JEE Main · 2021 · 27 Jul 2021 · Shift 1 · Environmental Chemistry

Eutrophication is the enrichment of a water body with nutrients such as nitrates and phosphates, which triggers a heavy growth of algae. The bloom mats the surface and cuts off light, and when the algae die the aerobic bacteria that decompose them consume the dissolved oxygen. The dissolved oxygen therefore falls rather than rises; once it is exhausted the decay continues anaerobically, and fish growth is already inhibited when the dissolved oxygen drops below about $6$ ppm. The statement that eutrophication leads to an increase in the oxygen level in water is not correct.