It is the natural aging of lakes due to nutrient enrichment of its water. It takes thousands of years for this process. The young lake is cold and clear supporting little life. As the time passes by it becomes warmer and encourages the growth of aquatic organisms as the nutrient gets introduced. The fertility increases, debris pile up, the lake grows shallower and warmer. Marsh plants start growing and fill the original basin. Eventually it converts into a land.
(b) Biological magnification:
It is increase in the concentration of the toxic substance of successive trophic levels. A few toxic substances present in industrial waste waters accumulate in organisms. Such as DDT and mercury. It takes place in aquatic ecosystem. The water with 0.003 pph DDT is the starting point and it gradually increases in organisms of successive levels upto 25 pph. This concentration disturbs the calcium metabolism in birds and causes thinning of egg shell which breaks prematurely and causes decline in bird population.
(c) Ground water depletion and ways for replenishment:
Ground water is the source of water for most of the human activities.
Regions that are having less annual rainfall depend on underground water for agricultural purposes-irrigation. Over exploitation of ground water for irrigation, construction, domestic use, industrial use has depleted it in many areas. The water level has gone down. This causing water crisis specially in summer and drought hit regions.
Its replenishment can be done by judiciously using the water at our disposal. Water must not be wasted at any shape/activities. Rain water harvesting must be practised by an large by everyone so that the ground water can be recharged annually. Also, reforestation, less deforestation must be in practice.