EutrophicationSpec D4.2.8
In short
Eutrophication is the enrichment of aquatic and marine ecosystems with nutrients, here from leaching of nitrogen and phosphate fertilizers. Algae grow rapidly, block light and die. Decomposers breaking down the dead matter respire aerobically, increasing biochemical oxygen demand (BOD). Oxygen levels fall and fish and other animals die.
Eutrophication is the enrichment of water with nutrients. Nitrogen and phosphate fertilizers leach out of farmland and run into rivers, lakes and coastal seas.
- Nitrate and phosphate leach from fields into the water.
- Algae and other producers grow rapidly, forming an algal bloom at the surface.
- The bloom blocks light, so plants below cannot photosynthesise and die; the short-lived algae die too.
- Bacteria decompose the dead organic matter, using oxygen in aerobic respiration.
- Biochemical oxygen demand rises and dissolved oxygen falls.
- Fish and other animals that need oxygen die, forming a low-oxygen 'dead zone'.
- Biochemical oxygen demand (BOD)
- The amount of dissolved oxygen used by microorganisms in a water sample over a set time while breaking down organic matter. A high BOD shows heavy organic pollution.
(opens full size in a new tab)Common mistake:
Fish die from lack of oxygen used up by decomposers, not because algae 'use up the oxygen' by photosynthesis. Algae produce oxygen while alive.
Written and checked against the IB Biology HL specification · Updated October 2026