Too much food for algae: Eutrophication is what happens when water is over-fed. Extra nitrates and phosphates make phytoplankton grow out of control: an algal bloom}}.
The points to remember
- Eutrophication happens in lakes, estuaries and coastal waters.
- The cause: extra mineral nutrients, especially nitrates and phosphates.
- The result: excessive growth of phytoplankton, an algal bloom.
- A bloom happens only if growth was limited before by too little phosphate or nitrate.
- In lakes, phosphate is usually the one in short supply; at sea, nitrate often is.
- Humans cause it by releasing fertilisers, sewage and detergents into water.
Remember it as: Feed the water, grow the algae.
Example (SL): Lake Erie, North America. In the 1960s phosphate from sewage and detergents poured into Lake Erie, the shallowest of the Great Lakes. Thick green blooms covered it every summer, and newspapers called it a 'dead lake'. Phosphate was the nutrient its algae had been short of.
Name the nutrient properly: Write 'nitrates' and 'phosphates' (or fertiliser, sewage, manure). 'Nitrogen' or 'phosphorus' on its own, or just 'run-off', does not name the pollutant.
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The nutrients come from many places at once. Most reach the water in rain that washes over fields and streets, so they are hard to trace to one pipe.
The points to remember
- Fertilisers washed off farmland: the largest source in most places.
- Manure and slurry from farm animals, especially large livestock farms.
- Sewage, treated or untreated, from towns and cities.
- Detergents that contain phosphates.
- Nitrogen oxides from vehicles and power stations, falling in rain.
- Urban run-off: fertiliser from lawns, parks and golf courses.
Reading the pies (SL). The Mississippi drains farms in 31 US states. Corn and soybean fields supply 52% of the nitrogen it carries, and pasture 37% of the phosphorus. Towns and sewage add only 9-12%: farming is the main cause.
Phosphorus travels on soil: Nitrate dissolves and washes straight into rivers. Phosphate sticks to soil, so it reaches rivers when soil is eroded: that is why grazed pasture supplies so much of it.
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Questions often name one source, such as untreated sewage or a crop grown near the coast, and ask for the problem it causes. Name the problem, then say how it happens.
Untreated sewage in a river
- Eutrophication: its nitrates and phosphates feed algal blooms.
- Hypoxia: bacteria decomposing it use up the oxygen.
- A rotten-egg smell of hydrogen sulphide, from decay with no oxygen.
- Pathogens: disease for swimmers, and in shellfish that filter the water.
- Cloudy water: less light, so less photosynthesis.
A crop farmed near the coast
- Fertiliser run-off feeds algal blooms, some of them toxic red tides.
- Pesticides poison coastal animals and can bioaccumulate.
- Eroded sediment clouds the water, so less light and productivity.
- Sediment smothers animals living on the sea bed, such as corals.
Example (SL): the River Thames, London. After heavy rain, old sewers overflow and untreated sewage pours into the Thames. In 2023, sewage spilled into England's rivers for about 3.6 million hours, bringing nutrients, low oxygen and germs that make swimmers ill.
Name the problem AND outline it: 'Eutrophication' alone is half an answer: add how, for example 'because sewage adds nitrates and phosphates'. 'Thermal pollution', or just 'loss of biodiversity', does not answer a sewage question.
A table of pollutants can show when eutrophication was most likely: look for the year when nitrate AND phosphate are both highest. Metals such as lead do not feed algae.
Once the bloom starts, a chain follows. Many questions want the whole chain, ending with what it does to the fish or animal named in the question (the next page explains it in full).
Link it to what the question asks: If the question asks about food production, your chain must end with fewer fish or shellfish to catch. Describing eutrophication without that link scores at most 3 of 4.
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How this comes up: Paper 2 Section A asks how fertiliser run-off harms a named animal [3]; Paper 1 asks for a year from a table [1] or a problem caused by sewage [2].
The Ganges river dolphin is an endangered dolphin that hunts fish in the rivers of India and Bangladesh. Farms along the rivers use large amounts of fertiliser.
Explain how fertiliser run-off from nearby farms could affect the population of Ganges river dolphins.
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