Edexcel iGCSE Biology-5.16-5.17 Biological Consequences of Water Pollution- Study Notes- New Syllabus
Edexcel iGCSE Biology-5.16-5.17 Biological Consequences of Water Pollution- Study Notes- New syllabus
Edexcel iGCSE Biology-5.16-5.17 Biological Consequences of Water Pollution- Study Notes -Edexcel iGCSE Biology – per latest Syllabus.
Key Concepts:
5.16 understand the biological consequences of pollution of water by sewage
5.17 understand the biological consequences of eutrophication caused by leached minerals from fertiliser
Understand the Biological Consequences of Pollution of Water by Sewage
🌱 Introduction
When sewage (wastewater from homes, toilets, and industries) is released into rivers, lakes, or seas without proper treatment, it causes serious water pollution.
This pollution affects aquatic life, oxygen levels, and overall ecosystem health.
🚰 What Sewage Contains
- Organic matter (human waste, food residues, detergents)
- Pathogenic microorganisms (bacteria, viruses, parasites)
- Chemicals & nutrients (like nitrates and phosphates)
All these together disturb the natural balance of the water body.
⚗️ Step-by-Step: What Happens When Sewage Enters Water
1. Increase in Organic Waste
- Sewage adds a huge amount of organic material (food for microorganisms).
- Microorganisms start decomposing this waste rapidly.
2. Bacterial Decomposition Increases
- Aerobic bacteria break down the organic matter, using up dissolved oxygen (DO) from the water.
- As oxygen levels fall, fish and aquatic animals start to suffocate and die.
- Key term: Biochemical Oxygen Demand (BOD) → the amount of oxygen needed by microorganisms to decompose organic matter.
- Higher BOD = More pollution = Less oxygen for fish.
3. Oxygen Depletion (Anoxic Conditions)
- If too much sewage is added, bacteria use up almost all oxygen.
- The water becomes anoxic (no oxygen).
- Only anaerobic bacteria survive now.
- These release toxic gases like methane (CH₄), ammonia (NH₃), and hydrogen sulfide (H₂S) → making the water foul-smelling and poisonous.
4. Death of Aquatic Organisms
- Fish, prawns, and other oxygen-dependent animals die off.
- Food chains collapse.
- Result: “Dead zones” – areas where no aquatic life can survive.
5. Eutrophication (Nutrient Overload)
- Sewage often contains nitrates & phosphates → causes algal bloom.
- Algae block sunlight → underwater plants die.
- When algae die, they are decomposed by bacteria → further oxygen loss.
- Cycle continues → water body chokes slowly.
6. Spread of Diseases
- Untreated sewage carries pathogens like E. coli, Salmonella, cholera bacteria, hepatitis viruses, etc.
- Causes waterborne diseases such as:
– Cholera
– Typhoid
– Dysentery
– Hepatitis A - Humans and animals get infected when they drink or bathe in contaminated water.
7. Long-Term Ecological Impact
- Biodiversity in polluted water decreases.
- Toxic substances can accumulate in fish → biomagnification.
- Rivers and lakes become unfit for recreation or drinking purposes.
📊 Summary Table
| Stage | Process | Biological Consequence |
|---|---|---|
| 1 | Sewage enters water | Adds organic matter and nutrients |
| 2 | Bacteria decompose waste | Oxygen used up (high BOD) |
| 3 | Oxygen level drops | Fish and aerobic life die |
| 4 | Anaerobic decay begins | Foul smell, toxic gases released |
| 5 | Nutrient enrichment | Algal bloom → eutrophication |
| 6 | Pathogens spread | Diseases in humans and animals |
⚠️ Effects at a Glance
| Category | Example Effects |
|---|---|
| Aquatic Life | Fish suffocation, biodiversity loss |
| Human Health | Cholera, typhoid, hepatitis |
| Ecosystem | Eutrophication, dead zones |
| Environment | Toxic gases, foul odour, unsafe water |
💡 Prevention & Control Measures
- Treat sewage before release – remove solids & pathogens.
- Biological treatment – using bacteria to clean wastewater.
- Reduce use of detergents & fertilisers – to limit nutrient pollution.
- Public awareness – promote hygiene and waste management.
