Why is "Saying 'the oxygen is used up' without saying what uses it up" a common mistake in Biological Consequences of Water Pollution?

Why it happens: Students learn the outcome (fish die) but not the cause, so they skip the microbe step. How to avoid it: Always state that the **microbes/decomposers respire** as they decompose the waste and **use up the dissolved oxygen** — the respiration link is the marking point. Source: Edexcel Biology examiner reports

Why is "Putting the eutrophication stages in the wrong order (oxygen used up before the algal bloom)" a common mistake in Biological Consequences of Water Pollution?

Why it happens: Students remember the steps but not the sequence, so they list them randomly. How to avoid it: Learn the fixed order: **algal bloom → blocks light → plants die → decomposers multiply → oxygen used up → fish die**. Source: Edexcel Biology examiner reports 2022–2024

Why is "Writing 'chemicals' or 'nutrients' instead of naming nitrate and phosphate" a common mistake in Biological Consequences of Water Pollution?

Why it happens: Students know fertiliser is involved but forget the exact minerals. How to avoid it: Name the two minerals specifically: **nitrate and phosphate**, leached from **fertiliser**. Source: Edexcel Biology examiner reports

Why is "Saying the algae (not the decomposers) use up the oxygen" a common mistake in Biological Consequences of Water Pollution?

Why it happens: The algal bloom is the obvious feature, so students assume the algae cause the oxygen loss directly. How to avoid it: The oxygen is used up by the **decomposers** that **respire** while breaking down the **dead plants and algae** — not by the living algae themselves. Source: Edexcel Biology examiner reports

Why is "Forgetting that sewage also spreads disease" a common mistake in Biological Consequences of Water Pollution?

Why it happens: Students focus on the oxygen/fish point and miss the second consequence. How to avoid it: Remember sewage has **two** consequences: it **lowers oxygen (fish die)** AND it **spreads disease** (it contains pathogens). Source: Edexcel Biology examiner reports

Why is "Saying the algal bloom 'kills the plants' without explaining the light/photosynthesis link" a common mistake in Biological Consequences of Water Pollution?

Why it happens: Students compress the step and skip the reason the plants die. How to avoid it: Explain that the bloom **blocks light**, so plants **cannot photosynthesise** and therefore die — the light and photosynthesis words are credited. Source: Edexcel Biology examiner reports

Ecology and the EnvironmentEdexcel IGCSE Science(Double Award)-Biology (4WSD0-1B)

Biological Consequences of Water Pollution

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Biological Consequences of Water Pollution — Edexcel International GCSE Science (Double Award) Biology (4WSD0-1B) Study Notes

Two of the most common ways humans pollute water are sewage and fertiliser run-off. Learn how raw sewage adds nutrients and microbes that use up dissolved oxygen (so fish die) and spread disease, and how leached nitrate and phosphate from fertilisers cause eutrophication: an algal bloom blocks light, plants die, decomposers multiply and use up the oxygen, and aquatic organisms suffocate. Covers Double Award Biology spec 4.16 and 4.17, assessed on Biology Paper 1 (4WSD0/1B).

What you’ll learn

Mapped to the Edexcel IGCSE 4SD0 syllabus (2026 onwards).

4.16 — Understand the biological consequences of pollution of water by sewage, including the role of microorganisms in decomposing waste, the consequent reduction of oxygen in the water, and the spread of disease.
4.17 — Understand the biological consequences of eutrophication caused by leached minerals from fertiliser.

Water pollution by sewage (spec 4.16)

Sewage adds nutrients and microbes that use up dissolved oxygen and spread disease.

Sewage is the waste water that drains from homes, farms and factories. It contains human waste, food remains and other organic matter, and it carries large numbers of microbes (microorganisms).

When raw, untreated sewage is released into a river, lake or sea, it pollutes the water in two important biological ways.

1. It uses up the dissolved oxygen.

Sewage adds plenty of nutrients (organic matter) and microbes to the water.
These microbes feed on and decompose the sewage.
As they decompose it they respire, and respiration uses up the dissolved oxygen in the water.
With far less dissolved oxygen available, aquatic organisms such as fish cannot respire and die.

2. It spreads disease.

Sewage often contains pathogens (disease-causing microbes) from human waste.
If this polluted water is drunk or used, it can spread diseases such as cholera and typhoid.

