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4.11.5Additional lessons

Eutrophication — IB Biology HL Revision Notes & Topical Questions

5 min read 5 parts Lesson 96 of 101

📚 IB Biology HL Hub / Topical Revision
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Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
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Section 1 of 5Causes

Eutrophication is an excessive nutrient enrichment of water that disrupts the ecosystem.

Fertiliser run-off
  • Fertiliser run-off Rain washes nitrates and phosphates from fields into streams and lakes, adding nutrients that were limiting growth.
Sewage
  • Sewage Untreated or poorly treated sewage adds organic matter and phosphates, particularly from detergents.
Livestock waste
  • Livestock waste Slurry from intensive animal farms is rich in nitrogen and can leak into drains and rivers.
Leaching
  • Leaching Soluble nitrates move down through soil into groundwater and then to nearby water bodies.
StageChange in the water
EarlyNutrient levels rise
MiddleAlgal bloom, light blocked
LateOxygen very low, fish die
Dead zoneBottom water with almost no oxygen

Section 2 of 5Sequence of Events

1
Extra nitrate and phosphate allow algae and cyanobacteria to grow rapidly, forming a dense bloom on the surface.
2
The bloom blocks light, so submerged plants cannot photosynthesise and begin to die.
3
Dead algae and plants are decomposed by aerobic bacteria, which multiply and use up dissolved oxygen.
4
Biochemical oxygen demand rises and the water becomes hypoxic, so fish and invertebrates suffocate.
5
Anaerobic conditions develop and release toxic products, and biodiversity of the water body falls sharply.

Section 3 of 5Management

Management
Prevention
  • Prevention Applying fertiliser in the right amount and at the right time reduces the nutrients lost to drains.
Buffer strips
  • Buffer strips Strips of vegetation between fields and rivers absorb nutrients before they reach the water.
Sewage treatment
  • Sewage treatment Removing phosphates and nitrates at the works reduces the load discharged into rivers.
Restoration
  • Restoration Removing sediment, aerating the water or harvesting algae can help a damaged lake recover.
Monitoring
  • Monitoring Regular tests for nitrate, phosphate, chlorophyll and dissolved oxygen give an early warning that a water body is becoming enriched.

Section 4 of 5Worked Example

Question

  • Dissolved oxygen in a lake falls from 9.0 mg dm⁻³ before a bloom to 2.7 mg dm⁻³ after decomposition.
  • Calculate the percentage decrease.

  1. Decrease = 9.0 − 2.7 = 6.3 mg dm⁻³.
  2. Percentage decrease = 6.3 ÷ 9.0 × 100 = 70%.

Answer A 70% decrease, low enough to harm most fish.

Section 5 of 5Exam Checklist & Tips

4.11.5 – Key Points for the Exam

  • Eutrophication Nutrient enrichment of water leading to excess growth of algae.
  • Algal bloom A rapid increase of algae on the water surface.
  • Biochemical oxygen demand The amount of oxygen used by microorganisms to break down organic matter in water.
  • Leaching Washing of soluble ions out of soil by water.
  • Hypoxia Very low dissolved oxygen concentration.

Common Mistakes

  • Stating that algae die from lack of oxygen; the oxygen loss comes from decomposers.
  • Saying nutrients themselves are toxic, when the damage is indirect.
  • Forgetting that light blockage kills submerged plants.
  • Omitting the link between algal death and the rise of aerobic bacteria.

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