Key Idea: Most marks in topic 4.4 come from managing eutrophication and the sources of water pollution. Learn those first, then BOD and water quality.
1. Eutrophication (most marks)
- Nitrates and phosphates from fertiliser and sewage cause an algal bloom.
- Algae block light, die and decompose: decomposers use up the oxygen.
- Fish die, decay adds more nutrients: positive feedback.
2. Three levels of management
- Change the activity: less fertiliser, phosphate-free detergents.
- Control the release: treat sewage, plant buffer strips along rivers.
- Clean up: remove nutrient-rich mud, restock plants and fish.
3. Sources and measuring
- Sources: sewage, farm and urban run-off, factories, rubbish and oil spills.
- BOD is the oxygen microbes use in 5 days at 20 °C: high BOD means much organic waste.
- Water quality: oxygen, pH, temperature, turbidity, nitrates and phosphates.
How it is examined:
| Question | How to score |
|---|---|
| [1] to [2] Identify a source of pollution from a figure | Name it and say what it adds. |
| [3] to [4] Outline the stages of eutrophication | Nutrients, bloom, death, decomposition, low oxygen. |
| [4] Explain how a management strategy works | Say which of the three levels it acts at. |
| [7] Evaluate strategies for eutrophication | All three levels, strengths, limits, judgement. |
| [9] Discuss a named pollution case | Location, source, impacts, management, both sides. |
Key terms, one line each:
| Term | In one line |
|---|---|
| Eutrophication | too many nutrients in water, causing algal blooms |
| BOD | the oxygen microbes use to break down organic matter |
| Point source | pollution from one place, such as a pipe |
| Non-point source | pollution from a wide area, such as farm run-off |
| Hypoxia | too little dissolved oxygen for most life |
Last-minute check: answer, then tap to see.
Which two nutrients cause eutrophication? Nitrates and phosphates.
Why does oxygen fall in a eutrophic lake? Decomposers use it to break down dead algae.
Which level is a buffer strip? Controlling the release of the pollutant.
High BOD means what? Lots of organic matter: polluted water.
IB-style questionExplain[4 marks]
After heavy rain, fertiliser washes from farms into a lake, and weeks later many fish die. Explain how the fertiliser led to the death of the fish.
Model answer plan
See the mark-by-mark plan — for / against / judgement, with marking guidance — in study mode.
Exam tips
- Write the eutrophication stages in order, one per point.
- Say which of the three management levels each strategy works at.
- For a named case, give location, source, impacts and management.
- Explain BOD as oxygen used by microbes, not oxygen in the water.