📘 Quick Recap
Sewage = organic waste + bacteria + nutrients
Bacteria decompose waste → oxygen drops → fish die
Anaerobic decay → foul gases
Nutrients → algal bloom → eutrophication
Pathogens → waterborne diseases
Overall → death of life + polluted, smelly water
🧠 Mnemonic: “SODAP” → Sewage → Oxygen drop → Death → Algae → Pathogens
Understand the Biological Consequences of Eutrophication Caused by Leached Minerals from Fertilisers
🌱 Introduction
Eutrophication is a process where excess minerals (mainly nitrates and phosphates) enter water bodies (rivers, lakes, ponds), leading to overgrowth of algae and serious damage to aquatic ecosystems.
These minerals usually come from fertilisers used in farming – when it rains, they leach (wash off) from soil into nearby water bodies.
💧 Step-by-Step: Process of Eutrophication
- Leaching of Minerals
Farmers use nitrate and phosphate fertilisers to increase crop yield.
When it rains, soluble minerals get washed from soil into rivers, lakes, or ponds.
This process is called leaching.
Trick: “Leaching = Leaving soil for lakes.” - Increase in Nutrient Levels (Nitrates & Phosphates)
The water body now becomes nutrient-rich, providing food for algae and aquatic plants.
Rapid nutrient enrichment is known as eutrophication. - Algal Bloom Formation
Algae grow excessively on the water surface forming a thick green layer (algal bloom).
This layer blocks sunlight from reaching underwater plants.
Result → Submerged plants cannot photosynthesise → they die. - Death of Aquatic Plants
Underwater plants die and decay due to lack of sunlight and competition for nutrients.
Dead plants become food for decomposing bacteria. - Increase in Bacterial Decomposition
Aerobic bacteria multiply rapidly to decompose dead plants and algae.
During decomposition, bacteria use up oxygen dissolved in water.
Term: Biochemical Oxygen Demand (BOD) → The oxygen needed for decomposition.
Higher BOD = More oxygen depletion. - Oxygen Depletion (Anoxic Conditions)
Oxygen levels drop sharply (hypoxia).
Water becomes anoxic → Fish and other aquatic animals suffocate and die.
Result: Formation of “Dead Zones” – areas where no aquatic life can survive. - Anaerobic Decay & Toxic Gases
Anaerobic bacteria produce foul-smelling, toxic gases like Methane (CH₄), Hydrogen sulfide (H₂S), Ammonia (NH₃).
Water becomes polluted, cloudy, and smelly. - Loss of Biodiversity
Many species die or migrate due to low oxygen.
Ecosystem balance is destroyed – few tolerant species dominate.
⚗️ Summary of the Eutrophication Process
| Step | Process | Biological Consequence |
|---|---|---|
| 1 | Leaching of nitrates/phosphates from soil | Nutrients enter water bodies |
| 2 | Nutrient enrichment | Promotes algal growth |
| 3 | Algal bloom forms | Blocks sunlight for underwater plants |
| 4 | Plants die | Lack of photosynthesis |
| 5 | Bacteria decompose dead matter | Oxygen levels fall |
| 6 | Oxygen depletion | Fish and aquatic life die |
| 7 | Anaerobic decay | Foul smell and toxic gases released |
| 8 | Long-term effect | Ecosystem collapse and biodiversity loss |
⚠️ Major Biological Consequences
| Category | Consequence |
|---|---|
| Plants | Submerged plants die due to lack of light |
| Animals | Fish and aquatic animals suffocate and die |
| Bacteria | Decomposers multiply rapidly, increasing oxygen demand |
| Ecosystem | Loss of biodiversity, formation of dead zones |
| Water Quality | Cloudy, smelly, unfit for human use |
💡 Real-Life Example
In agricultural regions, fertiliser run-off into lakes (e.g., Lake Erie in USA, parts of India) caused massive fish kills due to eutrophication.
Similar issues occur near farms using excess nitrogen-rich fertilisers.
🌎 Prevention & Control
- Use organic fertilisers (slow nutrient release).
- Buffer strips of grass between farms and rivers.
- Avoid overuse of chemical fertilisers.
- Treat farm wastewater before release.
- Educate farmers on sustainable agriculture.
📘 Quick Recap
Cause: Fertiliser minerals (nitrates & phosphates) leach into water.
Effect: Algal bloom → blocks sunlight → plants die → bacteria use oxygen → fish die.
Result: Low oxygen, foul water, loss of biodiversity.
Keyword: Eutrophication = “Too many nutrients → Too little life.”
Mnemonic: “Leach → Bloom → Breathe → Boom”
(Leaching → Algal bloom → Oxygen loss → Life collapse)