Environmental Issues

Eutrophication is the natural ageing of a lake through nutrient enrichment, but when human activities such as the discharge of nutrient-rich sewage and agricultural runoff dramatically accelerate it, the process is called cultural or accelerated eutrophication. The added nitrates and phosphates cause an explosive bloom of algae; when these algae die, their decomposition by aerobic microbes consumes large amounts of dissolved oxygen, raising the biochemical oxygen demand and causing fish and other aquatic organisms to suffocate. Biological magnification is incorrect because it refers to the increasing concentration of toxicants like DDT at successive trophic levels, not oxygen depletion. Ozone depletion concerns the thinning of stratospheric ozone by chlorofluorocarbons and is unrelated to lake oxygen. Thermal stratification is the layering of water by temperature and does not by itself cause nutrient-driven fish kills. As described in NCERT Class 12, Chapter 16 (Environmental Issues), domestic sewage and agricultural runoff are major causes of cultural eutrophication. The biochemical oxygen demand of the water rises sharply as microbes decompose the organic load, and a high BOD is therefore used as a practical indicator of the degree of organic pollution in a water body. Plausibility check: the chain of nutrients to algal bloom to decomposition to oxygen depletion is internally consistent with eutrophication, confirming the answer.