Explain eutrophication of water
Nutrient enrichment increases producer growth; decomposition then raises oxygen use and can lower dissolved oxygen until aquatic organisms die.
Exam scope and assessment notes
Extended only (Supplement) · Biology 0970, 2026–2028. Use the stated specimen features and evidence. Diagrams are schematic. Written practice is self-checked; human subject-teacher review, supervised practical competence and cumulative assessment remain pending. Cambridge states that IGCSE 0970 has the same subject content as IGCSE 0610, with 9–1 grading. This StudyForA pathway maps the 2026–2028 syllabus only. This 9–1 graded qualification is available only in a limited number of administrative zones. Confirm that your centre can enter you for this code and exam series with Cambridge International. Cambridge has published a separate syllabus for 2029. StudyForA has not reconciled that version yet; this page supports exams in 2026, 2027 and 2028 only.
Before you begin
- Know that organisms interact with other organisms and their environment.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What directly uses dissolved oxygen during the decomposition stage of eutrophication?
What you will learn
- Explain the ordered stages of eutrophication
- Link dissolved-oxygen reduction to the death of aquatic organisms
Start with increased nutrients
Nitrates and other ions enter water, often through fertiliser runoff or sewage. Increased nutrient availability supports faster growth of producers such as algae and aquatic plants. Dense growth may block light from reaching submerged producers.
The syllabus sequence continues when producers die: decomposers break down the dead material and increase their aerobic respiration. Decomposers use dissolved oxygen in the water.
Complete the oxygen-depletion chain
As aerobic respiration by decomposers increases, dissolved oxygen concentration falls. Organisms requiring dissolved oxygen may then die. The complete sequence is increased nitrate and other ions, increased producer growth, more decomposition after producer death, more aerobic respiration by decomposers, reduced dissolved oxygen and deaths of oxygen-requiring organisms.
A clear answer preserves the causal order. Nutrients do not directly remove oxygen; oxygen falls because decomposers use it during aerobic respiration as dead producer material is broken down.
Pause and explain
What is the first ecological effect after increased nitrate availability in eutrophication?
Worked example
A water sample has a nutrient increase followed by algal growth, decomposition and fish deaths. State the missing link between decomposition and fish deaths.
Show the worked solution
- Decomposers carry out aerobic respiration.
- That process uses dissolved oxygen.
Answer Aerobic respiration by decomposers uses dissolved oxygen, reducing the concentration available to fish and other oxygen-dependent organisms.
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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.
Write the full eutrophication sequence from nutrient input to death of oxygen-requiring organisms.
Give me a hint
Keep each event in causal order.
Compare my reasoning
- Nutrients increase producer growth; dead producers are decomposed.
- Decomposers respire aerobically, oxygen falls and organisms die.
Increased nitrate and other ions increase producer growth. Producers die; decomposition increases. Decomposers carry out more aerobic respiration, reducing dissolved oxygen. Organisms that require dissolved oxygen can then die.
Look for these in your work
- All six stages present
- Causal order correct
Explain why dissolved oxygen decreases during eutrophication.
Give me a hint
Identify the organisms and the respiration process.
Compare my reasoning
- Dead producers are decomposed by decomposers.
- Decomposers use oxygen during aerobic respiration.
When producers die, decomposers break down the material. Their increased aerobic respiration uses dissolved oxygen, lowering its concentration in the water.
Look for these in your work
- Decomposition identified
- Respiration linked to oxygen decrease
A student writes: “Nitrate kills fish by using up oxygen.” Rewrite the explanation accurately.
Give me a hint
Correct the direct-cause claim.
Compare my reasoning
- Nitrate and other ions first increase producer growth.
- When producers die, decomposers use oxygen during aerobic respiration.
Nitrate and other ions increase producer growth. When producers die, decomposers use oxygen during aerobic respiration, so dissolved oxygen falls and fish may die.
Look for these in your work
- Nutrients linked to producers
- Decomposers identified as oxygen users
Common mistakes
- Skipping the decomposition stage
- Saying nitrate directly consumes dissolved oxygen
- Reversing the order of producer growth and decomposition
What can you explain now?
What directly uses dissolved oxygen during the decomposition stage of eutrophication?
Compare with the explanation
Aerobic respiration by decomposers
Decomposers use oxygen as they break down dead producers.
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Make it stick
Recite the eutrophication chain and identify which organisms use oxygen.
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.