Eutrophication

Summary: Eutrophication is the process where excess nutrients (nitrates and phosphates from fertilisers) enter water bodies, causing algal blooms that deplete dissolved oxygen and kill aquatic life. Tags: igcse chemistry environment eutrophication keyword Created: 2026-07-14 Last Updated: 2026-07-14

Definition

Eutrophication is the enrichment of water bodies with nutrients (mainly nitrates and phosphates from agricultural fertilisers), leading to excessive plant and algal growth that ultimately depletes dissolved oxygen and kills aquatic organisms.

Process (Step by Step)

1. Fertiliser runoff: Rain washes nitrate (NO₃⁻) and phosphate (PO₄³⁻) fertilisers from farmland into rivers, lakes, and streams
2. Algal bloom: Excess nutrients cause algae and water plants to grow rapidly — a thick green layer forms on the water surface
3. Light blockage: The algal layer blocks sunlight from reaching underwater plants
4. Plant death: Underwater plants die (cannot photosynthesise without light)
5. Bacterial decomposition: Aerobic bacteria decompose the dead organic matter, consuming dissolved oxygen (O₂) in the water
6. Oxygen depletion: Dissolved O₂ levels drop dramatically
7. Aquatic death: Fish and other aquatic organisms die from hypoxia (oxygen starvation)

Sources of Eutrophying Nutrients

Prevention

• Use fertiliser sparingly and at appropriate times (avoid application before heavy rain)
• Maintain buffer zones of vegetation between farmland and water bodies
• Treat sewage before discharge into rivers
• Use phosphate-free detergents

See Also

• NPK Fertiliser — Fertilisers and their role in plant nutrition
• Haber Process and Ammonia — Ammonia production for fertilisers
• Climate Change and Greenhouse Gases — Related environmental chemistry
• Water Treatment — Removing pollutants from water
• IGCSE-Chem-Index