20.3 Pollution
- Syllabus
- 0610–2026–2027
- Topic
- 20.3
- Level
- —
Learning objectives
20.3.1Effects of untreated sewage and excess• Describe the effects of untreated sewage and excess fertiliser on aquatic ecosystems20.3.2Effects of non-biodegradable plastics• Describe the effects of non-biodegradable plastics, in both aquatic and terrestrial ecosystems20.3.3Sources and effects of pollution• Describe the sources and effects of pollution of the air by methane and carbon dioxide, limited to: the enhanced greenhouse effect and climate change20.3.4Process of eutrophication of water• Explain the process of eutrophication of water, limited to: • increased availability of nitrate and other ions • increased growth of producers • increased decomposition after death of producers • increased aerobic respiration by decomposers • reduction in dissolved oxygen • death of organisms requiring dissolved oxygen in water
Compare sewage and fertiliser pollution in water
Untreated sewage and excess fertiliser both alter aquatic ecosystems and can reduce dissolved oxygen, but they enter the system differently.
| Pollutant | Initial change | Effects on the ecosystem |
|---|---|---|
| untreated sewage | adds organic waste, microorganisms and mineral ions | decomposers increase and respire aerobically; dissolved oxygen falls, so oxygen-requiring organisms may die; pathogens may spread |
| excess fertiliser | nitrate and other ions wash into the water | producers grow rapidly; later decomposition and aerobic respiration reduce dissolved oxygen, so oxygen-requiring organisms may die |
In both cases, connect the pollutant to biological activity, then to oxygen concentration and organism survival.
Fertiliser does not kill fish by directly using oxygen. The oxygen is used during increased aerobic respiration by decomposers.
Explain why persistent plastics harm ecosystems
Non-biodegradable plastics are not broken down by decomposers, so they persist and accumulate in aquatic and terrestrial ecosystems.
| Aquatic ecosystems | Terrestrial ecosystems |
|---|---|
| animals may become entangled, injured or unable to move and breathe | animals may become trapped or strangled |
| swallowed plastic can block the digestive system, reduce feeding and cause death | ingestion can block digestion and reduce nutrition |
| microplastics can enter food chains and pass to consumers | plastic and associated chemicals can accumulate in organisms and food chains |
| persistent litter damages habitats | persistent litter can cover or damage habitats and soil environments |
The key property is persistence: non-biodegradable does not mean harmless. Large pieces and microplastics can cause different but connected effects.
Link greenhouse gases to sources and climate change
Methane and carbon dioxide are air pollutants that increase the enhanced greenhouse effect and contribute to climate change.
| Gas | Important human-related sources |
|---|---|
| carbon dioxide | burning fossil fuels; deforestation through burning and reduced removal by photosynthesis |
| methane | cattle and other livestock; rice farming; decomposition of waste in landfill |
Higher greenhouse-gas concentrations cause more outgoing infrared radiation to be absorbed and re-radiated in the atmosphere. Less energy escapes to space, strengthening the greenhouse effect and raising global temperatures.
The resulting climate change can alter rainfall and habitats, increase extreme conditions, melt ice and raise sea level.
The natural greenhouse effect makes Earth warm enough for life; pollution enhances it. Weather is a short-term condition, while climate change is a long-term shift in patterns.
Trace the six steps of eutrophication
Eutrophication is a causal sequence in which extra mineral ions ultimately reduce dissolved oxygen in water.
- The availability of nitrate and other ions increases.
- Producers grow more rapidly.
- After producers die, their decomposition increases.
- Decomposers carry out more aerobic respiration.
- The concentration of dissolved oxygen decreases.
- Organisms that require dissolved oxygen die.
The link between steps 3 and 5 is respiration: decomposers use dissolved oxygen while breaking down dead producers.
Keep the order exact. Producers first grow, then die and are decomposed; decomposers—not nitrate ions directly—remove the dissolved oxygen through aerobic respiration.