So a single source — raw sewage — both suffocates aquatic life (by removing oxygen) and spreads disease (by adding pathogens).

Exam tip. Many students write "the oxygen is used up" without saying why. The marking point is that microbes/microorganisms respire as they decompose the sewage and use up the dissolved oxygen. Always link the microbes to the oxygen loss.

Sewage adds nutrients and microbes to the water.
Microbes decompose the sewage and respire, using up dissolved oxygen.
Low oxygen → fish and other aquatic organisms die; sewage also spreads disease.

See the full worked example for biological consequences of water pollution →

What causes eutrophication (spec 4.17)

Nitrate and phosphate leached from fertiliser run into water and over-enrich it.

Farmers spread fertilisers on their fields to add minerals — especially nitrate and phosphate — so crops grow well. The problem comes when it rains.

Rainwater leaches (washes) the soluble nitrate and phosphate minerals out of the soil.
This mineral-rich water runs off into nearby rivers and lakes.
The water now contains far more nitrate and phosphate than normal.

This over-enrichment of the water with minerals is the starting point of eutrophication. The word simply means a body of water has become "well-nourished" with minerals — but as the next section shows, this is bad news for the living things in it.

Exam tip. The minerals named in the specification are nitrate and phosphate, and the source is fertiliser. Saying just "chemicals" or "pollution" will not score — name the minerals and link them to fertiliser leaching/run-off.

Fertilisers contain nitrate and phosphate minerals.
Rain leaches these minerals out of the soil into rivers and lakes (run-off).
The water becomes over-enriched with minerals — the start of eutrophication.

See the full worked example for biological consequences of water pollution →

The eutrophication sequence (spec 4.17)

Algal bloom → blocks light → plants die → decomposers use up oxygen → organisms die.

Once the water is rich in nitrate and phosphate, a chain of events follows. You must be able to put these steps in the correct order.

The extra minerals let algae grow very rapidly, forming an algal bloom that covers the surface of the water.
The algal bloom blocks light from reaching the plants growing below the surface.
Without light the underwater plants cannot photosynthesise, so they die.
Decomposers (microbes/bacteria) multiply to break down the large amount of dead plants (and dead algae).
As the decomposers respire they use up the dissolved oxygen in the water.
With little oxygen left, fish and other aquatic organisms die.

Here is the sequence shown as a diagram:

Eutrophication: nitrate and phosphate from fertiliser cause an algal bloom that blocks light; plants die, decomposers multiply and use up the dissolved oxygen, so fish and other organisms die.

Exam tip. The two stages that earn the most marks are "algal bloom blocks light so plants die" and "decomposers use up the dissolved oxygen so organisms die". Keep the order right — the algae block light before the decomposers cause the oxygen drop.

Algal bloom → blocks light → underwater plants die.
Decomposers multiply → respire → use up dissolved oxygen.
Low oxygen → fish and other aquatic organisms die.

See the full worked example for biological consequences of water pollution →

The common factor: dissolved oxygen

Both sewage and eutrophication kill aquatic life by lowering dissolved oxygen.

Notice that sewage pollution and eutrophication are different at the start but end the same way.

	Sewage (4.16)	Eutrophication (4.17)
Pollutant added	Sewage (nutrients + microbes)	Nitrate & phosphate from fertiliser
First effect	Microbes decompose the sewage	Algal bloom blocks light, plants die
Why oxygen drops	Microbes respire as they decompose	Decomposers respire as they break down dead plants
Final result	Fish die; disease spread	Fish and other organisms die

In both cases, microbes/decomposers respire and use up the dissolved oxygen, so aquatic organisms cannot respire and die. This is the idea examiners reward most often.

The amount of oxygen used by microbes to decompose pollution in water is sometimes called the biochemical oxygen demand (BOD) — the more organic pollution there is, the more oxygen the microbes use, and the lower the oxygen available for fish. You do not have to name BOD, but understanding it helps you explain why polluted water has so little oxygen.

Exam tip. If a question simply asks "explain why fish die in polluted water", the safe answer for both sewage and eutrophication is: microbes decompose the organic matter, respire and use up the dissolved oxygen, so the fish cannot respire and die.

Both pathways end with microbes using up dissolved oxygen.
Fish die because there is too little dissolved oxygen to respire.
Sewage adds a second harm — it also spreads disease.

