GCSE-ready- Home
- Lessons
- GCSE & IGCSE Biology
- Sewage and eutrophication
Sewage and eutrophication
🎬 The doodle video for this lesson is coming soon. Subscribe on YouTube to see it first.
Sewage and fertilisers in water can starve rivers and lakes of oxygen.
Sewage
Untreated sewage in water provides food for bacteria.
The bacteria multiply and use up the dissolved oxygen in aerobic respiration.
Fish and other animals die from lack of oxygen. Sewage can also spread pathogens, such as the bacteria that cause cholera.
The bacteria multiply and use up the dissolved oxygen in aerobic respiration.
Fish and other animals die from lack of oxygen. Sewage can also spread pathogens, such as the bacteria that cause cholera.
Eutrophication
1. Fertiliser minerals (nitrates and phosphates) are leached from fields into rivers and lakes.
2. Algae grow quickly (an algal bloom) and cover the surface.
3. The algae block light, so plants below cannot photosynthesise and die.
4. Decomposer bacteria feed on the dead plants and multiply.
5. The bacteria use up the oxygen in respiration.
6. Fish and other animals die.
2. Algae grow quickly (an algal bloom) and cover the surface.
3. The algae block light, so plants below cannot photosynthesise and die.
4. Decomposer bacteria feed on the dead plants and multiply.
5. The bacteria use up the oxygen in respiration.
6. Fish and other animals die.
Reducing the damage
Treat sewage before releasing it.
Use fertiliser carefully: the right amount, not before heavy rain, and not near water courses.
Use fertiliser carefully: the right amount, not before heavy rain, and not near water courses.
Why do fish die in eutrophication, even though algae release oxygen?
- The algae block light, so plants below die.
- Decomposer bacteria feeding on dead plants multiply hugely.
- Their respiration uses up more oxygen than is produced.
Answer: Bacteria decomposing dead plants use up the dissolved oxygen.
Sewage feeds bacteria that use up oxygen, so fish die; it also spreads disease. Eutrophication: fertilisers leach in, algae bloom, light is blocked, plants die, decomposers multiply and use up oxygen, fish die.
Check you have got it
Answer 6 quick questions with instant marking. If you get one wrong, GCSE-ready shows you why and gives you another go. It is free, and you do not need an account.