The process of nutrient enrichment of water and consequent loss of species diversity is called eutrophication.
Water polluted with inorganic fertilizers containing nitrates and phosphates or containing organic wastes causes eutrophication in water bodies.
The Decomposition of organic wastes also increases nutrients in the water and causes eutrophication.
During eutrophication, there is a luxuriant growth of algae, particularly blue-green in water. These form floating scums and water blooms.
The algae use oxygen at night and may deoxygenate the water enough to kill the fish and other animals.
The algal mat at the water surface may block light to the submerged plant.
The algal species quickly complete their life cycle and die thereby adding a lot of dead organic matter, and feed the oxygen-consuming bacteria.
They may be pushed on the shore by the wind and decompose, releasing foul gases, such as hydrogen sulfide.
Silt and decaying matter may accumulate and finally fill the lake or pond. This process is called senescence. It is a natural stage in the change of a lake into dry land.
Depending upon the climate, the size of the lake, and other factors, the natural ageing of a lake may span thousands of years.
However, pollutants from man activities example- effluents from industries and residential premises, etc. can radically accelerate the ageing process. This phenomenon is termed as cultural or accelerated eutrophication.
Control of Eutrophication:
Following measures can be adopted to combat the phenomenon of Eutrophication-
Wastewater must be treated before its discharge into water bodies.
To reduce the amount of nutrients in the water body, bacterial growth should be stimulated.
The dissolved nutrients can be removed by physio-chemical methods. For example- phosphorous can be removed by precipitation and nitrogen by nitrification or denitrification, electrodialysis, and ion exchange methods.