What is eutrophication?
Eutrophication means "too well fed". A lake, pond, river or sea gets more nutrients than it can handle. The two main nutrients are nitrogen (nitrates) and phosphorus (phosphates).
Algae and water plants need these nutrients to grow. In a healthy pond there is only a little, so growth stays in balance. When there is too much, algae grow out of control. A thick green layer is called an algal bloom.
Eutrophication can happen slowly by itself over thousands of years (natural). But most problems today are quick and are caused by people (cultural eutrophication).
Where do the extra nutrients come from?
- Farm fertilizer washed off fields by rain (runoff).
- Sewage and waste water from homes and towns that is not treated well.
- Detergents that contain phosphates.
- Animal waste from dairies and poultry farms.
- Factories and food-processing units that release waste water.
Some sources come from one pipe (point source, like a sewage drain). Others come from a wide area (non-point source, like rain washing many fields). Non-point sources are harder to control.
The chain of events
- Nutrients enter the water.
- Algae multiply and form a bloom on the surface.
- The bloom shades the water. Plants below cannot make food (no photosynthesis) and die.
- Dead algae and plants sink. Bacteria decompose them. This uses a lot of dissolved oxygen.
- Oxygen falls very low (hypoxia). Fish, crabs and other animals die. Water smells bad.
Scientists measure the oxygen that microbes need to rot the waste. It is called BOD (biochemical oxygen demand). High BOD means the water is badly polluted.
Why is it harmful?
- Fish kills and loss of water life (biodiversity).
- Some blooms (blue-green algae, or cyanobacteria) make toxins that can harm animals and people.
- Drinking water costs more to clean, and may taste and smell bad.
- Fishing, tourism and boating suffer.
- Large dead zones can form where rivers meet the sea.
How do we measure water quality?
Simple tests tell us if a pond is in trouble:
- Dissolved oxygen (DO): healthy water usually has more than 5 mg per litre. Below 2 mg per litre is called hypoxic (very low).
- Nitrate and phosphate levels (test kits).
- Chlorophyll: the green colour of algae. More chlorophyll means more algae.
- Turbidity or Secchi disc depth: how far down you can see. A bloom makes you see only a short way.
- BOD and pH.
How can we prevent and fix it?
- Use fertilizer in the right amount and at the right time. Do not spread it before heavy rain.
- Keep strips of grass, bushes and trees (buffer strips) between fields and water. They catch nutrients.
- Treat sewage so that nitrogen and phosphorus are removed before water goes to rivers.
- Use phosphate-free detergents.
- Build wetlands. Their plants take up nutrients.
- Repair a lake: remove algae and weeds, add air to the water (aeration), and stop new nutrients coming in. Stopping the source always comes first.
Try it: grow an algae bloom at home (safe version)
Take two clear jars. Fill both with pond or rain water (not for drinking). Add a tiny pinch of plant fertilizer to Jar B only. Put both on a bright windowsill. Look every day for 10 to 14 days. Jar B turns green first. That is a bloom in miniature. Wash your hands after, and pour the water on soil, not into a drain. Then slide the nutrient control in the 3D above and compare.
Key formulas and definitions
- Nutrients (N, P) ↑ → algae ↑ → light ↓ → dead matter ↑ → oxygen ↓
- Limiting nutrient: phosphorus in most fresh water, nitrogen in most sea water
- Healthy dissolved oxygen: more than 5 mg/L; hypoxic: below 2 mg/L
- BOD = oxygen microbes need to break down waste (high BOD = badly polluted)
- Runoff load = amount applied × fraction that runs off
Worked examples
1. List the steps that turn a clear pond into a dead zone.
Nutrients enter → algae bloom → light is blocked → plants and algae die and sink → bacteria decompose them and use up oxygen → fish die.
2. A farmer spreads 120 kg of fertilizer on a field next to a pond. 25% runs off into the pond. How much reaches the pond?
25% of 120 = 0.25 × 120 = 30 kg reaches the pond.
3. A pond has 8 mg/L of dissolved oxygen. After a bloom dies, the oxygen drops by 75%. What is the new value? Is it safe for fish?
75% of 8 = 6, so 8 − 6 = 2 mg/L. This is hypoxic (very low). Most fish cannot live in it.
4. Why does a bloom kill plants at the bottom even though the plants are still in water?
The green layer on top blocks sunlight. Plants need light for photosynthesis, so without light they cannot make food and they die.
5. Why is a buffer strip of grass between a field and a stream useful?
Rain water from the field must pass through the grass first. The roots and plants take up nitrogen and phosphorus, and soil particles settle, so fewer nutrients reach the stream.
6. A lake has 0.05 mg/L of phosphorus. It doubles three times in a few years. What is the final value?
Doubling three times is × 2 × 2 × 2 = × 8. 0.05 × 8 = 0.4 mg/L.
Common mistakes
- Thinking algae kill fish directly. Mostly it is the low oxygen after the algae die and rot.
- Thinking a bloom adds oxygen because algae do photosynthesis. By day they add some, but the night and the decay use far more.
- Blaming only farms. Sewage, detergents and factories also add nutrients.
- Thinking the water must look dirty. A bloom can look bright green and "pretty", yet the water is in trouble.