Human Impact on the Environment
Ecology and the Environment · 8 question types
Exam Frequency Analysis
Past paper frequency (2018 to 2024)
This topic accounts for approximately 10% of your exam marks.
Climate change, deforestation, and biodiversity loss are growing in frequency as contemporary issues.
Eutrophication is the over-enrichment of a body of water with nutrients, leading to the collapse of its ecosystem. It is the leading cause of dead zones in lakes, rivers and coastal seas worldwide.
The five-step sequence
- Excess nutrients (nitrate and phosphate) enter a body of water. Most commonly from agricultural fertilisers washed off fields by rain, or from sewage containing detergents.
- The extra nitrates and phosphates cause explosive growth of algae at the water surface (an algal bloom).
- The thick algal layer blocks sunlight from reaching the deeper-water plants. Those plants cannot photosynthesise and die.
- The dead plants and (later) the algae are broken down by decomposer bacteria. The decomposers multiply rapidly and use up the dissolved oxygen in the water as they respire.
- Fish, invertebrates and other aquatic animals suffocate because there is no longer enough oxygen in the water. The water body becomes a "dead zone".
Describe the sequence of events that leads to fish dying after fertiliser runoff
What comes up: questions ask you to describe or explain eutrophication, usually for 4–8 marks in the form of a step-by-step chain.
Write (five marks): (1) fertiliser/nitrates (and phosphates) wash off fields into the water body; (2) algae grow rapidly at the surface, forming an algal bloom that blocks light from reaching plants below; (3) the underwater plants die because they cannot photosynthesise; (4) decomposer bacteria break down the dead organic matter and their numbers increase; (5) the bacteria use up dissolved oxygen during respiration, so aquatic animals suffocate and die.
Watch out: it is the decomposer bacteria (through their respiration) that deplete the oxygen and kill the fish, not the algae directly. The mark scheme credits "bacteria/microorganisms respire" specifically for the oxygen-depletion step — writing "algae use up oxygen" will not score that mark.
Why fertilisers are the main culprit
Modern agriculture uses huge amounts of nitrogen-based fertiliser to maximise crop yields. Plants only absorb a fraction of the applied nitrate; the rest stays in the soil and is leached out by rainfall into rivers and groundwater. Even small fields can release enough nitrate to disturb a downstream lake.
Phosphates from older washing detergents used to make the problem worse; phosphate-free detergents have largely solved this part of the problem in the UK, but agricultural runoff remains the dominant source.
Where eutrophication happens
- Lakes and slow rivers in farming areas
- Coastal seas at the mouths of large rivers draining intensive agricultural land. The Gulf of Mexico has a dead zone the size of New Jersey every summer, fed by nitrate runoff from the Mississippi River
- The Baltic Sea, surrounded by farming countries, has experienced widespread eutrophication
Solutions
- Reduce fertiliser use by applying only as much as the crop will absorb, at the right time of year
- Buffer strips: leave strips of grass and trees along rivers and ditches to absorb runoff before it reaches the water
- Improve sewage treatment to remove nitrate and phosphate before discharging treated water
- Use phosphate-free detergents (now standard in the UK)
- Restore wetlands that naturally filter excess nutrients