A very fast crowd
Why Does a Pond Turn Green?
The green is millions of tiny living things, each too small to see, all doubling at once.
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Step 1 of 5
Algae are single living specks that feed on sunlight.
Each one is far too small to see. There are always some in any pond, drifting about, doing quietly what a leaf does.
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Step 2 of 5
They are held back by whatever they run out of first.
Usually that is nitrogen and phosphorus in the water. Sunlight is free, so the food supply is what sets the limit on how many there can be.
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Step 3 of 5
Give them more of that food and they double, and double again.
Fertiliser washed off a field, or a leaking drain, removes the limit. Doubling every day turns a handful into millions inside a week.
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Step 4 of 5
Once green, the water lets almost no light through.
The plants rooted on the bottom are put in the dark and die. Their remains feed bacteria, and the trouble compounds.
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Step 5 of 5
The real damage happens at night.
In daylight algae release oxygen, but in the dark they and the bacteria use it up. Before dawn the oxygen can run out, and the fish suffocate.
Some blooms are cyanobacteria rather than algae, and those can be poisonous — which is why a bright green pond carries warning signs.
The short version
A pond turns green when extra nutrients let tiny algae double unchecked — and the real harm is the oxygen they and the bacteria use up overnight.
Try it yourself
Look at the green in a pond against a white cup of the water. It is not a dye — it is specks, and it settles if left standing.