D4.2.8—Eutrophication
Eutrophication occurs when nitrate or phosphate enrichment drives algal blooms, decomposition, increased oxygen demand, hypoxia, and major aquatic biodiversity loss.
- Syllabus
- First assessment 2025
- Objective
- D4.2.8
- Level
- HL
Exam analysis
Chance of appearing5%of analysed past papers
Latest appearanceNovember 2025
Most common paperPaper3
Typical marks2–6
Common command terms
- Explain
- Discuss
Scoring notes
- Common mistake
- Saying algae directly use up all oxygen, instead of linking oxygen loss mainly to aerobic decomposition of dead organic matter.
Recent exam appearances
November 2025Paper1A ["HL"] · TZ340[ 1 ]D4.2.8—Eutrophication
November 2023Paper3 ["HL"] · TZ217(c)[ 3 ]D4.2.8—Eutrophication
May 2019Paper3 ["HL"] · TZ218[ 6 ]D4.2.8—Eutrophication
November 2018Paper3 ["HL"] · TZ016(b)[ 3 ]D4.2.8—Eutrophication
November 2017Paper3 ["HL"] · TZ018[ 6 ]D4.2.8—Eutrophication
Practice this objective
Coverage 2017–2025 · Updated 16 Jul 2026
Eutrophication Links Nutrients to Oxygen Loss
Eutrophication is enrichment by nutrients that stimulates algal growth and can deplete oxygen when biomass decomposes.
Nitrate or phosphate runoff increases primary production; decomposition consumes dissolved oxygen; hypoxia stresses or kills aquatic organisms and changes the food web.
Trace nutrient input, algal response, decomposition and oxygen before naming the ecological effect.; separate state, pressure, control loop and time
Fertilizer runoff causes an algal bloom; after algae die, bacteria respire intensely and fish suffocate in the oxygen-poor water. This gives a concrete prediction from the stated ecosystem.
Not every algal bloom is nutrient-driven, and oxygen recovery depends on flow, depth and further inputs. Interpret the result within the stated model and evidence limits.
Eutrophication exam focus
Assessment in practice
1–6 marksHow it is assessed
This objective is assessed through essay response, commonly using Explain / Discuss.
Command terms
Explain / Discuss
What earns marks
Build the answer around this relationship: Nitrate and phosphate enrichment commonly starts eutrophication in aquatic ecosystems.
Watch for
Saying algae directly use up all oxygen, instead of linking oxygen loss mainly to aerobic decomposition of dead organic matter.
Representative question
Question 1
Discuss the causes and consequences of eutrophication.
Core Stability and Change
Core D4.2 is secure when students can judge whether a system is being stabilized or pushed toward change. The route is: identify the stability support or disturbance, explain the mechanism, and state the ecosystem consequence using evidence.
- energy, nutrient cycling, diversity, and tolerance ranges maintain persistence
- Amazon deforestation and keystone removal can push systems toward instability
- harvest and agriculture require recovery, soil, nutrients, biodiversity, and monitoring
- eutrophication, biomagnification, and plastics harm ecosystems through specific mechanisms
Ecosystem Stability and Human Impact
Core transfer questions ask students to explain why an ecosystem remains stable or why a disturbance pushes it toward change. Strong answers do not list threats; they explain mechanisms such as lost rainfall recycling, trophic cascade, overharvest, nutrient enrichment, toxin biomagnification, plastic movement, or restoration through rewilding.
- Use stability requirements: energy input, nutrient cycling, biodiversity, genetic diversity, and abiotic tolerance ranges.
- Explain disturbance mechanisms such as Amazon tipping points, trophic cascades, overharvesting, agricultural damage, eutrophication, biomagnification, plastics, or rewilding.
- Support claims with evidence from controlled models, monitoring, food webs, or pollution pathways.
Concept essentials
- Nitrate and phosphate enrichment commonly starts eutrophication in aquatic ecosystems.
- Algal blooms increase organic matter that aerobic decomposers break down.
- Higher biochemical oxygen demand lowers dissolved oxygen and can kill aquatic organisms.