10.1.5 — Agricultural Runoff & Deoxygenation
Papers 3 & 4 Examination Practice
Examiner's Instructions
- • Answer all questions. Read each scenario carefully before answering.
- • Use precise Cambridge keyword terminology — vague answers lose marks.
- • Click "Reveal Answer" to check your response after attempting each question.
- • Click "Mark Scheme" to see the exact examiner point allocation.
- • ⚠️ Examiner Traps are shown in amber — these are the most common errors on this topic.
The Chemicals
5 questions · 10 marks
Name two types of chemical compounds from fertilisers that commonly wash into rivers and lakes.
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⚠️ Examiner Trap
Do not write 'nitrogen' alone — the examiner wants the dissolved ionic form: nitrates. Nitrogen gas (N₂) does not dissolve in river water.
Name one common household source of phosphates that can enter rivers.
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⚠️ Examiner Trap
Students often forget non-agricultural sources. Fertilisers are not the only phosphate source — detergents from household waste are equally important.
State two physical properties of nitrates that make them particularly dangerous as water pollutants.
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⚠️ Examiner Trap
Students confuse solubility with reactivity. Nitrates are dangerous BECAUSE they are soluble — they spread everywhere in water.
Explain why both nitrates AND phosphates are needed together for algal bloom to occur, rather than just one alone.
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⚠️ Examiner Trap
Do not simply say 'they are both nutrients.' Explain the specific biological role of each — nitrogen for proteins, phosphorus for DNA/energy.
State two conditions, other than the presence of nitrates and phosphates, that encourage algal bloom in a lake.
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⚠️ Examiner Trap
Students only name chemical factors. Remember: algae also need physical conditions — warmth and sunlight are equally important triggers.
The Consequence
5 questions · 12 marks
What is the main environmental consequence of excess nitrates and phosphates entering a lake?
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⚠️ Examiner Trap
Do not write 'pollution' as your answer — it is too vague. The specific term is DEOXYGENATION. This is the word that earns the mark.
Describe the sequence of events, from fertiliser entering a lake to deoxygenation of the water. Use the terms: algae, microorganisms, aerobic respiration, decomposition.
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⚠️ Examiner Trap
Many students stop at 'algae grow' and forget the crucial middle steps. The OXYGEN is not removed directly by algae — it is removed by BACTERIA decomposing dead algae. This is the most common error on Paper 4.
Explain why dissolved oxygen levels in a lake DECREASE during the decomposition stage of eutrophication.
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⚠️ Examiner Trap
Students say 'algae use up oxygen.' Wrong! Algae PRODUCE oxygen during photosynthesis. It is the DECOMPOSING BACTERIA that consume the oxygen — a crucial distinction.
Predict what happens to dissolved oxygen levels as algae levels INCREASE during early bloom. Explain your answer.
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⚠️ Examiner Trap
This is a counterintuitive question. Students assume oxygen always drops during bloom — but in the EARLY stage, actively photosynthesising algae actually produce oxygen. The crash comes LATER during decomposition.
Suggest and explain one economic argument FOR and one argument AGAINST restricting phosphates in detergents.
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⚠️ Examiner Trap
Paper 4 'suggest' questions require REASONING, not just a one-word answer. Always link your point to a consequence: 'X happens, THEREFORE Y results economically.'
The Victim
5 questions · 13 marks
Explain how the deoxygenation of water causes the death of fish.
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⚠️ Examiner Trap
Do not say fish 'drown' — fish do not breathe air, so they cannot drown. The correct term is that fish SUFFOCATE due to lack of dissolved oxygen in the water.
State two types of aquatic organisms (other than fish) that would be killed by deoxygenation.
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⚠️ Examiner Trap
Some bacteria SURVIVE deoxygenation — anaerobic bacteria actually thrive without oxygen. Do not say 'all organisms die.' The question specifies aerobic aquatic organisms.
Explain how the deoxygenation of a lake could affect organisms that live OUTSIDE the water. Use the term 'food chain' in your answer.
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⚠️ Examiner Trap
Students answer only about what happens IN the water. This question specifically asks about organisms OUTSIDE the water — think about what eats the fish: birds, otters, humans who fish commercially.
State two ways in which eutrophication could affect the quality of drinking water taken from the lake.
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⚠️ Examiner Trap
Think beyond just 'it looks bad.' Examiners want specific impacts: nitrate poisoning, algal toxins, treatment costs — concrete consequences, not vague 'it's polluted.'
A student claims: 'The real killer in eutrophication is the algae — they use up all the oxygen.' Evaluate this claim and write a corrected, scientifically accurate explanation of what actually removes the oxygen from the water.
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⚠️ Examiner Trap
This is the #1 examiner trap for this topic. The most commonly confused point in the entire eutrophication topic. The agent of deoxygenation is BACTERIA (during decomposition), NOT algae (which produce oxygen). If you write 'algae remove the oxygen' you will score ZERO for this question.
Eutrophication — The Golden Chain (Must Memorise)
🎯 The key distinction: BACTERIA remove the oxygen, not algae. Algae produce oxygen. Remember this and you will not fall into the #1 examiner trap.
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