See the full worked example for biological consequences of water pollution →

How it’s examined

Assessed on Biology Paper 1 (4WSD0/1B), 2 hours, 110 marks, untiered (Double Award has no separate Biology Paper 2). Common question shapes: 'state two effects of sewage on a river' (2 marks), 'explain why fish die when sewage enters water' (3 marks), 'put the stages of eutrophication in order' (3–4 marks), 'name the minerals that cause eutrophication' (1–2 marks), and an extended 'describe how leached fertiliser leads to the death of fish in a lake' (5–6 marks). Reliable marks come from the keywords nutrients and microbes, decompose, respire / use up dissolved oxygen, fish die, spread disease, nitrate and phosphate, leached from fertiliser, algal bloom, blocks light, plants die, decomposers multiply and lack of oxygen.

Sources: Pearson Edexcel International GCSE Science (Double Award) 4SD0 specification — Issue 4, September 2024 (valid for 2026 onwards); Pearson Edexcel International GCSE Biology 4BI1 specification — Issue 4 (shared Biology Paper 1 content); Pearson Edexcel 4SD0 / 4BI1 examiner reports 2022–2024. Last reviewed 2026-06-04.

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Step-by-step worked examples — Biological Consequences of Water Pollution

Step-by-step solutions to past-paper-style questions on biological consequences of water pollution, written exactly the way a tutor would explain them at the board.

