2.3 Plant nutrients
Plant nutrients are inorganic substances — mainly nitrogen and phosphorus compounds— that are essential for normal plant growth. Nitrogen and phosphorus come from human and animal faeces, detergents and fertilizers, and are not removed by standard water treatment processes or by sewage treatment. If the concentration of these nutrients in water gets too high they can cause a rapid proliferation of algae—an algal bloom (Figure 8). This can discolour the water, give it a bad taste and smell, and may produce a green scum on the surface. When they die, the algae sink and decay, forming large amounts of natural organic material with a high oxygen demand. The water becomes deoxygenated and polluted even further. This type of pollution is not usually a problem in British rivers but it does affect some reservoirs, lakes and canals.