There is something odd about eutrophication that catches people out every year: it starts with more nutrients, more growth and more life, and ends with a lake that nothing can live in. Getting the marks depends on being able to explain that turn.
📚 What you need to know
Eutrophication starts with artificially large inputs of nutrients such as nitrates and phosphates.
This causes excess growth of plants and phytoplankton – an algal bloom.
The bloom blocks sunlight, so plants below the surface can no longer photosynthesise and die.
Decomposing bacteria increase in number and use up dissolved oxygen as they respire aerobically.
This creates an increased biochemical oxygen demand (BOD).
Low dissolved oxygen means fish and insects cannot survive, producing a dead zone.
Where the nutrients come from
When lakes, rivers, estuaries and coastal waters receive artificially large inputs of nutrients – typically nitrates and phosphates – the result is excess growth of plants and phytoplankton such as algae.
The usual source is the one from the previous page: mineral ions from excess fertilisers leaching from farmland into waterways. The fertiliser was intended to increase plant growth in a field, and that is precisely what it does when it arrives in a lake. The problem is not that it stops working. The problem is that it keeps working, in the wrong place.
The sequence, step by step
Eutrophication questions are almost always asking for this chain in order. Learn it as six steps and you will not lose your place.
Steps 5 and 6 are where most marks are won and lost. The oxygen loss is a respiration problem, not a photosynthesis problem.
The steps in words
The excess nutrients cause a huge growth of algae, known as an algal bloom. This bloom sits across the water surface and blocks out sunlight, stopping it penetrating below. Aquatic plants below the surface therefore start to die, because they can no longer photosynthesise. The algae themselves also begin to die once competition for nutrients becomes too intense.
As aquatic plants and algae die in increasing numbers, decomposing bacteria feed on the dead organic matter and increase in number as well. Those bacteria respire aerobically, and in doing so they use up the dissolved oxygen in the water.
The term to use
The respiring bacteria create an increased biochemical oxygen demand, or BOD – a measure of how much dissolved oxygen the organisms in the water are consuming.
As a result the availability of dissolved oxygen rapidly decreases, and aquatic organisms such as fish and insects may be unable to survive. Where there is not enough oxygen to support aquatic life, the area is called a dead zone. Dead zones occur in both oceans and freshwater.
Once the blue line drops below the dashed one, the fish and insects go. That region is the dead zone.
A question that gives you a graph like this often asks why the oxygen falls after the algal population has already peaked. The answer is the lag: the oxygen is consumed by decomposers respiring on the dead algae, so the crash cannot happen until there is a large mass of dead material to decompose.
🧠
Two deaths, in order
The plants below die first, from lack of light. Then the algae die, from competition for nutrients. Both lots of dead material feed the bacteria, and it is the bacteria that take the oxygen. Three groups, three fates, one chain.
Worked examples
WE 1
Outline the process
Outline how the leaching of fertiliser from farmland can lead to the death of fish in a nearby lake. (5 marks)
Point 1: the input
Nitrates and phosphates leach from the farmland into the lake, giving an artificially large input of nutrients.
Point 2: the bloom
This causes excess growth of algae and phytoplankton – an algal bloom.
Point 3: light
The bloom blocks sunlight from penetrating below the surface, so submerged plants cannot photosynthesise and die.
Point 4: decomposers
Decomposing bacteria feed on the dead plants and algae and increase in number, respiring aerobically and using up dissolved oxygen.
Point 5: the outcome
Dissolved oxygen falls rapidly, so fish are unable to survive.
Nutrients, bloom, no light, decay, no oxygenfive marks means five distinct steps – do not merge the bloom and the light-blocking into one
WE 2
Explain a term
Explain what is meant by biochemical oxygen demand, and why it increases during eutrophication. (3 marks)
Point 1: the meaning
BOD is a measure of the amount of dissolved oxygen consumed by the organisms in a body of water.
Point 2: why it rises
Large numbers of plants and algae die, providing dead organic matter for decomposing bacteria.
Point 3: the mechanism
These bacteria increase in number and respire aerobically, so the demand for dissolved oxygen rises sharply.
More dead matter means more bacteria means more respirationBOD is about oxygen being used, not about oxygen being present
WE 3
Interpret a graph
A graph shows the algal population in a lake peaking in June, while dissolved oxygen concentration reaches its minimum in August. Explain this time difference. (3 marks)
Point 1: what the peak means
The June peak is the maximum algal population, when the bloom is at its largest.
Point 2: the delay
Oxygen is not consumed by the algae themselves but by decomposing bacteria feeding on the algae and plants after they die.
Point 3: the mechanism
A large mass of dead material only accumulates after the bloom collapses, so the bacterial population and their aerobic respiration peak later, giving the August minimum.
The oxygen crash follows the dead material, not the living algaethis is the single most common data-question on eutrophication – learn the lag
💡 Exam tips
Name the nutrients: nitrates and phosphates.
Use the term algal bloom, and say it blocks light rather than “covers the lake”.
State clearly that the oxygen is consumed by bacteria respiring aerobically.
Use biochemical oxygen demand (BOD) where the question invites a technical term.
Finish with dead zone if the question asks about the final state.
For graph questions, explain the lag between the algal peak and the oxygen minimum.
⚠ Common mistakes
Saying the algae use up the oxygen. They photosynthesise; the bacteria take the oxygen.
Saying the fertiliser poisons the fish. The nutrients are not toxic – the oxygen loss kills them.
Forgetting the light-blocking step. It is what kills the submerged plants.
Writing that the algae die because of the lack of light. They die from competition for nutrients.
Describing BOD as the amount of oxygen in the water. It is the amount being consumed.
Assuming dead zones only occur in lakes. They occur in oceans as well.
Up next: The Effects of Pollution – a different way for a pollutant to do damage, in which the substance does not have to be present in large amounts at all.
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