Here is the strange thing about eutrophication: it starts by adding nutrients to a lake, and ends with almost nothing alive in it. Fertiliser is supposed to make things grow. Follow the chain carefully and you will see exactly where it turns from growth into suffocation — and the turning point is not where most students think.
📘 What you need to know
Eutrophication follows artificially large inputs of nutrients — especially nitrates and phosphates — into lakes, rivers, estuaries and coastal waters.
The usual source is fertiliser leaching from farmland into waterways.
Excess nutrients cause excess growth of plants and phytoplankton, producing an algal bloom.
The bloom blocks sunlight, so submerged plants can no longer photosynthesise and die.
Decomposing bacteria feed on the dead matter, increase in number, and respire aerobically.
This creates a high biochemical oxygen demand (BOD), so dissolved oxygen falls rapidly.
Fish and insects may be unable to survive, creating dead zones in both freshwater and the oceans.
The chain, one link at a time
Eutrophication is a sequence, and exam marks are awarded for the links between steps rather than for the steps themselves. Learn it in order and never skip the middle.
Steps 1 to 3 are about growth. Steps 4 to 6 are about death and decay. The switch happens when the bloom shades out the plants beneath it.
Walking through it in words
When a body of water receives an artificially large input of nutrients, plants and phytoplankton such as algae grow in excess. This growth, an algal bloom, forms a dense layer at the surface that blocks sunlight from penetrating below it. Aquatic plants under the surface can therefore no longer photosynthesise, so they start to die. The algae themselves also begin to die once competition for nutrients becomes too intense.
As plants and algae die in increasing numbers, decomposing bacteria feed on the dead organic matter and increase in number too. Those bacteria respire aerobically, which means they use up the dissolved oxygen in the water. The result is a high biochemical oxygen demand (BOD), and the availability of dissolved oxygen falls rapidly. Aquatic organisms such as fish and insects may then be unable to survive, producing dead zones where there is not enough oxygen to support aquatic life.
The single most common mistake is writing that the algae use up the oxygen. They do not — while alive they produce it. The oxygen is consumed by the bacteria decomposing the dead material. If your answer never mentions bacteria, it will not score full marks.
Definition to learnBiochemical oxygen demand (BOD) = the amount of dissolved oxygen used by micro-organisms decomposing organic matter in a sample of water
A high BOD is a sign of heavy organic pollution, which is why water companies measure it. It tells you how much oxygen the bacteria in that water are about to remove.
What the data looks like
Data questions on eutrophication almost always show three variables measured over time, and the marks come from spotting the lag between them. Nothing happens simultaneously.
Oxygen keeps falling after the algae have already peaked and started to decline. That delay is the decomposition step, and explaining it is usually worth a mark on its own.
Where the nutrients come from
Agricultural fertiliser. The minerals in synthetic fertilisers are highly soluble, so rainfall dissolves them and leaches them out of the soil into rivers and lakes.
Sewage and untreated waste water, which is rich in both nutrients and organic matter.
Manure run-off from intensive livestock farming.
The prevention measures are the ones from the previous page: apply fertiliser in small volumes, avoid applying it when rain is forecast, and prefer organic fertilisers, which release nutrients more slowly.
Worked examples
WORKED EXAMPLE 1
Explain how the leaching of nitrate fertiliser from a field can lead to the death of fish in a nearby lake. [5]
Step 1: nutrients arrive
Nitrates are highly soluble, so rainfall washes them from the soil into the lake.
Step 2: the bloom
Excess nutrients cause rapid growth of algae and phytoplankton — an algal bloom at the surface.
Step 3: light is blocked
The bloom prevents light reaching the submerged plants, which can no longer photosynthesise and die. The algae die too as competition for nutrients intensifies.
Step 4: decompositionDecomposing bacteria feed on the dead organic matter and increase in number, respiring aerobically and using up dissolved oxygen.
Step 5: the outcomeBOD rises and dissolved oxygen falls, so fish cannot respire and diefive marks, five links — count the steps in the question and make sure you have that many
WORKED EXAMPLE 2
A student measures dissolved oxygen in a lake and finds it is still falling three weeks after the algal population has peaked. Explain this observation. [3]
Step 1: what the peak means
The algal peak marks the point at which algae start to die in large numbers, not the point of maximum damage.
Step 2: the delay
Dead algae and plants must first be decomposed, and the bacterial population takes time to build up.
Step 3: the consequenceAerobic respiration by the growing bacterial population continues to remove oxygen, so the level keeps falling after the algal peakwhenever two curves peak at different times, the explanation is almost always a time lag in a biological process
WORKED EXAMPLE 3
Explain why a high biochemical oxygen demand indicates polluted water. [2]
Step 1: what BOD measures
BOD is the amount of dissolved oxygen used by micro-organisms decomposing organic matter in the water.
Step 2: the inferenceA high BOD means a large amount of organic matter and many decomposers, so the water is heavily polluted and oxygen is being depletedhigh BOD equals low oxygen available for fish — the two always move in opposite directions
💡 Exam tip
Write the chain in order, and never jump from “algal bloom” straight to “fish die”.
Name the bacteria as the organisms using the oxygen. This is the marking point most often missed.
Use the terms algal bloom, BOD and dead zone explicitly.
Explain the light-blocking step properly: the bloom stops light reaching submerged plants.
For data questions, comment on the time lag between the curves.
Say nitrates and phosphates are highly soluble when explaining how they reach the water.
⚠ Common mix-up
Saying the algae use up the oxygen. Living algae release oxygen. The bacteria decomposing them consume it.
Saying the algae poison the fish. Some blooms do release toxins, but the mechanism you are asked about is oxygen depletion.
Skipping the plant death step. Light-blocking, then plant death, then decomposition — all three are needed.
Confusing BOD with dissolved oxygen. High BOD means oxygen is being used, so the dissolved level is low.
Assuming eutrophication is only a freshwater problem. Coastal dead zones are extensive too.
Thinking a dead zone has no oxygen at all. It has too little to support aquatic life, which is not quite the same claim.
Up next: The Effects of Pollution — pollutants that do not get diluted as they move through a food chain, but concentrated.
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