(a) Eutrophication. The process by which a body of water becomes barren either by natural means or by pollution, extensively rich in dissolved nutrients. It results in the increased primary productivity that often leads to seasonal deficiency in dissolved oxygen.

Algal bloom. Phosphorus and nitrates dissolved in water act as nutrients and accelerate the growth of algae that may form a mat on the water surface. It is termed algal bloom.

Effects. 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 plants. The algae may die and sink, and feed the oxygen-consuming bacteria. They may be pushed on to the shore by wind and decompose, releasing foul gases, such as hydrogen sulphide. 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 and it becomes barren, ultimately death of lake occurs.

(b) Biological magnification (Bio-magnification). The phenomenon in which the harmful pollutants (such as pesticides) enter the food chain and get concentrated more and more at each successive trophic level of organisms is called biological magnification.

This phenomenon is well known for mercury and DDT. Figure on the side shows biomagnification of DDT in an aquatic food chain. Zooplankton (tiny floating animals) in the food chain, accumulated modest levels of DDT. However, small fish, forming the next level of the food chain, must eat zooplankton several times their own weight and thus they accumulated more of DDT. In this manner, the concentration of DDT magnified at successive trophic levels, starting with 0.003 ppb (ppb = parts per billion) in water it ultimately reached 25 ppm (ppm = parts per million) in fish-eating birds.

High concentrations of DDT disturb calcium metabolism in birds. It causes thinning of egg shell and their premature breaking. Thus, there is decline in bird population.

Biomagnification of DDT in an aquatic food chain.

(c) Ground water depletion and ways for its replenishment

Underground water is more pure and safe for drinking. It is getting polluted

particularly in industrial towns. The common sources of underground water pollution are sewage and industrial effluents spilled over the ground. The fertilizers and pesticides used in fields also act as pollutants. Pollution is also increased due to seepage from refuse dumps, septic tanks and seepage pits.

Method to control. Sewage and factory wastes should be treated to clean them before their release into water sources.