Eutrophication:

It is a process whereby lakes, estuaries, or slow-moving streams receive excess nutrients that stimulate excessive plant growth. This extra plant growth, often called an algal bloom, reduces dissolved oxygen in the water when dead plant material decomposes and can cause other organisms to die. Typically, it is such nutrients as ammonia, nitrate, phosphate and other mineral nutrients that cause eutrophication.

Nutrients can come from many sources, such as fertilizers applied to crops, golf courses, and lawns; deposition of nitrogen from the atmosphere; erosion of soil containing nutrients; and raw sewage.

a) Nitrates are easily removed from soils by water. Rain often washes fertilizers containing nitrates from fields, lawns and golf courses into rivers.

b) If other nutrients are present along with the nitrate, the river becomes eutrophic.

c) Algae use the nutrients, growing and reproducing rapidly. An algal bloom develops, with thick layers of algae produced. Some of the algae is shaded and die.

d) Bacteria decompose the dead algae, removing oxygen from the water.

e) Aquatic insects, fish and more algae die due to lack of oxygen.