The shape of the lesson
By the end of the lesson, learners can
- Sewage, fertiliser and eutrophication: Choose a source, excess fertiliser or untreated sewage, and press Wash it in.
- Plastics that last: Move the time slider from 0 to 100 years.
- The enhanced greenhouse effect: Drag the slider (or the point on the graph) to add carbon dioxide and methane to the atmosphere.
- Eutrophication (Extended): ions up, producers up, producers die, decomposers up, oxygen down, animals die.
- The oxygen is used by decomposers, not by the algae.
- Plastics: non-biodegradable; eaten, entangling, microplastics in food chains.
- Carbon dioxide: fossil fuels and deforestation. Methane: cattle, rice, landfill.
One picture every idea returns to
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.
Forty-five minutes, phase by phase
| Time | Phase | What happens in the room | Grouping |
|---|---|---|---|
| 0 to 5 min | Hook: predict | Open with the question in the title, Too much of a good thing.. Fertiliser feeds crops and sewage is mostly natural waste, yet both can empty a lake of fish. Put the figure on the board: it is the one picture every idea in the lesson returns to. | Whole class, then pairs |
| 5 to 13 min | Teach and check: Sewage, fertiliser and eutrophication | Choose a source, excess fertiliser or untreated sewage, and press Wash it in. Project tab 3 of the lab (Eutrophication of a lake) and let a student drive it while the class predicts. | Whole class, then pairs |
| 13 to 21 min | Teach and check: Plastics that last | Move the time slider from 0 to 100 years. Project tab 4 of the lab (Non-biodegradable plastics on a beach and in the sea) and let a student drive it while the class predicts. | Pairs |
| 21 to 30 min | Teach and check: The enhanced greenhouse effect | Drag the slider (or the point on the graph) to add carbon dioxide and methane to the atmosphere. Project tab 5 of the lab (Carbon dioxide, methane and the enhanced greenhouse effect) and let a student drive it while the class predicts. | Anticipation / Reaction Guide |
| 30 to 36 min | Check | The four Stage 1 questions from the student lesson, answered on mini whiteboards or by show of fingers. Re-teach the one most get wrong. | Individual, then whole class |
| 36 to 43 min | Plenary | The exam-style questions from Stage 2, one at a time: thirty seconds alone, then a random call. The feedback lines in the key give the examiner's wording. | Individual, random call |
| 43 to 45 min | Exit and homework | Set the worksheet below (6 questions, 24 marks). Learners self-assess against the objectives before they leave. | Individual |
Protect the parts that carry the learning
The sequence carries about fifty minutes of material, so the lab is driven by one student while the class predicts rather than explored freely. Times are a guide.
Protected: the check at 30 minutes. It is the only individual read on whether the key idea landed, and it is not cut.
Cut first if the build overruns: the last check in The enhanced greenhouse effect, never the check or the plenary.
In a 60 minute block: run worksheet questions 1 to 3 in class as a Rally Coach set and model both the hardest worked example and its check live.
Anticipation / Reaction Guide
Before the lesson, students mark five statements true or false. After the teaching they mark them again and justify any change of mind in one sentence each. The statements come from the examiner traps below.
The full structure, with its variations and the other twelve, is on the cooperative-learning page.
What to head off, and how
| Trap learners fall into | Teaching move that pre-empts it |
|---|---|
| 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. |
| Breaking into tiny pieces is not the same as being decomposed. | Microplastics are still plastic: no decomposer has broken them down. |
| 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. |
Pollution: worksheet
6 questions, 24 marks. Written in the style of the papers; no past paper question is reproduced.
Extended. Fertiliser is washed from a field into a lake. Explain how this can lead to the death of fish in the lake.
Dissolved oxygen was measured at points along a river below a pipe releasing untreated sewage.
| Distance downstream / m | 0 (above pipe) | 50 | 200 | 500 | 1000 |
|---|---|---|---|---|---|
| Dissolved oxygen / mg per dm3 | 9.5 | 3.0 | 2.2 | 5.8 | 9.0 |
(a) Describe the changes in dissolved oxygen. (b) Explain why the oxygen falls below the pipe. (c) Suggest why it recovers further downstream.
State the sources of carbon dioxide and methane from human activities, and describe how an increase in these gases causes climate change.
Describe the effects of non-biodegradable plastics in (a) aquatic and (b) terrestrial ecosystems.
Explain why untreated sewage reduces the dissolved oxygen in a river faster than the same amount of excess fertiliser.
Suggest three ways to reduce eutrophication of rivers near farmland.
Answer key and marking notes
- The fertiliser increases the concentration of nitrate and other ions in the water.
- This causes increased growth of producers, especially algae (an algal bloom).
- Algae block the light, so plants below die; the algae also die later.
- Decomposers (bacteria) increase and break down the dead producers, so there is more decomposition.
- The decomposers carry out more aerobic respiration, so the dissolved oxygen concentration falls.
- Fish and other organisms that need dissolved oxygen die.
- (a) It falls sharply from 9.5 to 3.0 just below the pipe, reaches its lowest (2.2) at 200 m, then rises back to 9.0 by 1000 m.
- (b) The sewage provides organic matter, so decomposer bacteria multiply.
- Their aerobic respiration uses up the dissolved oxygen.
- (c) The organic matter is used up so fewer decomposers respire; oxygen dissolves from the air (and is produced by photosynthesis), so levels recover.
- Carbon dioxide: burning fossil fuels; deforestation (burning trees, fewer trees photosynthesising).
- Methane: cattle (digestion), rice paddy fields, decaying waste in landfill.
