Eutrophication is a chain of cause and effect: extra nutrients enter water, plants and algae multiply, light is blocked, plants die, bacteria multiply and use up oxygen, and animals die. Questions give marks for each link, so a two-step answer loses most of them.
It follows on from linking decomposition with nutrient recycling and sits inside human influence on ecosystems.
What is the chain, link by link?
- Source: fertiliser runoff or sewage raises the nitrate and phosphate levels in the water.
- Growth: with more nutrients, algae grow fast and form a dense layer near the surface (an algal bloom).
- Light: the bloom blocks light from reaching submerged plants.
- Death: those plants cannot photosynthesise and die.
- Decomposers: bacteria feed on the dead plants and algae and increase in number.
- Oxygen: the bacteria respire aerobically and use up dissolved oxygen.
- Consequence: fish and other aerobic organisms cannot get enough oxygen and die, so biodiversity falls.
Learn it as a story with a cause attached to each arrow. Every arrow answers the question “why does that lead to this?”
Worked example
Invented data for practice. A student tests a stream above and below a farm on the same day.
| Site | Nitrate (mg/dm³) | Dissolved oxygen (mg/dm³) |
|---|---|---|
| Upstream of the farm | 2 | 8.0 |
| Downstream of the farm | 14 | 2.5 |
Question: Use the data to explain the difference in oxygen between the two sites.
Step 1, compare: nitrate is 14 ÷ 2 = 7 times higher downstream. Oxygen fell from 8.0 to 2.5, a drop of 5.5 mg/dm³.
Step 2, percentage: 5.5 ÷ 8.0 × 100 = 68.75%, so about 69% lower.
Step 3, chain: the higher nitrate downstream fits runoff from the farm. More nitrate lets algae and plants grow faster, and a dense bloom blocks light so plants die. Bacteria decompose the dead plants and respire aerobically, which uses dissolved oxygen.
Step 4, limit: one day and two sites show a link, not proof that the farm is the only cause.
The mistake to watch for
A common error is to skip the middle of the chain or to misplace the oxygen use.
Mistaken answer: “The fertiliser kills the fish because it is poisonous, and the algae use up all the oxygen.”
Two slips: the question is about nutrient enrichment, not poison, and the main oxygen use comes from decomposing bacteria respiring, not from the algae.
The correction is to write the bacteria link explicitly: dead material, then more bacteria, then aerobic respiration, then oxygen falls.
Check yourself
Try these first, then open each answer.
1. Put in order: fish die; algal bloom; nitrate rises; bacteria use oxygen; light blocked.
Show answer
Nitrate rises, algal bloom, light blocked (plants die), bacteria use oxygen, fish die.
2. An invented lake has 9.0 mg/dm³ dissolved oxygen in spring and 3.0 in late summer after a bloom. Calculate the percentage decrease.
Show answer
Decrease = 9.0 − 3.0 = 6.0. Percentage = 6.0 ÷ 9.0 × 100 = 66.7%, so about 67% lower.
3. Explain why submerged plants die during an algal bloom.
Show answer
The dense algae near the surface block light. Submerged plants cannot photosynthesise enough to make their own food, so they die.
Where this leads next
Once the chain is automatic, practise choosing between actions to reduce such a problem in comparing conservation options using specified ecological criteria. Test the whole module in the practice set. Before that, assessing evidence for a habitat change helps with the data step.
If you can recite the chain but lose marks when it is hidden inside a data question, a teacher can work from your own answers in online one-to-one Biology tuition.