Define/Explain the following terms:
$(i)$ Eutrophication
$(ii)$ Jhum cultivation

(N/A) $(i)$ Eutrophication: The natural aging of a lake by biological enrichment of its water. In a young lake,the water is cold and clear,supporting little life. With time,streams draining into the lake introduce nutrients such as nitrogen and phosphorus,which encourage the growth of aquatic organisms. As the lake's fertility increases,plant and animal life burgeons,and organic remains begin to be deposited on the lake bottom. Over centuries,as silt and organic debris pile up,the lake grows shallower and warmer,with warm-water organisms supplanting those that thrive in a cold environment. Marsh plants take root in the shallows and begin to fill in the original lake basin. Eventually,the lake gives way to large masses of floating plants (bog),finally converting into land.
$(ii)$ Jhum cultivation: Also known as 'slash and burn' agriculture. In this practice,the farmers cut down the trees of the forest and burn the plant remains. The ash is used as a fertilizer and the land is then used for farming or cattle grazing. After cultivation,the area is left uncultivated for several years so as to allow its recovery.