- These gases absorb infrared (heat) radiated by the Earth and radiate it back, so more heat is trapped: the enhanced greenhouse effect.
- Global temperatures rise, causing climate change: melting ice, rising sea levels, extreme weather.
- (a) Animals such as turtles, seabirds and fish swallow plastic, blocking the gut so they starve.
- (a) Animals become entangled in nets and bags and drown or are injured; microplastics enter food chains.
- (b) Plastic litter spoils habitats and can trap or be eaten by land animals.
- (b) Plastic is not decomposed, so it builds up in landfill, and toxic chemicals can leak into soil and water.
- Sewage contains organic matter that decomposers can feed on straight away.
- So decomposer numbers and their aerobic respiration increase immediately, using up oxygen.
- Fertiliser only supplies ions; oxygen falls only after producers have grown, died and begun to decompose.
- Apply fertiliser at the correct amount and time, not before heavy rain.
- Leave strips of uncultivated land or hedges beside rivers to absorb run-off.
- Treat sewage fully before releasing it into rivers (or use organic fertiliser such as manure carefully).
8 multiple-choice questions on this lesson
From the topic question bank. Assign them in the app for a marked score in your gradebook, or run the topic as a class game.
Untreated sewage enters a river. Which change in the river is most likely just downstream?
- ADissolved oxygen falls and fish die
- BDissolved oxygen rises and fish increase
- CThe water becomes clearer and colder
- DBacteria in the water are all killed
Which gas is released mainly from cattle, rice paddy fields and landfill sites?
- AMethane
- BCarbon dioxide
- COxygen
- DNitrogen
A seabird chick is found dead with its stomach full of plastic fragments. Which statement best explains its death?
- AThe plastic filled its gut so it could not take in enough food
- BThe plastic was digested and released too much energy
- CThe plastic decomposed inside it and used up its oxygen
- DThe plastic made the chick grow too heavy to fly
What is the enhanced greenhouse effect?
- AExtra greenhouse gases from human activity trap more heat, raising global temperature
- BThe natural trapping of heat by the atmosphere that keeps the Earth warm
- CDamage to the ozone layer that lets more ultraviolet light reach the Earth
- DHeat released directly into the air by power stations and vehicles
Which is a likely consequence of climate change caused by the enhanced greenhouse effect?
- AMelting ice and rising sea levels flooding low-lying land
- BA thicker ozone layer protecting crops from sunlight
- CLower global temperatures leading to more glaciers
- DLess carbon dioxide dissolving in the oceans
During eutrophication, which organisms use up most of the dissolved oxygen in the water?
- ADecomposer bacteria respiring aerobically
- BAlgae photosynthesising at the surface
- CFish swimming faster in warm water
- DWater plants growing on the lake bed
Nitrate and dissolved oxygen were measured in a lake each week after fertiliser run-off. Nitrate peaked in week 1 and oxygen was lowest in week 7. Why is there a delay of several weeks?
- AAlgae must grow and die before decomposers can multiply
- BNitrate takes six weeks to dissolve in the lake water
- CFish use up the oxygen slowly over the six weeks
- DOxygen is used directly by the nitrate ions as they react
Two identical ponds receive the same mass of fertiliser. Pond X has a fountain that bubbles air through it; pond Y does not. Which result is most likely?
- AFewer fish die in pond X because oxygen is replaced
- BMore fish die in pond X because algae grow faster
- CNo algae grow in pond X because air removes nitrate
- DFish die at the same rate in both ponds
Quiz key
Sewage provides organic matter for decomposer bacteria, which multiply and use up dissolved oxygen in aerobic respiration, so fish die. Oxygen does not rise; the water becomes cloudier, not clearer; and bacteria increase rather than die.
Methane comes from the digestion of food in cattle, from waterlogged rice fields and from decaying waste in landfill. Carbon dioxide comes mainly from burning fossil fuels and deforestation; oxygen and nitrogen are not greenhouse gases released by these sources.
Plastic cannot be digested, so it stays in the stomach, giving a false feeling of fullness and blocking the gut; the chick starves. Plastic is not digested, so it releases no energy, and it does not decompose. A chick is not killed by the weight of plastic preventing flight.
The enhanced greenhouse effect is the extra warming caused when human activity increases the carbon dioxide and methane in the atmosphere, so more heat is trapped. The natural greenhouse effect is the background trapping of heat; ozone damage is a separate problem; and heat released directly is not how greenhouse gases work.
Higher temperatures melt ice caps and glaciers and make seawater expand, so sea levels rise and low land floods. The ozone layer is not thickened by greenhouse gases; global temperatures rise, not fall; and more carbon dioxide in the air means more dissolves in the oceans, not less.
When the producers die, decomposer bacteria multiply and break down the dead material, carrying out aerobic respiration that uses up dissolved oxygen. Photosynthesising algae release oxygen; fish are victims of the low oxygen, not the cause; and the water plants below die because they are shaded.
Extra nitrate first increases growth of producers; only when they die is there more dead matter for decomposers, whose numbers then rise and use up oxygen. Each step takes time. Nitrate dissolves quickly; fish are not the main oxygen users; and nitrate ions do not react with oxygen in this way.
The fish die because decomposers use up dissolved oxygen. Bubbling air into pond X keeps adding oxygen, so fewer fish die. Air does not make algae grow faster or remove nitrate, and the difference in oxygen supply means the two ponds will not behave the same.
What to project
The student lesson embeds these tabs. Open them full screen on the projector; every slider has a drag handle on the stage.
Slides for this lesson
A projectable deck for every pack is on the way. Until then, present the student lesson full screen: its sections, figure and lab tabs follow the same order as the plan above.