Eutrophication is the story of a lake being fed to death. Nothing toxic is added — just plant food. But the chain of events that follows can strip a water body of oxygen and kill almost everything in it. This is the most examined process in the whole sub-topic, so learn the sequence properly.
📚 What you need to know
Eutrophication happens when a water body receives large amounts of mineral nutrients, mainly nitrates and phosphates.
Nitrate mostly comes from agricultural run-off. Phosphate comes from detergents, sewage and fertiliser.
The sequence: nutrient enrichment → rapid plant and algal growth → algal bloom → light blocked → plants below die → bacteria decompose them → dissolved oxygen crashes → aquatic life dies.
The oxygen is used up by aerobic decomposers, not by the algae themselves.
Hypoxia is low oxygen; anoxia is a complete lack of it. Severe cases create dead zones.
It is a positive feedback system: decomposition releases nutrients, which feed more growth, which produces more decomposition.
What starts it
In a healthy lake, nutrients are the limiting factor. Algae would love to grow faster but there is not enough nitrogen and phosphorus to let them. Remove that limit and growth explodes.
Nutrient
Main sources
Why it reaches the water
Nitrate
Synthetic and organic fertilisers, animal slurry, sewage
Very soluble, so it leaches through soil and washes off fields with rain
Phosphate
Fertilisers, some detergents, untreated or partly treated sewage
Sticks to soil particles, so it arrives attached to eroded sediment as well as dissolved
If a question asks which nutrient matters more, say it depends on the system: phosphate is usually the limiting nutrient in freshwater, nitrate more often in coastal seas. Naming the limiting factor shows real understanding.
The sequence, step by step
🧩 Learn this chain in order
Nutrient enrichment. Excess nitrate and phosphate enter the water from run-off or sewage.
Fast plant growth. Macrophytes such as duckweed and water hyacinth spread across the surface.
Algal bloom. Algae and phytoplankton reproduce rapidly and can cover the water completely.
Light is blocked. The surface mat stops sunlight reaching submerged plants.
Submerged plants die. They can no longer photosynthesise, so they stop producing oxygen and then die. Algae die too once nutrients run short.
Decomposition. Aerobic bacteria break down the huge mass of dead material and consume dissolved oxygen doing it.
Oxygen crash. DO falls, causing hypoxia or anoxia. Fish and invertebrates die or leave.
Photosynthesis by submerged plants normally adds oxygen to the water. Shading removes that supply at exactly the moment decomposition raises demand.
Why it is a positive feedback loop
The reason eutrophication is so hard to reverse is that it feeds itself. Decomposing the dead algae releases the nutrients locked inside them straight back into the water, ready to fuel the next bloom.
Because nutrients are recycled internally, a lake can keep blooming for years after the fertiliser run-off has been reduced. Sediment holds a nutrient store.
Positive does not mean good. In systems language, positive feedback amplifies a change and drives the system away from equilibrium. Negative feedback is the stabilising one. Examiners test this wording deliberately.
Hypoxia, anoxia and dead zones
Three terms, learn them exactlyHypoxia = low dissolved oxygen — too little for most aquatic life Anoxia = severe or complete absence of dissolved oxygen Dead zone = an area where oxygen is too low to support aquatic life
A useful memory hook: hypo- means below normal (think hypothermia), and an- means without (think anaemia). Hypoxia is a shortage. Anoxia is nothing left.
Dead zones form where nutrient-rich river water meets the sea and the water column separates into layers. Large seasonal dead zones are well documented where major rivers drain intensive farmland into enclosed or slow-mixing seas.
Consequences for people
Ecosystem service
What eutrophication does to it
Fisheries
Sudden fish kills, then long-term decline in stocks and lost income for fishing communities
Recreation and aesthetics
Green, murky water, bad smells from decay, foam and slime; swimming and boating become unpleasant
Drinking water
Treatment becomes harder and more expensive; some blooms release toxins into supplies
Human health
Toxins from some blooms can cause illness through drinking water or contaminated shellfish
Biodiversity
Tolerant species dominate, sensitive species disappear, so diversity falls even where total biomass is high
EXAM PRACTICE
A lake next to arable farmland shows nitrate rising from 4 to 26 mg per litre over ten years, while summer dissolved oxygen falls from 9 to 3 mg per litre. Explain the link between the two trends. [5]
Start with the cause
Rising nitrate comes from fertiliser run-off leaching off the arable land into the lake.
Then the biological response
Nitrate is a limiting nutrient, so removing that limit lets algae and phytoplankton grow rapidly, forming a surface bloom.
Then the shading step
The bloom blocks light, so submerged plants cannot photosynthesise. They stop adding oxygen, then die.
Then the oxygen step
Aerobic bacteria decompose the dead algae and plants, using dissolved oxygen in respiration.
Oxygen demand rises while oxygen supply falls, so summer DO drops to hypoxic levelsFive marks, five links. Write the chain as separate sentences and the marks tick themselves off.
💡 Exam tip
Never stop at “the algae use up the oxygen”. Name the decomposing bacteria as the main consumers — that is the marking point.
Mention that shading also removes an oxygen source, not only that it kills plants. Two effects, two marks.
Use “positive feedback” and explain why it amplifies. Do not just label it.
Summer readings are usually the worst: warm water holds less oxygen and decomposition speeds up.
If given data, quote figures from it. “Nitrate rose from 4 to 26 mg per litre” earns credit that “nitrate went up” does not.
⚠️ Common mix-up
Saying algae use up all the oxygen while alive. Algae photosynthesise by day; the crash comes from decomposing them after death.
Calling eutrophication a pollutant. It is a process. The pollutants are nitrate and phosphate.
Mixing hypoxia and anoxia. Low vs. none.
Thinking positive feedback is beneficial. It means self-reinforcing, and here it makes things worse.
Assuming the lake recovers as soon as fertiliser use stops. Nutrients stored in sediment keep the loop running for years.
Up next: Managing Water Pollution — the three levels of intervention, and why the cheapest fix is nearly always the earliest one.
Want this explained one-to-one?
Book a free session with an experienced IB ESS tutor and get your trickiest topics made simple.
