DR JEREMY BURGESS / SCIENCE PHOTO LIBRARY DR JEREMY BURGESS / SCIENCE PHOTO LIBRARY
Polluted ditch. A ditch overgrown with duckweed, Lemna minor. In the background stand some cows. The duckweed is a sign of phosphate pollution, as the chemical stimulates rapid growth of this plant. The sources of the phosphates are artificial fertilisers and animal dung. The growth of the duckweed on the surface prevents sunlight from reaching other plants. When they and the duckweed growth die, the bacteria which decompose them use up much of the oxygen dissolved in the ditch water. Without oxygen, many of the species resident in the ditch also cannot survive. The water becomes stagnant and largely barren. This process is called eutrophication.
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