Water pollution, eutrophication and biomagnification
Sewage, fertilisers, pesticides, heat and oil pollute water. Extra nutrients cause eutrophication; chemicals like DDT build up along food chains by biomagnification.
Major water pollutants
| Pollutant | Examples | Sources | Effects |
|---|---|---|---|
| Infectious agents (germs) | Bacteria, viruses and other parasites | Excreta of people and animals | Spread water-borne diseases |
| Organic chemicals | Pesticides, detergents, oil | Farm, industrial and household waste | Biomagnification |
| Inorganic chemicals and fertilisers | Acids, alkalis, metals, salts; nitrates and phosphates | Industrial waste, household cleaners, runoff from fields | Water unfit to drink; eutrophication |
| Radioactive materials | Radioactive uranium, thorium and iodine | Ore mining and processing, nuclear power plants, some rocks | Can cause genetic disorders |
| Heat | Hot water | Power plants, industrial cooling | Death of fish (thermal pollution) |
| Oil slick | Petroleum | Leaking oil ships | Marine life dies for lack of dissolved oxygen |
Draw it: eutrophication step by step
Eutrophication means 'well fed': too much food for algae ends up starving the water of oxygen.
How biomagnification happens
- 1DDT and other pesticides are sprayed on crops or used in small amounts to kill mosquitoes.
- 2Rain washes them into soil, ponds and rivers, where tiny plants and algae take them in.
- 3★These chemicals are non-biodegradable: they are not broken down and not removed from the body, so they stay in it.
- 4Each animal eats many organisms from the level below, so the chemical builds up at every trophic level.
- 5★The top consumers, such as fish-eating birds, collect the highest amount. Birds then lay eggs with weak, thin shells, and their numbers fall.
- 6DDT has even been found in cow's milk and mother's milk, where it can harm newborn babies.
★ key step
Worked example: how much does DDT build up?
Your NIOS lesson gives DDT levels in a food chain: water 0.2 ppm, algae 77 ppm, fish 500 to 600 ppm, pelican 1700 ppm. How many times more concentrated is DDT in (a) the fish and (b) the pelican than in the water?
- Concentration factor = level in the organism ÷ level in the water.
- (a) Fish: 500 ÷ 0.2 = 2500 and 600 ÷ 0.2 = 3000, so 2500 to 3000 times.
- (b) Pelican: 1700 ÷ 0.2 = 8500 times.
(a) 2500 to 3000 times; (b) 8500 times: the top consumer carries the most
Controlling water pollution
- Use less water by changing the way things are made or done.
- Treat waste water and recycle as much of it as possible.
- Limit the amount of waste water let out into rivers and lakes.
- Use fertilisers and pesticides only in the advised amounts, so less washes into water bodies.
Fertiliser washed from fields into a village pond often turns it green with algae and leaves the fish gasping. Using fertiliser only in the advised amount saves the farmer money and protects the pond.
1 marks: “Name the process in which a pond, rich in nutrients from fertilisers, gets covered with algae.” Eutrophication.
3 marks: “Lake water holds only a trace of DDT, yet herons that eat the lake's fish carry thousands of times more. Name the process, explain how it happens, and say why the DDT is not removed from their bodies.” Biomagnification. The pesticide (DDT) is non-biodegradable, so it is not broken down or excreted. Each animal eats many from the level below, so the level rises at each step and is highest in the top consumer; birds lay weak eggshells.
3 marks: “Describe the steps by which eutrophication leads to the death of fish in a lake.” Fertilisers and sewage add nitrates and phosphates; algae grow fast; algae die; decomposers use up dissolved oxygen while breaking them down; fish die of lack of oxygen.
Feed the water, kill the fish (eutrophication). Up the chain, up the poison (biomagnification).