Write critical notes on the following:
$(a)$ Eutrophication
$(b)$ Biological magnification
$(c)$ Groundwater depletion and ways for its replenishment

(A) Eutrophication: It is the natural ageing process of a lake caused due to nutrient enrichment. It is accelerated by the runoff of nutrients such as animal wastes,fertilizers,and sewage from land,which leads to an increased fertility of the lake. As a result,it causes a tremendous increase in the primary productivity of the ecosystem. This leads to an increased growth of algae,resulting in algal blooms. Later,the decomposition of these algae depletes the supply of oxygen,leading to the death of other aquatic animal life.
$(b)$ Biological magnification: To protect crops from various diseases and pests,a large number of pesticides are used. These pesticides reach the soil and are absorbed by plants along with water and minerals. Due to rain,these chemicals can also enter water sources and the bodies of aquatic plants and animals. As a result,chemicals enter the food chain. Since these chemicals cannot be decomposed,they keep on accumulating at each trophic level. The maximum concentration is accumulated at the top carnivore's level. This increase in the concentration of pollutants or harmful chemicals with an increase in the trophic level is called biological magnification. For example,as shown in the figure,$DDT$ concentrations increase from $0.04\,ppm$ in phytoplankton to $13.8\,ppm$ in large fish.
$(c)$ Groundwater depletion and ways for its replenishment: The level of groundwater has decreased in recent years. The source of water supply is rapidly diminishing each year because of an increase in population and water pollution. To meet the demand for water,water is withdrawn from water bodies such as ponds,rivers,etc. As a result,the source of groundwater is depleting. This is because the amount of groundwater being drawn for human use is more than the amount replaced by rainfall. Lack of vegetation cover also results in very small amounts of water seeping through the ground. An increase in water pollution is another factor that has reduced the availability of groundwater.
Measures for replenishing groundwater:
$1.$ Preventing over-extraction of groundwater.
$2.$ Optimizing water use and reducing water demand.
$3.$ Rainwater harvesting.
$4.$ Preventing deforestation and planting more trees.