Question

Eutrophication of water bodies leading to killing of fishes is mainly due to non-availability of-
A. Food
B. Light
C. Essential minerals
D. Oxygen

Answer 1

Hint: Eutrophication is a phenomenon wherein there is too much enrichment of essential nutrients like nitrates, phosphates etc. in water bodies which promotes the growth of planktons, plants, algae and other microbes. Because of the accumulation of algae, the water bodies are depleted of many important substitutes and lead to death of fishes.

Complete answer:
Option A Food: Food is still available for fishes even if the water bodies get eutrophied as there are even other sources to obtain nutrients at the bottom of water.
So, A is the incorrect option
Option B Light: Because of nutrient enrichment algae tends to cover the entire or part of water bodies leading to overall decrease in clarity of water but that does not deplete it of light. Presence of an algal layer decreases water clarity and purity but is not a factor of killing fishes.
So, B is not the correct option.
Option C Essential minerals: Yes, eutrophication is due to enrichment of water bodies with nutrients like nitrogen or phosphorus which paves the way for more algae or plankton growth. This leads to eutrophication but not death as fishes can survive with other available food.
So, C is not the correct option.
Option D Oxygen: Due to eutrophication, there is excessive growth of algae in water bodies. Since algae photosynthesise and reduce overall oxygen in this process fishes cannot survive this oxygen depletion and hence are killed.
So, D is the correct option.

Hence, Option D is the correct answer.

Note:
When water bodies are enriched with nitrates or phosphates (from industry, fertilizers etc) algal blooms start appearing which consume most of the dissolved oxygen from water resulting in hypoxic conditions and fishes are killed due to suffocation and lack of oxygen.