Polluted river. A river infested with duckweed (^ILemna^i ^Iminor^i) due to pollution by fertilisers from surrounding farmland. This effect is called eutro- phication, and is a common cause of freshwater pollution. Fertilisers rich in nitrates and phos- phates are washed out of farmland by rain and carried into streams and rivers, where they stimulate overgrowth of weeds and algae. The water soon becomes choked by vegetation, which blocks out light and exhausts the oxygen supply as it decays. Very little life survives in the stagnant water, apart from anaerobic bacteria which create the familiar "bad egg" smell. Photographed at Fairhaven Garden Trust, Norfolk, UK.