Eutrophication and water pollution – lake, catchment and indicator species
BiologyEcology & EnvironmentAges 17–18
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Sign in to playA lake and its catchment: add fertiliser runoff from farmland, sewage from a town, copper from a mine or acid rain, then follow the eutrophication sequence – nitrate and phosphate rise, algae bloom, light is blocked, water plants die, bacteria decompose them and BOD rises, dissolved oxygen falls and fish die. Hour-by-hour oxygen, BOD, nitrate, phosphate, turbidity and pH graphs and an invertebrate sample with a biotic index. Try the solutions: buffer strips, sewage treatment, phosphate stripping and liming.
Lesson: Water pollution: eutrophication, acid rain and heavy metals; indicator species of water quality
What it shows
Eutrophication happens when nitrate and phosphate from fertiliser runoff and sewage make algae multiply rapidly. The bloom shades the lake bed, so rooted water plants cannot photosynthesise and die. Aerobic bacteria decompose the dead algae and plants and use up dissolved oxygen, so the biochemical oxygen demand (BOD) rises and oxygen falls, especially near the bottom and before dawn. Fish and sensitive invertebrates such as stonefly and mayfly nymphs die, while sludge worms and rat-tailed maggots survive, which is why invertebrates are used as indicator species.
How to use
Set Nitrogen fertiliser on farmland, Sewage from the town, Copper in mine water and Rain pH, then press Play to run 180 days and watch the seven steps light up. Change Graph to see oxygen and BOD, nutrients, living things or turbidity and pH. Read the water-quality table and the invertebrate sample, then test the solutions: Buffer strips, Sewage treatment and Lime the lake.
Parameters you can change
- Nitrogen fertiliser on farmland 0–300 kg N/ha/yr
- Buffer strips along the streams
- Sewage from the town 0–30 thousand people
- Sewage treatment None, Secondary (activated sludge), Tertiary: plus phosphate stripping
- Copper in mine water 0–100 µg/L
- Rain pH 3.5–6.5
- Rock in the catchment Granite (low in calcium), Limestone (neutralises acid)
- Lime the lake
- Water temperature 5–30 °C
- Graph Oxygen and BOD, Nitrate and phosphate, Living things, Turbidity and pH
Questions to explore
- Why do the fish die on calm, overcast days even though the lake is still full of algae?
- Secondary sewage treatment cuts the BOD sharply, so why does the lake stay eutrophic?
- Which invertebrate groups disappear first when oxygen near the lake bed falls, and why?
