Eutrophication can reduce water quality, deplete dissolved oxygen, kill aquatic organisms, lower biodiversity, and disrupt ecosystem services. These impacts form a causal sequence beginning with excessive nutrient inputs.
Level: SL and HL core, Subtopic 4.4 Water pollution.
The Mechanism and Its Impacts
Eutrophication occurs when an aquatic ecosystem receives excessive mineral nutrients, especially nitrates and phosphates. Major sources include agricultural fertilizer runoff and sewage discharge.
| Stage | Process | Ecosystem impact |
|---|---|---|
| Nutrient enrichment | Nitrate and phosphate availability rises. | Phytoplankton and algae grow rapidly, producing an algal bloom. |
| Light reduction | Dense growth blocks sunlight. | Submerged plants photosynthesize less and die, reducing habitat and food. |
| Decomposition | Dead algae and plants are decomposed by microorganisms. | Microbial respiration raises biochemical oxygen demand (BOD). |
| Oxygen depletion | Dissolved oxygen falls. | Fish and invertebrates suffocate or migrate; sensitive species disappear. |
| System disruption | Food webs and species interactions change. | Pollution-tolerant species dominate; fisheries, recreation, and drinking-water supplies deteriorate. |
A common misconception is that eutrophication is simply an algal bloom. The bloom is only one stage; the most damaging effects often follow decomposition and oxygen depletion. Some blooms release toxins, but not every bloom is toxic.
The Chesapeake Bay in the United States is a named example where nutrient runoff contributes to seasonal oxygen-poor zones, harming organisms and fisheries. This creates an environmental justice issue because fishing communities may lose income and food despite producing little of the pollution.
Global warming, stratification, sewage disposal, and eutrophication can interact to increase (low-oxygen) or (oxygen-free) water.