1Why sewage kills fish (Getting started)
Getting started• 4WSD0/1B style — short answer• sewage, oxygen, microbes, spec-4.16
Question
Raw sewage is released into a river. (a) What do microbes do to the sewage? (b) Which gas in the water is used up as a result? (c) State one consequence for the fish. (3 marks)
Step-by-step solution
1. Step 1
  (a) (1). The microbes decompose (break down) the sewage.
2. Step 2
  (b) (1). The dissolved oxygen in the water is used up (as the microbes respire).
3. Step 3
  (c) (1). With too little oxygen, the fish die (cannot respire).
Answer
(a) Microbes decompose the sewage. (b) Dissolved oxygen. (c) The fish die because there is not enough oxygen to respire.
Examiner tip
Part (b) must say 'dissolved oxygen' — just 'oxygen' is usually accepted, but 'air' is not. Link the oxygen loss to the microbes respiring.
2Name the eutrophication minerals (Getting started)
Getting started• 4WSD0/1B style — short answer• eutrophication, nitrate, phosphate, fertiliser, spec-4.17
Question
(a) Name the two minerals leached from fertiliser that cause eutrophication. (b) How do these minerals get from a field into a river? (2 marks)
Step-by-step solution
1. Step 1
  (a) (1). The minerals are nitrate and phosphate.
2. Step 2
  (b) (1). Rain leaches them out of the soil and they run off into the river.
Answer
(a) Nitrate and phosphate. (b) Rainwater leaches them out of the soil and they run off into the river.
Examiner tip
Both minerals are needed for the first mark. 'Chemicals' or 'nutrients' alone is too vague — examiners want nitrate and phosphate named.
3Put eutrophication in order (Building confidence)
Building confidence• 4WSD0/1B style — structured• eutrophication, algal-bloom, order, spec-4.17
Question
The stages of eutrophication are listed in the wrong order: plants die, decomposers use up oxygen, algal bloom blocks light, fish die. Write them in the correct order. (3 marks)
Step-by-step solution
1. Step 1
  Start with the algae (1). Extra minerals first cause an algal bloom that blocks light.
2. Step 2
  Light is blocked (1). Without light the plants die (cannot photosynthesise).
3. Step 3
  Oxygen runs out (1). Decomposers use up the oxygen breaking down the dead plants, so the fish die.
Answer
Algal bloom blocks light → plants die → decomposers use up oxygen → fish die.
Examiner tip
The order mark is lost if the oxygen loss is placed before the algal bloom, or if 'plants die' and 'algae bloom' are swapped. The algae must block the light first.
4Sewage and the spread of disease (Building confidence)
Building confidence• 4WSD0/1B style — structured• sewage, disease, pathogens, spec-4.16
Question
A village uses river water for drinking. Upstream, untreated sewage enters the river. (a) Explain why this could make the villagers ill. (b) Apart from disease, state one other biological consequence of the sewage on the river. (3 marks)
Step-by-step solution
1. Step 1
  (a) pathogens (1). Sewage contains pathogens / disease-causing microbes from human waste.
2. Step 2
  (a) spread (1). Drinking the contaminated water spreads disease (e.g. cholera/typhoid) to the villagers.
3. Step 3
  (b) (1). Microbes decompose the sewage and use up the dissolved oxygen, so fish die.
Answer
(a) The sewage contains pathogens (disease-causing microbes) from human waste, so drinking the water spreads diseases such as cholera. (b) Microbes decompose the sewage and use up the dissolved oxygen, so fish die.
Examiner tip
Two separate consequences of sewage are credited here: spreading disease (pathogens) AND lowering oxygen (microbes respiring). Keep them distinct.
5Explain death of fish from fertiliser run-off (Stretch)
Stretch• 4WSD0/1B style — extended• eutrophication, fertiliser, oxygen, spec-4.17
Question
Fertiliser is spread on a field next to a lake. A few weeks later, many fish in the lake are found dead. Explain, in full, how the fertiliser caused the fish to die. (5 marks)
Step-by-step solution
1. Step 1
  Minerals enter (1). Rain leaches nitrate and phosphate from the fertiliser into the lake.
2. Step 2
  Algal bloom (1). The extra minerals make algae grow rapidly, forming an algal bloom that blocks light.
3. Step 3
  Plants die (1). Underwater plants cannot photosynthesise without light, so they die.
4. Step 4
  Decomposers (1). Decomposers multiply and break down the dead plants, respiring as they do so.
5. Step 5
  Oxygen used up (1). This uses up the dissolved oxygen, so the fish cannot respire and die.
Answer
Rain leaches nitrate and phosphate from the fertiliser into the lake. The extra minerals cause an algal bloom that blocks the light, so underwater plants cannot photosynthesise and die. Decomposers multiply to break down the dead plants and use up the dissolved oxygen as they respire, so the fish cannot respire and die.
Examiner tip
Five clear marking points follow the chain: minerals → algal bloom blocks light → plants die → decomposers → oxygen used up → fish die. The two 'oxygen' links (decomposers respire; fish can't respire) are where weaker answers lose marks.
6Compare sewage and eutrophication (Stretch)
Stretch• 4WSD0/1B style — extended• sewage, eutrophication, compare, oxygen, spec-4.16, spec-4.17
Question
Pollution by sewage and eutrophication both reduce the number of fish in a river. (a) State one way the cause of each is different. (b) Explain why both end up killing the fish in the same way. (4 marks)
Step-by-step solution
1. Step 1
  (a) sewage (1). Sewage adds organic waste, nutrients and microbes directly to the water.
2. Step 2
  (a) eutrophication (1). Eutrophication is caused by nitrate and phosphate leached from fertiliser.
3. Step 3
  (b) microbes respire (1). In both cases microbes/decomposers respire as they break down organic matter.
4. Step 4
  (b) oxygen used up (1). Their respiration uses up the dissolved oxygen, so the fish cannot respire and die.
Answer
(a) Sewage adds organic waste, nutrients and microbes directly to the water, whereas eutrophication is caused by nitrate and phosphate leached from fertiliser. (b) In both, microbes/decomposers respire as they break down organic matter, using up the dissolved oxygen, so the fish cannot respire and die.
Examiner tip
The high-mark idea in (b) is the shared mechanism: microbes respiring → dissolved oxygen used up → fish die. Naming different causes in (a) but the same oxygen-loss outcome in (b) secures full marks.

Model Answers — Biological Consequences of Water Pollution

High-scoring sample answers for biological consequences of water pollution on the Pearson Edexcel IGCSE Biology Paper 1 (4WSD0/1B) paper, with examiner-style notes mapping each response to the mark scheme and assessment objectives.

Question 1
4WSD0/1B style2 marks
State two biological consequences of releasing untreated sewage into a river. (2 marks)
Model answer
Microbes decompose the sewage and use up the dissolved oxygen, so fish (and other aquatic organisms) die (1).

The sewage contains pathogens, so it spreads disease (1).
Why this scores
Two distinct marking points: oxygen depletion (fish die) and disease spread. Listing two versions of the same idea (e.g. 'fish die' and 'animals die') scores only once.
Question 2
4WSD0/1B style3 marks
Name the two minerals that cause eutrophication and state where they come from. (3 marks)
Model answer
The two minerals are nitrate (1) and phosphate (1). They are leached from fertiliser (which has been spread on farmland) and run off into the water (1).
Why this scores
Both minerals are needed for two of the marks; the third is for the source (leached from fertiliser/run-off). 'Nutrients' alone does not name the minerals.
Question 3
4WSD0/1B style5 marks
Explain how releasing sewage into a river leads to the death of fish. (5 marks)
Model answer
Sewage adds nutrients (organic matter) and microbes to the water (1).

