal expansion and land-ice melt, shift species ranges, disrupt currents, stratify water and stress reefs. Separate evidence for global warming from local short-term variability. Ocean acidification occurs as atmospheric CO2 dissolves; it forms hydrogen ions and hydrogen carbonate, lowers pH and reduces carbonate-ion availability. This can impair hard-coral and shelled-organism calcification. A shell mass investigation must control shell type, surface area, solution pH, volume, temperature and time.
Conservation decisions
Conservation aims to maintain or enhance biodiversity. The IUCN Red List can help prioritise, but project viability also depends on local evidence, costs, enforcement, social effects and timescale. Options include marine protected and no-take areas, captive breeding/release, local and global legislation such as CITES and the IWC moratorium, UNESCO biosphere reserves, marine zoos/aquaria, ecotourism and invasive-species control. Marine ecosystems cross national borders, so international cooperation matters but may be incomplete. Evaluate proposals against stated data and acknowledge uncertainty.
Sources for this lesson
Pause and explain
Why can lower ocean pH harm a calcifying organism?
Worked example
A shell experiment places equal masses of empty mollusc shells in solutions at pH 8.0 and 6.5. The lower-pH group loses more mass. Explain the result and give two controls.
Show the worked solution
- More dissolved CO2 can increase hydrogen-ion concentration and lower pH.
- Lower pH reduces carbonate-ion availability and can dissolve calcium-carbonate shell material.
- Control shell type/surface area, starting mass, solution volume, temperature and exposure time.
Answer The lower pH solution is more acidic, which can reduce carbonate availability and increase loss of calcium-carbonate shell material. Keep shell species and surface area, initial mass, solution volume, temperature and time comparable; repeats and measured pH improve reliability.
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 toxin is measured at 0.02 mg kg−1 in plankton, 0.3 mg kg−1 in small fish and 2.4 mg kg−1 in a predator. Describe the pattern and name one reason to verify before inferring biomagnification.
Give me a hint
Compare trophic positions, then check sampling, diet and concentration units.
Compare my reasoning
- Describe increasing measured concentration at higher trophic levels.
- Check that organisms and concentrations are comparable on the same basis.
- Consider diet, age, exposure and sample size as alternative influences.
The measured concentration rises from plankton to small fish to predator, consistent with biomagnification. To support that inference, confirm comparable units and tissues, adequate samples and trophic positions; age, exposure and diet can also affect concentration.
Look for these in your work
- Describes the trend accurately
- Uses trophic-level reasoning
- Checks comparability and alternatives
Explain the chemical link between rising atmospheric CO2 and reduced carbonate availability in seawater.
Give me a hint
Trace dissolved CO2 to hydrogen ions and hydrogen carbonate; connect to pH.
Compare my reasoning
- State that atmospheric CO2 dissolves into seawater.
- Explain that reactions produce hydrogen ions and hydrogen carbonate.
- Relate increased hydrogen ions to lower pH and reduced carbonate ions.
More atmospheric CO2 can dissolve in the ocean and react with water, increasing hydrogen ions and hydrogen carbonate. Hydrogen ions lower pH and shift carbonate chemistry, reducing carbonate-ion availability for organisms that build calcium-carbonate shells or skeletons.
Look for these in your work
- Traces CO2 into seawater
- Includes hydrogen ions and hydrogen carbonate
- Connects pH to carbonate availability
A coastal authority considers a no-take marine protected area. Identify one ecological benefit, one possible social cost and a monitoring design to evaluate it.
Give me a hint
Use a before/after comparison and a comparable site outside the protected area.
Compare my reasoning
- Describe how reduced extraction may allow populations or habitat to recover.
- Identify effects on fishing access or local livelihoods.
- Compare repeated ecological and social measures inside and outside before and after designation.
Reduced extraction may help stocks or habitat recover, while restrictions can affect fishing income and food access. A before-after-control-impact design with comparable protected and unprotected sites, repeated stock/habitat measures and livelihood data can help evaluate outcomes; enforcement and spillover also matter.
Look for these in your work
- Explains a plausible ecological benefit
- Acknowledges social cost
- Proposes a comparison over time
Common mistakes
- Confusing bioaccumulation within one organism with biomagnification along a food chain
- Calling every plastic fragment biodegradable
- Treating ocean warming and acidification as the same process
- Claiming a conservation project works without feasibility and outcome evidence
What can you explain now?
Why can lower ocean pH harm a calcifying organism?
Compare with the explanation
Hydrogen ions reduce carbonate availability needed for calcium-carbonate structures
State the mechanism, use relevant evidence and include suitable units. Explain what the evidence cannot establish.
After trying it yourself, choose your next review. This is your self-assessment.
Your review choice appears on Today. Sign in to sync it across devices.
Make it stick
How are ocean acidification and global warming linked to CO2 but different in their main mechanism?
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.