By the end of this lesson
- Describe the sequence of events in eutrophication.
- Explain why decomposer respiration reduces oxygen.
- Suggest ways to reduce eutrophication.
BIG QUESTION
How can fertiliser that helps crops grow end up killing fish?
Extra nitrate and phosphate trigger a chain of events in which the decomposers, not the algae, remove the oxygen.
Minerals enter the water
Fertilisers add nitrate ionsMineral ions that supply plants with nitrogen to make amino acids and then proteins; a lack causes stunted growth and often yellowing, especially of older leaves. and phosphateMineral ions that supply plants with phosphorus, used to make DNA (nucleic acids), ATP and cell membranes; a lack can cause poor root growth. ions to soil. When too much is applied, or heavy rain follows application, soluble ions are leachingWashing soluble mineral ions, such as nitrates, out of the soil by rain water. through the soil or run off into rivers and lakes.
Untreated sewage and animal waste can add the same minerals.
Algal bloom and light
The minerals allow rapid growth of algae near the surface, called an algal bloomA rapid growth of algae in water, often caused by fertiliser runoff, that blocks light from plants below.. The algae block light from reaching plants deeper in the water.
These plants cannot photosynthesise enough and die. Algae also die as minerals run out or overcrowding occurs.
Decomposers deplete oxygen
Dead plants and algae are food for aerobic decomposerAn organism, such as a bacterium or fungus, that breaks down dead organic matter and waste, returning nutrients to the soil. (bacteria). Their numbers increase and their respirationA series of enzyme-controlled chemical reactions in cells that release energy from nutrient molecules, such as glucose; it is not the same as breathing. uses up dissolved oxygen.
Fish and other aerobic animals cannot get enough oxygen and die. Their bodies add to the decaying material, making conditions worse.
Reducing eutrophication
Farmers can apply fertilisers in appropriate amounts and at the right time, avoid application before heavy rain, and leave buffer strips of vegetation beside watercourses.
Removing phosphates during sewage treatment also reduces eutrophication.
Sewage pollution and eutrophication
Both lead to low oxygen but begin differently.
| Feature | Sewage pollution | Eutrophication from fertiliser |
|---|---|---|
| Main input | Organic matter (and pathogens). | Mineral ions such as nitrate and phosphate. |
| First change | Bacteria feed directly on the organic matter. | Algal bloom blocks light. |
| Cause of oxygen fall | Bacterial respiration. | Bacterial respiration decomposing dead plants and algae. |
| Health risk | Pathogens in drinking water. | Some algal blooms release toxins. |
Language that earns precision
- eutrophication
- Enrichment of water with mineral ions, leading to excessive plant and algal growth and oxygen depletion.
- leaching
- Washing of soluble ions through soil by water.
- algal bloom
- A rapid increase in algae in a body of water.
Common misconceptions
Claim: The algae use up the oxygen.
Correction: Algae photosynthesise; the major oxygen fall is caused by decomposer bacteria respiring.
Claim: Fertiliser poisons the fish.
Correction: Fish die because dissolved oxygen falls.
Claim: Eutrophication happens instantly.
Correction: It is a sequence of events over days or weeks.
Match the answer to the command word
Give the sequence in order: minerals → algae → light blocked → plants die → decomposers → respiration → oxygen falls → fish die.
Write “bacteria respire and use up oxygen”, not “the algae suffocate the fish”.
Recall, apply, then justify
Explain how fertiliser leaching into a lake can cause fish to die.
Hint
Five steps in sequence.
Suggested marking points
- Nitrate/phosphate ions cause rapid growth of algae (algal bloom).
- Algae block light from plants below.
- Plants cannot photosynthesise and die.
- Bacteria decompose the dead plants and their respiration uses up oxygen.
- Fish cannot get enough oxygen and die.
Suggest two ways a farmer could reduce eutrophication of a nearby river.
Hint
Amount, timing and barriers.
Suggested marking points
- Use the correct amount of fertiliser / do not over-apply.
- Avoid spreading before rain / leave a buffer strip by the river.
Dissolved oxygen in a eutrophic lake is higher in the afternoon than just before dawn (illustrative data). Explain this pattern.
Planning hint
Photosynthesis in daylight, respiration all the time.
What a strong design includes
- During the day, algae and plants photosynthesise and release oxygen.
- Photosynthesis can exceed respiration in daylight.
- At night there is no photosynthesis.
- All organisms, including large bacterial populations, continue respiring.
- Oxygen is used without being replaced overnight.
- So oxygen is lowest just before dawn, when fish deaths are most likely.