The microbes feed on and decompose the sewage (1).

As they decompose it the microbes respire (1).

Their respiration uses up the dissolved oxygen in the water (1).

With too little oxygen left, the fish cannot respire and die (1).
Why this scores
The full chain — sewage adds nutrients/microbes → microbes decompose and respire → dissolved oxygen used up → fish die — scores all five. The link 'microbes respire to use up the oxygen' is the point most often missed.
Question 4
4WSD0/1B style6 marks
Describe, in the correct order, the sequence of events in eutrophication after nitrate and phosphate enter a lake. (6 marks)
Model answer
The extra nitrate and phosphate cause algae to grow rapidly, forming an algal bloom (1).

The algal bloom blocks the light from reaching the plants below (1).

The underwater plants cannot photosynthesise and so die (1).

Decomposers (microbes) multiply to break down the dead plants (and dead algae) (1).

The decomposers respire and use up the dissolved oxygen in the water (1).

With little oxygen left, the fish and other aquatic organisms die (1).
Why this scores
Six marking points must be in order: algal bloom → blocks light → plants die → decomposers multiply → oxygen used up → organisms die. Marks are lost if the oxygen depletion is placed before the algal bloom or the decomposers.
Question 5
4WSD0/1B style — extended open response8 marks
A farmer spreads fertiliser on a field beside a lake. Some weeks later the lake water turns green and many fish die. Explain fully how the use of fertiliser led to the death of the fish. (8 marks)
Model answer
How the minerals reach the lake

The fertiliser contains the minerals nitrate and phosphate (1).

Rainwater leaches these minerals out of the soil and they run off into the lake (1).

The eutrophication sequence

The extra minerals make algae grow very rapidly, forming an algal bloom (this is why the water turns green) (1).

The algal bloom blocks the light reaching the plants below the surface (1).

The underwater plants cannot photosynthesise, so they die (1).

Decomposers (microbes) multiply to break down the large amount of dead plant material (1).

As the decomposers respire they use up the dissolved oxygen in the water (1).

With too little oxygen, the fish cannot respire and die (1).
Why this scores
Full marks require the complete chain: nitrate/phosphate leached → algal bloom → blocks light → plants die → decomposers multiply → respire and use up oxygen → fish die. Using the 'green water' clue to identify the algal bloom shows good application.
Question 6
4WSD0/1B style — extended open response9 marks
A student says: 'Sewage and fertiliser pollution are completely different — only sewage can kill fish because only sewage contains microbes.' Using your knowledge of water pollution, explain why this statement is wrong. (9 marks)
Model answer
The statement is wrong because both types of pollution can kill fish, and the killing mechanism is essentially the same.

Sewage pollution (4.16)

Sewage adds nutrients and microbes to the water (1).

The microbes decompose the sewage and respire, using up the dissolved oxygen (1).

So the fish cannot respire and die; sewage also spreads disease (1).

Eutrophication from fertiliser (4.17)

Nitrate and phosphate are leached from fertiliser into the water (1).

These cause an algal bloom that blocks light, so underwater plants die (1).

Decomposers (microbes) multiply to break down the dead plants (1).

The decomposers respire and use up the dissolved oxygen, so the fish die (1).

Why the statement is wrong

Fertiliser pollution does involve microbes — the decomposers that break down the dead plants are microbes (1).

In both cases the fish die because microbes respire and use up the dissolved oxygen, so it is not true that only sewage can kill fish (1).
Why this scores
The evaluative top band needs both mechanisms set out AND the explicit correction: eutrophication also relies on microbes (decomposers), and both pollutants kill fish by lowering dissolved oxygen. Stating the shared oxygen-loss mechanism secures the highest marks.

Key Definitions and Keywords — Biological Consequences of Water Pollution

Definitions to memorise and the exact keywords mark schemes credit for biological consequences of water pollution answers — sharpened from recent examiner reports for the 2026 Pearson Edexcel IGCSE Biology Paper 1 (4WSD0/1B) sitting.

