Rescue the Lake: Nutrients, Algae and Oxygen
Follow the chain from nutrient pollution to dead fish: nutrients feed algae, dead algae feed bacteria, bacteria use up dissolved oxygen, and fish need oxygen to live.
Before you try
What if a lake turned bright green? Is that a sign of more life or less?
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What happened?
Move a control to see what changes and why.
A single well-mixed lake with relative nutrient and algae units and rounded oxygen rates; real lakes stratify and vary by depth.
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Try these ideas
- Run 90 days with the defaults and watch the oxygen gauge.
- Raise sewage treatment to 90% and run again.
- Warm the water to 35 °C and see how much sooner the fish struggle.
Why it works
Nitrates and phosphates from fertiliser and sewage are food for algae as much as for crops. Fed too well, algae explode into a green bloom that looks like life but ends in death: when the algae die and sink, bacteria decompose them and use up the dissolved oxygen. Fish need about 4 mg/L; below 2 they suffocate. This chain is called eutrophication, and warm water makes it worse because it holds less oxygen and speeds decay.
Bengaluru’s Bellandur lake, with its foam and fires, and stretches of the Yamuna show what untreated nutrients do. The fixes are upstream: treat sewage, use fertiliser carefully and keep plants along the banks; aerators only buy time. In this simulator you set the fertiliser, the sewage and its treatment, the temperature and an aerator, run the days, and watch the algae, the oxygen gauge and the fish.
Check your understanding
Question 1 of 4
An algal bloom kills fish because
The nutrients that cause blooms mostly come from
Warm water makes eutrophication worse because it
The most lasting fix for a green lake is
Explain it in your own words
What changed? Why did it happen? Where might you notice this in everyday life?
For parents & teachers
- Leave a jar of pond water in sunlight with a pinch of fertiliser and watch it for a week.
- Ask: why does a lake sometimes smell of rotten eggs?
- Find out where your town’s sewage goes.