Eutrophication is the single most examined process in this topic, and it has a quirk that catches people out: the water fills up with photosynthesising algae, and then everything suffocates. If you can explain that apparent contradiction properly, you can answer any eutrophication question thrown at you.
📚 What you need to know
Eutrophication is nutrient enrichment of a water body — usually by nitrate and phosphate — leading to excessive growth of algae and aquatic plants.
Nitrate mostly comes from agricultural run-off; phosphate mostly from sewage and detergents.
The sequence: nutrients in → rapid algal and plant growth → light blocked → plants die → bacteria decompose them → dissolved oxygen crashes → aquatic life dies.
It is driven by positive feedback: each round of death and decay releases nutrients that fuel the next round.
Hypoxia = low oxygen. Anoxia = oxygen effectively gone. A dead zone is water where too little oxygen remains to support most life.
Consequences hit fisheries, recreation, drinking water and human health.
What eutrophication actually is
Nutrients are not pollutants in themselves. Plants need nitrogen and phosphorus, and a healthy lake contains both. Eutrophication happens when they arrive in far larger amounts than the system evolved to handle, so growth is limited by nothing at all.
Definition to learn
eutrophication = enrichment of a water body with mineral nutrients, causing excessive growth of algae and aquatic plants
Nitrate (NO3−) — very soluble, so it washes off farmland with the first heavy rain after fertiliser is applied.
Phosphate (PO43−) — arrives in sewage and in detergents, and also binds to soil particles that erode into rivers.
Phosphate is usually the limiting nutrient in freshwater, and nitrate in the sea. You do not need that for a definition mark, but it explains why phosphate-free detergents made such a difference to lakes.
The sequence, step by step
🧩 From fertiliser to dead zone
Nutrient enrichment. Excess nitrate and phosphate reach the water from run-off, sewage or detergents.
Rapid plant growth. Aquatic plants such as duckweed and water hyacinth spread across the surface.
Algal bloom. Phytoplankton multiply until a green layer covers the water.
Light is blocked. Submerged plants can no longer photosynthesise, so they die — and the oxygen they used to release is lost.
Decomposition. Bacteria break down the dead plants and, soon after, the dead algae. Their aerobic respiration consumes dissolved oxygen.
Oxygen crash. Dissolved oxygen falls to hypoxic or anoxic levels. Fish and invertebrates die or leave, and a dead zone forms.
Notice where the damage is: not at the surface, where growth is booming, but in the dark water underneath it.
Why it speeds up: positive feedback
Eutrophication does not settle down on its own. Decomposing the dead algae releases the nitrogen and phosphorus locked in their tissue straight back into the water, where it fuels the next bloom. The loop reinforces itself, which is the definition of positive feedback.
This is why a lake can stay in a poor state for years after the fertiliser has stopped arriving — the nutrients are already in the system, cycling.
Hypoxia, anoxia and dead zones
Get these three exactly right.Hypoxia is a low dissolved oxygen concentration — stressful, and enough to drive sensitive species out. Anoxia is the severe case, with oxygen effectively absent. A dead zone is an area of water where oxygen is too low to support most aquatic life, and they now form seasonally at the mouths of many major rivers.
A common wrong answer: “the algae use up all the oxygen”. Algae do respire, and at night a dense bloom does pull oxygen down. But the main oxygen sink is the decomposer bacteria working on the dead material. Say that, and say why.
What eutrophication costs
Area affected
Consequence
Why it happens
Fisheries
Sudden fish kills, then long-term loss of stocks
Oxygen falls below the level fish can tolerate, and eggs and young are hit hardest
Biodiversity
Sensitive species disappear; a few tolerant ones dominate
Only organisms that cope with low oxygen and heavy shading survive
Recreation
Swimming, boating and angling become unpleasant or unsafe
Green murky water, foam, slime and the smell of decay
Drinking water
Treatment becomes harder and more expensive
Algae clog filters, and some blooms release toxins that must be removed
Human health
Illness from contact with, or drinking, affected water
Certain cyanobacteria produce toxins harmful to people, pets and livestock
Worked examples
WORKED EXAMPLE
A lake beside farmland turns bright green in summer, and by late summer dead fish are found. Explain how fertiliser use led to the fish deaths. [5]
Step 1: get the nutrients in
Rain washes excess nitrate and phosphate off the fields into the lake.
Step 2: growth
With nutrients no longer limiting, algae and aquatic plants multiply rapidly and form a bloom across the surface.
Step 3: the shading
The green layer blocks light, so submerged plants cannot photosynthesise. They die, and the oxygen they supplied is lost.
Step 4: the oxygen sink
Bacteria decompose the dead plants and algae. Their aerobic respiration removes dissolved oxygen from the water.
Step 5: the deathsOxygen falls to hypoxic levels, below what fish need to respire, so they suffocate. Larger, active fish are affected first.Nutrients → bloom → shading → decomposition → hypoxia → fish kill
WORKED EXAMPLE
Outline how positive feedback makes eutrophication worse over time. [3]
Name the loop
Nutrients cause growth; growth is followed by mass death; decomposition of that dead material releases nutrients again.
Show the amplificationEach cycle returns nutrients to the water, so the next bloom can be as large or larger even if no new fertiliser arrives.Give the consequence
Oxygen demand keeps rising, so hypoxia deepens and lasts longer each season.
A self-reinforcing loop, not a one-off eventPositive feedback = change amplified. Do not read it as “beneficial”.
💡 Exam tip
Learn the sequence as a chain of six links and write it in order. Marks are awarded per link, not for the general idea.
Always name decomposer bacteria as the main users of oxygen. That single word separates a top answer from an average one.
Use the correct terms: hypoxia, anoxia, dead zone, algal bloom, positive feedback. Precision reads as understanding.
If given data, link the axes. Rising nitrate in spring, peak algal cover in summer, minimum oxygen a few weeks later.
Mention that recovery is slow because nutrients keep cycling in the sediment. It is the reason management is difficult.
⚠ Common mix-up
“The algae use up all the oxygen.” Their decomposers are the bigger sink. Blame the bacteria and explain why.
“The bloom blocks oxygen from the air.” The real problem is that it blocks light, killing the plants that produce oxygen below.
Positive feedback is not a good thing. It means amplified, not beneficial.
Eutrophication is not the same as an algal bloom. The bloom is one stage in the process, not the whole thing.
Not all nutrient enrichment is human-made. Lakes enrich naturally over long timescales; it is the speed of the human version that causes the damage.
Forgetting phosphate. Many answers mention only nitrate, and lose the mark for the detergent and sewage source.
Up next: Managing Water Pollution — the three levels of management, how they map onto the stages of pollution, and how to evaluate them for marks.
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