Sewage
Examiner keyword
Waste water from homes, farms and factories containing human waste, organic matter and large numbers of microbes.
Decompose
Examiner keyword
To break down dead organic matter (such as sewage or dead plants); carried out by microbes that respire as they feed.
Dissolved oxygen
Examiner keyword
Oxygen dissolved in water that aquatic organisms such as fish need for respiration; used up by microbes decomposing pollution.
Eutrophication
Examiner keyword
The over-enrichment of water with minerals (especially nitrate and phosphate from fertiliser) that leads to an algal bloom, oxygen depletion and the death of aquatic organisms.
Leaching
Examiner keyword
The washing of soluble minerals (such as nitrate and phosphate) out of the soil by rainwater, which can then run off into rivers and lakes.
Algal bloom
Examiner keyword
A rapid growth of algae covering the surface of water, caused by excess nitrate and phosphate; it blocks light from reaching plants below.
Decomposer
Examiner keyword
A microorganism (bacterium or fungus) that breaks down dead organic matter; decomposers respire and use up dissolved oxygen.
Fertiliser
A substance added to soil to supply minerals such as nitrate and phosphate so crops grow well; run-off causes eutrophication.

Common Mistakes and Misconceptions — Biological Consequences of Water Pollution

The traps other students keep falling into on biological consequences of water pollution questions — taken from recent Pearson Edexcel IGCSE Biology Paper 1 (4WSD0/1B) examiner reports and mark schemes — and how to avoid them.

✕Saying 'the oxygen is used up' without saying what uses it up
Edexcel Biology examiner reports
Why it happens
Students learn the outcome (fish die) but not the cause, so they skip the microbe step.
How to avoid it
Always state that the microbes/decomposers respire as they decompose the waste and use up the dissolved oxygen — the respiration link is the marking point.
✕Putting the eutrophication stages in the wrong order (oxygen used up before the algal bloom)
Edexcel Biology examiner reports 2022–2024
Why it happens
Students remember the steps but not the sequence, so they list them randomly.
How to avoid it
Learn the fixed order: algal bloom → blocks light → plants die → decomposers multiply → oxygen used up → fish die.
✕Writing 'chemicals' or 'nutrients' instead of naming nitrate and phosphate
Edexcel Biology examiner reports
Why it happens
Students know fertiliser is involved but forget the exact minerals.
How to avoid it
Name the two minerals specifically: nitrate and phosphate, leached from fertiliser.
✕Saying the algae (not the decomposers) use up the oxygen
Edexcel Biology examiner reports
Why it happens
The algal bloom is the obvious feature, so students assume the algae cause the oxygen loss directly.
How to avoid it
The oxygen is used up by the decomposers that respire while breaking down the dead plants and algae — not by the living algae themselves.
✕Forgetting that sewage also spreads disease
Edexcel Biology examiner reports
Why it happens
Students focus on the oxygen/fish point and miss the second consequence.
How to avoid it
Remember sewage has two consequences: it lowers oxygen (fish die) AND it spreads disease (it contains pathogens).
✕Saying the algal bloom 'kills the plants' without explaining the light/photosynthesis link
Edexcel Biology examiner reports
Why it happens
Students compress the step and skip the reason the plants die.
How to avoid it
Explain that the bloom blocks light, so plants cannot photosynthesise and therefore die — the light and photosynthesis words are credited.

Past paper style quiz

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Biological Consequences of Water Pollution

Detailed Notes

What you’ll learn

How it’s examined

1Why sewage kills fish (Getting started)

2Name the eutrophication minerals (Getting started)

3Put eutrophication in order (Building confidence)

4Sewage and the spread of disease (Building confidence)

5Explain death of fish from fertiliser run-off (Stretch)

6Compare sewage and eutrophication (Stretch)

Sewage

Decompose

Dissolved oxygen

Eutrophication

Leaching

Algal bloom

Decomposer

Fertiliser

✕Saying 'the oxygen is used up' without saying what uses it up

✕Putting the eutrophication stages in the wrong order (oxygen used up before the algal bloom)

✕Writing 'chemicals' or 'nutrients' instead of naming nitrate and phosphate

✕Saying the algae (not the decomposers) use up the oxygen

✕Forgetting that sewage also spreads disease

✕Saying the algal bloom 'kills the plants' without explaining the light/photosynthesis link

Past paper style quiz