PollutionCambridge IGCSE Biology: Revision notes

Untreated sewage contains high levels of organic matter and nitrate ions, which enter water bodies and trigger a damaging chain of events.

When sewage enters an aquatic ecosystem:

• The organic matter provides a rich food source for decomposers (bacteria and fungi)
• Decomposer populations increase rapidly and respire aerobically
• Aerobic respiration consumes dissolved oxygen at a much faster rate than it can be replenished
• Dissolved oxygen concentration decreases sharply
• Aquatic organisms that require high dissolved oxygen levels (fish, macroinvertebrates) die from oxygen depletion
• This creates anaerobic conditions where only anaerobic decomposers survive
• Dead organisms accumulate, further reducing water quality

The effects are particularly severe because sewage is a point source of pollution, meaning it enters from one specific location and creates a localised dead zone.

Eutrophication is the process by which excess nutrients (especially nitrate and phosphate ions) cause aquatic ecosystems to become depleted of oxygen and biologically dead.

The eutrophication process occurs in this sequence:

1. Increased nitrate and other ions from fertiliser runoff enter water bodies
2. Increased producer (algae) growth – excess nitrogen promotes rapid algal growth (algal bloom)
3. Increased decomposition – when algae die, decomposers break down the dead biomass
4. Increased aerobic respiration by decomposers – breaking down large quantities of dead algae requires intense aerobic respiration
5. Reduction in dissolved oxygen – respiration consumes oxygen faster than it can be replaced; dissolved oxygen levels drop dramatically
6. Death of organisms requiring dissolved oxygen – fish and other aerobic aquatic life suffocate and die

Key difference from sewage: Eutrophication is caused by excess nutrients triggering algal growth, whereas sewage directly provides organic matter for decomposer respiration.

Consequences:

• Loss of biodiversity in the affected water body
• Creation of anaerobic dead zones
• Collapse of food chains that depend on oxygen-requiring organisms

Non-biodegradable plastics are synthetic polymers that cannot be broken down by natural decomposers and persist in ecosystems indefinitely.

Effects in aquatic ecosystems:

• Entanglement – marine animals (seals, dolphins, seabirds) become tangled in plastic bags, nets, and fishing line, causing injury or death
• Ingestion – animals mistake plastic fragments and microplastics for food, causing internal injury, blockages, and malnutrition
• Habitat damage – accumulated plastic debris damages coral reefs and smothers the seabed
• Bioaccumulation – toxic chemicals used in plastic manufacture (e.g. BPA, phthalates) are absorbed by organisms and accumulate up food chains
• Fragmentation – large plastic items break into microplastics (particles <5mm) that enter food chains at every level

Effects in terrestrial ecosystems:

• Soil contamination – plastic waste breaks down slowly, releasing toxins into soil and reducing fertility
• Reduced decomposition – presence of plastic inhibits natural decomposition processes
• Plant damage – roots become entangled; plastics physically block plant growth
• Toxin release – degradation of plastics releases harmful chemicals that poison organisms
• Habitat obstruction – plastic debris creates physical barriers and reduces available habitat

Why non-biodegradable plastics are problematic:

• Decomposers lack enzymes to break down synthetic polymers
• Persistence means effects accumulate over decades or centuries
• No natural mechanism for removal from ecosystems

Methane (CH₄) and carbon dioxide (CO₂) are the two major greenhouse gases released by human activity.

Sources of carbon dioxide:

• Combustion of fossil fuels – burning coal, oil, and natural gas in power stations, vehicles, and industries
• Respiration – increased human population respires, releasing CO₂
• Deforestation – removes trees that absorb CO₂; decomposition of felled trees releases stored carbon
• Industrial processes – cement and steel production release CO₂ as a byproduct

Sources of methane:

• Livestock farming – cattle and sheep produce methane during anaerobic digestion in their ruminant stomachs
• Rice paddies – anaerobic conditions in flooded fields allow methanogenic bacteria to produce methane
• Landfill sites – anaerobic decomposition of waste produces methane
• Natural gas extraction and leakage – methane escapes during drilling, transport, and distribution
• Wetlands – natural sources, but increasingly affected by human activity

Why these gases matter:

• Both are potent greenhouse gases that trap heat in the atmosphere
• Methane is approximately 25–28 times more effective at trapping heat than CO₂ over a 100-year period
• Concentrations of both have increased dramatically since the Industrial Revolution due to human activity

The enhanced greenhouse effect is the increase in Earth's natural greenhouse effect caused by human emissions of greenhouse gases.

How the natural greenhouse effect works:

1. Solar radiation enters Earth's atmosphere and heats the surface
2. The surface re-radiates this energy as infrared radiation (heat)
3. Greenhouse gases in the atmosphere absorb some of this infrared radiation
4. This trapped heat warms the atmosphere and surface (this is the natural greenhouse effect, which is essential for life)

The enhanced greenhouse effect:

• Human activities have increased atmospheric concentrations of CO₂ and methane
• Higher concentrations of greenhouse gases trap more infrared radiation than normal
• More heat is retained in the atmosphere and at the surface
• Global average temperatures increase beyond the natural baseline

Effects of climate change:

• Rising global temperatures – average temperatures increase, affecting weather patterns
• Melting of polar ice and glaciers – leads to sea-level rise
• Ocean acidification – CO₂ dissolves in seawater, forming carbonic acid, which damages marine ecosystems (corals, shell-forming organisms)
• Changing precipitation patterns – some regions experience drought, others flooding
• Ecosystem disruption – species cannot adapt quickly enough; habitats are destroyed
• Increased extreme weather events – hurricanes, heatwaves, and storms become more frequent and intense

Key distinction:

• The natural greenhouse effect is essential and keeps Earth warm enough for life (~15°C)
• The enhanced greenhouse effect is a problem because it is warming the planet too quickly, disrupting ecosystems and human societies