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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

Coverage 2017–2025 · Updated 16 Jul 2026

A Fair Mesocosm Test Changes One Factor and Tracks a Response

Design element Nutrient-enrichment test
independent variable phosphate added to treatment only
control addition equal volume of water without phosphate
controlled variables starting culture, vessel, light, temperature, mixing and sampling
dependent variables algal cell density and dissolved oxygen over time
reliability replicate treatment and control vessels

The design must also prevent escape or disposal of organisms and nutrients into natural water, and it must protect organisms from avoidable heat, starvation or oxygen stress.

Matched phosphate-treatment and control mesocosms receive equal light and stirring; only the treatment receives phosphate while algal cell density and oxygen concentration are logged.

Eutrophication Converts Nutrient Enrichment into Oxygen Loss

  1. Nitrate and phosphate inputs remove a nutrient limit on algae or cyanobacteria.
  2. A bloom reduces light penetration, so submerged plants die.
  3. Bloom cells also die, increasing dead organic matter.
  4. Decomposer populations and aerobic respiration increase.
  5. Biochemical oxygen demand rises as dissolved oxygen is consumed.
  6. Hypoxia or anoxia kills or excludes oxygen-dependent animals.

Algae do not directly “use up all the oxygen” simply by growing. The key oxygen-loss step is respiration, especially by decomposers processing dead biomass.

A nutrient-enrichment sequence shows nitrate and phosphate runoff, an algal bloom, shading and plant death, decomposer growth, higher biochemical oxygen demand, falling dissolved oxygen and fish death or migration.

BOD Measures Oxygen Consumed while Organic Matter Is Decomposed

Biochemical oxygen demand (BOD) is the amount of dissolved oxygen used by aerobic biological activity while organic matter in a measured volume of water is broken down.

  1. Measure dissolved oxygen in a known water sample.
  2. Incubate the sealed sample in darkness at 20 °C for five days.
  3. Measure dissolved oxygen again.
  4. Initial oxygen − final oxygen gives the five-day BOD.

Darkness prevents photosynthesis from replacing oxygen during incubation. High BOD indicates much respirable organic matter and/or strong decomposer activity, not high oxygen availability.

Use matched volume, temperature, time and handling. BOD is an indirect pollution measure and does not identify the pollutant by itself.

Eutrophication Changes More Than Dissolved Oxygen

Ecosystem property Typical eutrophication change
light penetration falls as algal biomass and turbidity rise
submerged macrophytes decline when photosynthesis becomes light-limited
producer diversity often falls as bloom-forming species dominate
food-chain length can shorten as habitats and oxygen-sensitive consumers disappear
net production may rise initially even while stability and diversity fall

Early nutrient enrichment can produce high primary production; later death, decomposition and hypoxia create the ecological damage. Sampling at only one time point can miss this sequence.

High productivity is not the same as a healthy or stable ecosystem. Composition, oxygen regime and food-web structure must also be judged.

Human Use Is Sustainable Only When Ecological Recovery Remains Possible

measure stock and ecosystem condition → keep removal and pollution within recovery capacity → monitor delayed and indirect effects → reduce pressure or restore processes when indicators deteriorate

Pressure Mechanism to name
excess nutrient input bloom → dead biomass → decomposer respiration → high BOD → hypoxia
persistent toxin retention within organisms plus concentration across trophic levels
marine plastic persistence, fragmentation, entanglement, ingestion and chemical transfer
harvesting removal relative to recruitment, growth, age structure and habitat condition
degraded habitat connectivity, species interactions and succession restored through rewilding

A resource being renewable, a crop yield being high or a restoration site becoming greener is not enough. Sustainability claims require evidence that future ecological capacity is retained.

Eutrophication exam focus

[ 6 ]

Discuss the causes and consequences of eutrophication.

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.