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JEE Advanced levelSingle correct optionDifficulty 3 of 3ev147

Arrange the events of the eutrophication of a lake in the correct order: (i) fish die, (ii) fertiliser and detergent run-off adds nitrate and phosphate, (iii) dissolved oxygen falls and BOD rises, (iv) a dense algal bloom develops, (v) dead algae are decomposed by aerobic bacteria.

  1. A(ii), (iv), (v), (iii), (i)
  2. B(iv), (ii), (v), (i), (iii)
  3. C(ii), (iii), (iv), (v), (i)
  4. D(ii), (v), (iv), (i), (iii)

Answer: A

Nutrient input (ii) triggers the algal bloom (iv). The algae die and are decomposed by aerobic bacteria (v), which consumes oxygen so that DO falls and BOD rises (iii). The lack of oxygen then kills the fish (i).
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