Eutrophication of water bodies leading to killing of fishes is mainly due to non-availability of:

Options

A. food
B. light
C. essential minerals
D. oxygen

Answer

**Step-by-Step Solution:**

1. **Understanding Eutrophication**: Eutrophication is the process where water bodies, such as lakes, become enriched with nutrients (especially nitrogen and phosphorus), leading to excessive growth of algae, known as algal blooms.

**Hint**: Think about what causes an increase in nutrients in water bodies.

2. **Sources of Nutrients**: Nutrients can enter water bodies from various sources, including agricultural runoff, sewage discharge, and industrial waste. These nutrients promote algal growth.

**Hint**: Consider the types of human activities that contribute to nutrient pollution.

3. **Effects of Algal Blooms**: The excessive growth of algae can block sunlight from reaching aquatic plants, which are crucial for photosynthesis. This can lead to a decline in plant life.

**Hint**: Reflect on how light availability affects aquatic ecosystems.

4. **Decomposition and Oxygen Consumption**: When the algae die, they are decomposed by bacteria. This decomposition process consumes a significant amount of dissolved oxygen in the water.

**Hint**: Think about the relationship between decomposition and oxygen levels in water.

5. **Impact on Aquatic Life**: The decrease in dissolved oxygen levels can lead to hypoxia (low oxygen conditions), which can be fatal for fish and other aquatic organisms that rely on oxygen for survival.

**Hint**: Consider what happens to fish and other aquatic animals when oxygen levels drop.

6. **Conclusion**: Therefore, the killing of fishes due to eutrophication is mainly due to the non-availability of oxygen, as it is consumed during the decomposition of dead algae.

**Final Answer**: The correct answer is oxygen.

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