ADAM HART-DAVIS / SCIENCE PHOTO LIBRARY ADAM HART-DAVIS / SCIENCE PHOTO LIBRARY
A biologist checks the pollution level in a stream by monitoring both the animals living in it and the concentration of dissolved oxygen. Here, he is 'kick sampling', the standard method for collecting animals. He stands his net vertically and kicks the bottom mud 20 times just upstream of it. Loose material is displaced and washed into the net for collection and examination. This stream is badly polluted, hence he finds much sewage fungus but no animals except for a few bloodworms. The pollution is due to the oxidation of organic matter. The resulting low oxygen content encourages anaerobic growth but makes the water unsuitable for higher animals.
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