Fertiliser feeds crops and sewage is mostly natural waste, yet both can empty a lake of fish. Plastics are useful because they last, which is exactly why they harm wildlife. Carbon dioxide and methane are natural gases, but the extra amounts released by people are warming the planet. The step-by-step process of eutrophication is Extended content.
Untreated sewage and excess fertiliser entering rivers and lakes cause algae to grow rapidly and then oxygen levels to fall, killing fish and other animals. Extended: this process is eutrophication: more nitrate and other ions, more growth of producers, more decomposition after they die, more aerobic respiration by decomposers, less dissolved oxygen and death of organisms that need it. Non-biodegradable plastics are not broken down by decomposers, so they harm animals on land and in water for many years. Extra carbon dioxide and methane cause the enhanced greenhouse effect and climate change.
Fig. 1: the six steps of eutrophication (Extended)
Section 01
Sewage, fertiliser and eutrophication.
Choose a source, excess fertiliser or untreated sewage, and press Wash it in. Follow the graph of nitrate, algae, bacteria and oxygen over 12 weeks, and watch the fish.
Effects (Core): excess fertiliser or untreated sewage in water causes rapid growth of algae, which cover the surface; plants below die; the water loses its dissolved oxygen; fish and other aquatic animals die. Sewage can also carry pathogens.
Extended.The six steps: (1) increased availability of nitrate and other ions; (2) increased growth of producers, especially algae; (3) producers die (plants below are shaded and cannot photosynthesise, algae die later), so there is increased decomposition; (4) decomposers (bacteria) multiply and carry out more aerobic respiration; (5) dissolved oxygen in the water is reduced; (6) organisms requiring dissolved oxygen, such as fish, die.
Sewage in the sim: it adds organic matter that decomposers feed on directly, so the bacteria rise straight away and oxygen falls sooner, with a smaller algal bloom.
Examiner trap
Fish do not die because the algae "use up the oxygen". In the light algae produce oxygen. The oxygen is used up by the huge numbers of decomposers respiring aerobically as they break down the dead producers.
Section 02
Plastics that last.
Move the time slider from 0 to 100 years. Paper, banana peel and an apple core are biodegradable: decomposers break them down. The plastic bottles and bags are not.
Non-biodegradable: decomposers cannot break most plastics down, so they stay in the environment for decades or centuries, slowly breaking into microplastics.
Aquatic ecosystems: turtles, seabirds and fish swallow plastic, which blocks the gut so they starve; animals become entangled in nets, bags and rings and drown or are injured; microplastics are taken in by small animals and pass along food chains.
Terrestrial ecosystems: litter and landfill spoil habitats; animals are trapped in or eat plastic; toxic chemicals from plastic can leak into soil and water.
Examiner trap
Breaking into tiny pieces is not the same as being decomposed. Microplastics are still plastic: no decomposer has broken them down.
Section 03
The enhanced greenhouse effect.
Drag the slider (or the point on the graph) to add carbon dioxide and methane to the atmosphere. Watch how much of the infrared heat is trapped and the global temperature.
Gas
Main sources from human activity
Carbon dioxide
burning fossil fuels (power stations, vehicles, industry); deforestation, especially burning trees
Methane
cattle and other ruminant livestock; rice paddy fields; decaying organic waste in landfill sites
Greenhouse effect: light from the Sun warms the Earth; the Earth radiates heat (infrared); carbon dioxide and methane absorb some of it and radiate it back, keeping the Earth warm enough for life.
Enhanced greenhouse effect: extra carbon dioxide and methane from human activity trap more heat, so the mean global temperature rises.
Climate change: melting ice caps and glaciers, rising sea levels and flooding of low land, more extreme weather (droughts, storms), changes in where species can live, and loss of habitats.
Examiner trap
The greenhouse effect itself is natural and necessary. The problem is the enhanced greenhouse effect caused by extra gases. Also, it is not the same as the hole in the ozone layer.
Stage 1 · Learn
Check what the sim just showed you
Four quick checks. Each correct answer earns XP and records this skill on your profile.
Quick check+10 XP
In the eutrophication sim with excess fertiliser, which rises first after the fertiliser is washed in?
Quick check+10 XP
In the sim, what happens to the oxygen line when the bacteria line peaks?
Quick check+10 XP
In the plastics sim, after 10 years what is left on the beach and in the sea?
Quick check+10 XP
In the greenhouse sim, what happens when you increase the carbon dioxide and methane?
Stage 2 · Exam
Exam-style questions
Unlocks once the four checks above are done. Worth more XP, written to IGCSE Paper 2 and Paper 4 standard.
Finish the four checks above to unlock the exam questions
Exam style+20 XP
Extended: put these steps of eutrophication in order. P: decomposers respire more. Q: algae grow rapidly. R: dissolved oxygen falls. S: nitrate ions increase.
Exam style+20 XP
Extended: why do plants at the bottom of a lake die during eutrophication?
Exam style+20 XP
Which human activity is a major source of methane?
Exam style+20 XP
Why are non-biodegradable plastics a long-term problem in the sea?
Skill unlocked
Pollution, mastered.
This skill is now recorded on your profile. Keep the chain going.