What is pollution?
Pollution is the release of substances or energy into the environment in amounts that cause harm. Pollutants can be nutrients (too much of something that is normally useful), toxic chemicals such as heavy metals and pesticides, organic waste such as sewage,plastics, or even heat and noise. Lakes and rivers are especially vulnerable because pollution from a whole catchment drains into them.
Eutrophication step by step
- Fertiliser or sewage washes nitrates and phosphates into the water.
- Algae, no longer limited by nutrients, multiply rapidly: an algal bloom turns the water green.
- The algae block sunlight, so plants deeper in the water cannot photosynthesise and die.
- Algae and plants die in huge numbers. Decomposing bacteria multiply and use up dissolved oxygen.
- Oxygen falls so low that fish and other animals suffocate: a fish kill. Low-oxygen “dead zones” also form in coastal seas, such as the Gulf of Mexico.
Why oxygen matters
Fish, insects and other water animals breathe oxygen dissolved in water. Even cool, clean water holds only about 8–10 milligrams per litre, compared with about 270 mg in a litre of air, and warm water holds less. Most fish are stressed below about 5 mg/L and die below about 2 mg/L. That is why oxygen is one of the most important measures of water health.
Bioaccumulation and biomagnification
Some pollutants, such as mercury, lead and the pesticide DDT, are persistent: they are not broken down and are stored in body tissue instead of being excreted. Over an animal’s lifetime they build up (bioaccumulation). Because each predator eats many prey, concentrations multiply at each step of the food chain (biomagnification). Top predators, including people who eat large fish such as tuna or swordfish, can receive concentrations thousands or millions of times higher than in the water. In the 1950s, mercury from a factory poisoned thousands of people in Minamata, Japan, who ate contaminated seafood.
Other kinds of pollution
- Plastic pollution entangles wildlife and breaks into microplastics found in food chains worldwide.
- Thermal pollution from power stations warms rivers, reducing how much oxygen water can hold.
- Acid rain from sulfur and nitrogen oxides acidifies lakes and damages forests.
- Oil spills coat birds and animals and poison shoreline life.
Measuring water quality
Scientists measure dissolved oxygen, nutrient levels, water clarity, pH and temperature. Living things also act asindicator species: mayfly and stonefly larvae need clean, well-oxygenated water, while sludge worms and rat-tailed maggots tolerate heavy pollution. Counting them reveals water quality over time.
Reducing pollution
Solutions include treating sewage, applying fertiliser only when and where crops need it, plantingbuffer strips of vegetation and restoring wetlands along rivers to trap nutrients, and strict limits on industrial discharges. International agreements such as the Minamata Convention on Mercury (2013) aim to reduce mercury worldwide. Nutrient problems can clear within a few years once inputs stop; persistent chemicals can linger for decades.
Key terms
- Algal bloom
- A rapid increase in algae, often turning water green.
- Dissolved oxygen
- Oxygen gas dissolved in water, measured in mg/L.
- Biomagnification
- Increasing concentration of a persistent pollutant at each level of a food chain.
Common misconceptions
- “More nutrients make a lake healthier.” Excess nutrients cause blooms that can suffocate the lake.
- “Algae add oxygen, so blooms are good.” Their decomposition uses more oxygen than they produce.
- “If water looks clear, it’s safe.” Mercury and many chemicals are invisible.
Key takeaways
- Nutrient pollution causes eutrophication: algal bloom → decomposition → oxygen loss → fish kills.
- Fish need at least about 5 mg/L of dissolved oxygen to stay healthy.
- Persistent pollutants such as mercury biomagnify, reaching the highest levels in top predators.
- Reducing pollution at the source, plus buffer strips and treatment, lets ecosystems recover.
Model note: a simplified lake with average daily rates; real lakes vary with temperature, depth, seasons and day–night cycles. Mercury factors of about 5 times per food-chain level are typical but vary widely between lakes.