Eutrophication
Big idea: Eutrophication is when excess nutrients cause algal blooms that eventually suffocate aquatic life. It creates dead zones - areas where almost nothing can survive.
The eutrophication process (learn this sequence!)
- Nutrient input - nitrates and phosphates enter water (fertilizers, sewage)
- Algal bloom - excess nutrients cause rapid algae growth
- Light blocked - algae on surface blocks sunlight to plants below
- Plant death - submerged plants die without light
- Decomposition - bacteria decompose dead matter, using oxygen
- Oxygen depletion - dissolved oxygen falls (hypoxia)
- Fish kills - animals die or flee the low-oxygen zone
- Dead zone - area becomes lifeless
PAST PAPER ALERT (Paper 1, 4 marks): Explain how nutrient pollution could impact aquatic food production in the Gulf of Mexico. You MUST link eutrophication to FOOD PRODUCTION (fish deaths, dead zones reducing fisheries). Max 3 marks if you only describe eutrophication without the food link!
Major dead zones
- Gulf of Mexico - approximately 15,000 km squared, from Mississippi River nutrients
- Baltic Sea - one of largest, from agricultural runoff
- Chesapeake Bay - USA, improving with management
- Lake Erie - recurring blooms from farm runoff
Sources of nutrient pollution
- Agricultural fertilizers - largest source globally
- Sewage and wastewater - contains N and P from human waste and detergents
- Animal waste - concentrated from factory farms
- Atmospheric deposition - N from vehicle and industrial emissions
- Urban runoff - fertilizers from lawns and gardens
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IB-style question — how eutrophication kills fish [4]
A shallow bay receives heavy fertiliser run-off from nearby farms every spring.
Explain the chain of events by which these extra nutrients can cause fish in the bay to die. [4]
How to answer it, step by step
- Nutrients feed algae
• Extra nitrate and phosphate make algae grow fast (an algal bloom).
• A thick algal mat blocks sunlight, so underwater plants die. - Dead matter strips the oxygen
• Bacteria decompose the dead algae and plants, using up dissolved oxygen (BOD rises).
• With little oxygen left (hypoxia), the fish suffocate and die.
Final answer
Examiners want the full sequence: nutrients → algal bloom → light blocked → decomposition → oxygen drop → fish die.