Eutrophication is the enrichment of a lake, estuary, or coastal water with mineral nutrients, especially nitrates and phosphates. It is commonly caused by nutrient-rich runoff and effluent from human activities, although natural enrichment can occur slowly.
How Eutrophication Develops
- Excess nitrates and phosphates enter the aquatic system from fertilizers, livestock waste, sewage, or urban and industrial effluent.
- Greater nutrient availability increases phytoplankton and algal growth, potentially producing an algal bloom.
- The bloom reduces light penetration. Submerged plants photosynthesize less and may die.
- When algae and plants die, they add organic matter for decomposers.
- Aerobic microorganisms decompose this material and consume dissolved oxygen through cellular respiration. This raises biochemical oxygen demand (BOD).
- Oxygen concentrations fall, creating hypoxic or severe anoxic conditions.
- Fish and oxygen-dependent invertebrates may die or migrate, reducing biodiversity and disrupting food webs. Some blooms also release toxins harmful to wildlife and humans.
| Cause | How it adds nutrients |
|---|---|
| Agricultural runoff | Rain washes nitrate and phosphate fertilizers into rivers and lakes. |
| Livestock waste | Manure introduces nitrogen compounds, phosphates, and organic matter. |
| Poorly treated sewage | Effluent supplies nutrients and decomposable organic matter. |
| Urban or industrial runoff | Water carries nutrient-containing wastes into aquatic systems. |
| Natural accumulation | Weathering, erosion, and succession enrich water gradually. |
A common misconception is that fertilizers directly remove oxygen. They do not: nutrients stimulate biomass growth, and the later decomposition of that biomass consumes dissolved oxygen.
IB Exam Technique
This is core IB ESS Topic 4.4: Water pollution content for SL and HL. For an “explain” question, give the complete causal chain: nutrient input, algal bloom, reduced light and plant death, decomposition, increased BOD, oxygen depletion, then organism death or migration. Apply this mechanism to unfamiliar water-quality data in Paper 1 or Paper 2 Section A. Name both the source and the resulting ecological consequence.