Eutrophication is the process by which excess nutrients — mainly nitrates and phosphates — enter a body of water and trigger explosive growth of algae. The subsequent decay of this algal bloom removes so much oxygen from the water that fish and other aquatic organisms suffocate and die, destroying the ecosystem.
What is eutrophication and what causes it?
The word eutrophication comes from the Greek for "well-nourished." In ecology, it describes the over-enrichment of water with nutrients, leading to a chain of biological events that depletes oxygen.
The main sources of excess nutrients are:
- Agricultural fertilisers — nitrates and phosphates spread on fields dissolve in rainwater and run off into rivers, streams, and lakes. This is called leaching.
- Untreated or poorly treated sewage — human and animal waste adds nitrates, phosphates, and organic matter directly to waterways.
- Detergents — phosphate-containing detergents washed into drains can reach watercourses.
- Atmospheric deposition — nitrogen oxides from vehicle exhausts dissolve in rain and fall into water bodies.
Of these, agricultural runoff is the most significant cause of eutrophication in the UK.
What happens step by step during eutrophication?
The sequence of events follows logically from each cause. Learning it as a causal chain is the best way to answer longer exam questions.
| Step | What happens | Why |
|---|---|---|
| 1 | Excess nitrates and phosphates enter the water | Fertiliser leaching or sewage input |
| 2 | Algal bloom — algae and surface plants grow rapidly | Nutrients are now the limiting factor no longer; light and warmth drive growth |
| 3 | Algae form a dense surface mat, blocking sunlight | Upper layer of algae shades everything below |
| 4 | Submerged aquatic plants die | They cannot photosynthesise without light |
| 5 | The massive biomass of dead algae and plants is decomposed by aerobic bacteria | Decomposers respire and multiply explosively |
| 6 | Aerobic bacteria use up the dissolved oxygen in the water | Biochemical oxygen demand (BOD) soars |
| 7 | Fish, invertebrates, and other aerobic organisms die from oxygen depletion | Known as deoxygenation or oxygen crash |
The ecosystem collapses from diverse and productive to nearly lifeless within weeks in severe cases.
Why do the bacteria, not the algae, cause oxygen depletion?
This is a common point of confusion. Living algae actually produce oxygen through photosynthesis — so the bloom itself does not immediately deplete oxygen. The critical step is what happens when the algae die and are broken down.
Decomposer bacteria use aerobic respiration to break down the dead organic matter. The sheer quantity of dead algae provides an enormous food source, causing a population explosion of bacteria. Their collective respiration consumes dissolved oxygen far faster than it can be replaced from the atmosphere or by photosynthesis. The water becomes anoxic (oxygen-free), killing all aerobic life.
How does eutrophication affect biodiversity?
Before eutrophication, a healthy lake or river supports a food web including aquatic plants, invertebrates, fish, and birds that feed on them. Eutrophication compresses this diversity:
- Species lost: most fish species, most invertebrates, submerged aquatic plants, and the birds that depend on them.
- Species that survive or increase briefly: algae, some surface-feeding birds, and anaerobic bacteria (which thrive in oxygen-free water and produce toxic gases such as hydrogen sulphide).
- Long-term result: a turbid, foul-smelling body of water dominated by decomposers and algae, with very low biodiversity.
This is why eutrophication is classified as a serious form of water pollution even though the input (a nutrient) is not itself toxic.
What can be done to prevent or reverse eutrophication?
Prevention is far easier than reversal:
- Reduce fertiliser application — precision farming and controlled-release fertilisers limit runoff. Buffer strips of unfarmed land beside waterways slow leaching.
- Improve sewage treatment — tertiary treatment removes phosphates before effluent reaches rivers.
- Phosphate-free detergents — reducing phosphate loads entering the drainage system.
- Restoration — dredging accumulated sediment, introducing aerators to raise dissolved oxygen, or removing algae mechanically can help recovery but are expensive.
Once a water body has shifted to a eutrophic state, it often remains in that state for years or decades — the nutrients accumulate in the sediment and continue feeding algal growth long after the original input stops.
Frequently asked questions
What is eutrophication in GCSE biology?
Eutrophication is the over-enrichment of water with nutrients, particularly nitrates and phosphates, which triggers rapid algal growth. When the algae die and decompose, aerobic bacteria use up the dissolved oxygen, causing fish and other aquatic organisms to suffocate. It is most commonly caused by agricultural fertiliser leaching into rivers and lakes, and it reduces biodiversity severely.
Why is eutrophication caused by fertilisers?
Fertilisers contain high concentrations of nitrates and phosphates, which are essential plant nutrients. When it rains, these soluble ions are washed off farmland and leach into nearby watercourses. In water, they act as nutrients for algae in the same way they act for crops on land, triggering exponential algal growth. The scale of modern agriculture means that even modest leaching rates can deliver enough nutrients to cause serious eutrophication.
Why do fish die during eutrophication?
Fish die because the water becomes deoxygenated. After an algal bloom, the algae eventually die. Aerobic bacteria decompose the enormous mass of dead organic matter, consuming dissolved oxygen in the process. When oxygen levels drop below around 3 mg per litre, most fish species cannot survive. The die-off is typically sudden and can kill a entire fishery in days once the oxygen crash begins.
How do you remember the steps of eutrophication for the exam?
A reliable sequence to memorise: nutrients in → algae bloom → algae block light → plants below die → bacteria decompose dead material → bacteria use up oxygen → aerobic animals die. The key misunderstanding to avoid is blaming the algae directly for oxygen depletion — it is the bacteria decomposing them that cause the oxygen crash. Markers award specific marks for stating this mechanism clearly.
For Socratic GCSE biology with Professor Darwin — tracing eutrophication from a molecule of nitrate all the way to ecosystem collapse — visit aitutors.me.