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
- Sewage is wastewater produced from homes, hospitals, industries and other human activities.
- It contains human waste, food waste, detergents, chemicals and large numbers of microorganisms.
- If untreated sewage is released into rivers, lakes or seas, it causes water pollution.
- Sewage pollution affects the quality of water and harms aquatic organisms.
- It also increases the spread of disease and disturbs the balance of aquatic ecosystems.
What is Sewage?
Definition: Sewage is wastewater containing human waste, household waste and other organic materials that must be treated before being released into the environment.
What Happens When Sewage Enters Water?
- When untreated sewage enters a river or lake:
- It adds a large amount of organic matter to the water.
- Bacteria and other decomposers begin breaking down this organic matter.
- During decomposition, these microorganisms use oxygen for aerobic respiration.
- As more oxygen is used, the amount of dissolved oxygen in the water decreases.
- This is the main reason why sewage pollution is harmful to aquatic life.
Stage 1: Increase in Decomposer Activity
- Sewage contains large amounts of biodegradable organic waste.
- Decomposer bacteria feed on this waste.
- Their population increases rapidly because plenty of food is available.
- More bacteria means more respiration takes place.
Stage 2: Dissolved Oxygen Decreases
- As bacteria respire:
- They remove oxygen dissolved in the water.
- Oxygen levels continue to fall.
- The water may no longer contain enough oxygen for aquatic animals.
- This is called oxygen depletion.
Stage 3: Aquatic Animals Are Affected
- Most aquatic animals, such as fish, insect larvae, crabs and freshwater shrimps, need dissolved oxygen for respiration.
- When oxygen levels become very low:
- Fish find it difficult to breathe.
- Their activity decreases.
- Growth and reproduction are affected.
- Many organisms may die due to lack of oxygen.
Stage 4: Death of Aquatic Organisms
- If oxygen levels continue to decrease:
- Large numbers of fish and other aquatic animals may die.
- Sensitive species disappear first.
- Only organisms that can survive in low-oxygen conditions remain.
- This changes the balance of the ecosystem.
Stage 5: Growth of Harmful Microorganisms
- Untreated sewage may contain disease-causing microorganisms such as bacteria, viruses and protozoa.
- These pathogens can:
- Contaminate drinking water.
- Spread diseases to humans.
- Infect animals that depend on polluted water.
Stage 6: Damage to Aquatic Ecosystems
- Continuous sewage pollution can:
- Reduce water quality.
- Destroy habitats.
- Kill aquatic plants and animals.
- Disrupt food chains.
- Reduce biodiversity.
- Over time, the ecosystem becomes less stable.
⚠️ Biological Consequences of Sewage Pollution
1. Reduction in Dissolved Oxygen
- Decomposer bacteria use oxygen during respiration.
- Oxygen levels in water decrease.
- Less oxygen is available for aquatic organisms.
2. Death of Fish and Other Aquatic Animals
- Fish cannot obtain enough oxygen.
- Large numbers may die if oxygen levels become too low.
- Populations of aquatic animals decrease.
3. Increase in Disease
- Pathogens present in sewage can spread diseases through contaminated water.
- This affects humans, domestic animals and wildlife.
4. Reduced Biodiversity
- Sensitive species disappear.
- Only tolerant species survive.
- Biodiversity decreases.
5. Disruption of Food Chains
- Loss of organisms affects predators and prey.
- Population balance is disturbed.
6. Poor Water Quality
- Water becomes less clean.
- Less suitable for aquatic life.
- Unsafe for drinking unless treated.
🌍 Why Should Sewage Be Treated?
- Remove harmful microorganisms.
- Reduce organic waste.
- Prevent oxygen depletion.
- Protect aquatic organisms.
- Improve water quality.
- Reduce spread of disease.
- Maintain healthy aquatic ecosystems.
📊 Biological Consequences of Sewage Pollution
| Consequence | Explanation |
|---|---|
| Increase in decomposers | Bacteria feed on organic waste and multiply rapidly. |
| Oxygen depletion | Bacteria use dissolved oxygen during respiration. |
| Death of aquatic animals | Fish and other organisms cannot obtain enough oxygen. |
| Spread of disease | Pathogens in sewage infect humans and animals. |
| Reduced biodiversity | Sensitive species die, leaving fewer organisms. |
| Disrupted food chains | Loss of organisms affects ecosystem balance. |
| Poor water quality | Water becomes unsafe for aquatic life and humans. |
🌱 Sequence of Events in Sewage Pollution
| Step | What Happens? |
|---|---|
| 1 | Untreated sewage enters a river or lake. |
| 2 | Decomposer bacteria feed on organic waste. |
| 3 | Bacteria respire and use dissolved oxygen. |
| 4 | Oxygen levels in water decrease. |
| 5 | Aquatic organisms struggle to respire. |
| 6 | Many organisms die, reducing biodiversity and disrupting the ecosystem. |
📘 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
- Fertilisers are added to soil to increase crop growth by providing minerals like nitrates and phosphates.
