Eutrophication

Waste from factories, agrochemicals used in farming, and domestic sewage containing excreta, urine and detergents raise the concentration of nitrate (NO₃⁻) and phosphate (PO₄³⁻) ions in reservoirs. Algae then grow enormously and form a green scum floating on the water. This condition is called eutrophication.

When the overgrown algae die, bacteria act on them in airless conditions and release harmful gases such as hydrogen sulphide (H₂S), ammonia (NH₃) and methane (CH₄). This also causes an unpleasant smell, and the living things of the reservoir die.

Harmful effects of eutrophication:

• The water loses its clearness.
• The water of the reservoir can no longer be used.
• Aquatic plants and animals die, so biodiversity falls.
• The reservoir loses its beauty.

Why too much food kills a lake

Nitrate and phosphate are fertilisers: in a field they feed the crop, and washed into a lake they feed the algae. The algae form a mat that shades the water plants below, which die. When the algae themselves die, bacteria decomposing them use up the oxygen dissolved in the water. Fish and other animals then suffocate, and the airless water produces foul gases. So it is not a poison that kills the lake but too much nourishment, which is what the name means: eu-trophication, good feeding.