4.4.6—Eutrophication sequence
- Syllabus
- First assessment 2026
- Objective
- 4.4.6
- Level
- HL
Build the Oxygen-Collapse Chain
Excess limiting nutrients can trigger bloom → shading → death → decomposition → high BOD → hypoxia or anoxia.
Aerobic decomposers consume oxygen as dead biomass accumulates. Sediment and decomposition can recycle nutrients, feeding another bloom and reinforcing the decline.
A summer algal bloom dies; bacteria use oxygen overnight, fish surface for air, and bottom water becomes hypoxic.
Death adds organic matter; decomposition raises oxygen demand and lowers DO.
Nutrient enrichment and hypoxia are stages in a chain, not synonyms.