- Sometimes, rain washes (leaches) these minerals from soil into nearby rivers and lakes.
- This leads to an excessive growth of algae in water bodies.
- The process of nutrient enrichment followed by oxygen depletion is called eutrophication.
- It mainly affects aquatic ecosystems and can kill many organisms living there.
What is Eutrophication?
Definition
- Eutrophication is the process in which water bodies become rich in nutrients, causing excessive growth of algae and a drop in oxygen levels.
- It usually starts when fertiliser minerals (nitrates/phosphates) enter water systems through runoff or leaching.
⚙️ Step-by-Step Process of Eutrophication
1. Fertiliser Run-off or Leaching
- Farmers use fertilisers to improve crop yield.
- Rainwater washes nitrates and phosphates from the soil.
- These minerals enter rivers, ponds, and lakes.
2. Algal Bloom Formation
- Extra nutrients in water act like food for algae.
- Algae multiply rapidly on the surface.
- This forms a thick layer called an algal bloom.
3. Blocking of Sunlight
- The algal bloom covers the water surface.
- Sunlight cannot reach deeper water plants.
- Submerged plants cannot photosynthesise properly.
4. Death of Aquatic Plants
- Lack of light stops photosynthesis.
- Plants die due to lack of food production.
- This increases dead organic matter in the water.
5. Increase in Decomposers
- Bacteria and fungi begin breaking down dead plants.
- Decomposers multiply rapidly.
- They use oxygen for respiration.
6. Oxygen Depletion
- Decomposers consume large amounts of dissolved oxygen.
- Oxygen levels in water drop sharply.
- This leads to low oxygen (hypoxic) conditions.
7. Death of Aquatic Animals
- Fish and other aquatic animals need oxygen for respiration.
- Low oxygen levels cause suffocation.
- Many organisms die, especially fish and invertebrates.
8. Final Ecosystem Collapse
- Most aquatic life is destroyed.
- Water becomes foul-smelling and lifeless.
- Biodiversity in the water body decreases significantly.
🧬 Biological Consequences of Eutrophication
1. Oxygen depletion
- Dissolved oxygen in water decreases.
- Aquatic animals cannot carry out aerobic respiration.
- Leads to mass fish deaths.
2. Loss of biodiversity
- Sensitive species die due to lack of oxygen.
- Only a few tolerant organisms may survive.
- Overall species variety decreases.
3. Disruption of food chains
- Death of producers (plants) affects herbivores.
- Loss of herbivores affects carnivores.
- Entire aquatic food web becomes unstable.
4. Death of aquatic plants
- No light for photosynthesis.
- Plants die and increase decomposition.
- Further worsens oxygen shortage.
5. Increase in decomposers
- Bacteria population increases.
- More oxygen is used in decomposition.
- This speeds up oxygen depletion.
6. Poor water quality
- Water becomes cloudy, green, and foul-smelling.
- It becomes unsuitable for drinking or farming use.
📊 Eutrophication Summary Flow
| Stage | What Happens |
|---|---|
| Fertiliser runoff | Nitrates and phosphates enter water |
| Algal bloom | Rapid growth of algae on surface |
| Light blocked | Submerged plants cannot photosynthesise |
| Plant death | Aquatic plants die |
| Decomposition | Bacteria break down dead matter |
| Oxygen drop | Oxygen used up in respiration |
| Animal death | Fish and aquatic organisms die |
🌍 Why Eutrophication is Harmful
- It destroys aquatic ecosystems.
- It reduces oxygen levels in water.
- It kills fish and other organisms.
- It disrupts natural food chains.
- It reduces biodiversity in freshwater habitats.
- It makes water unsuitable for human use.
📝 Key Points
- Eutrophication is caused by leached fertiliser minerals (nitrates and phosphates) entering water bodies.
- These nutrients cause rapid algal growth (algal bloom).
- Algae block sunlight, leading to death of aquatic plants.
- Decomposers break down dead plants and use up oxygen.
- Oxygen levels fall, causing death of fish and other aquatic animals.
- The result is a low-oxygen, lifeless aquatic ecosystem with reduced biodiversity.
🌎 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)