Which of the following is an indicator of pollution of water bodies?

Options

A. Level of oxygen
B. Presence of algae
C. Presence of decomposers
D. Bacteria present in the faeces of mammals living in water

Answer

To determine which of the following is an indicator of pollution in water bodies, we can analyze the options provided.

### Step-by-Step Solution:

1. **Understanding Water Pollution Indicators**: Water pollution indicators are specific signs or measurements that can help us assess the quality of water and the level of pollution present in it.

2. **Evaluating Option 1 - Level of Oxygen**:
- The level of dissolved oxygen in water is generally stable, with some variations.
- While it is important for aquatic life, it cannot be solely relied upon as an indicator of pollution.
- However, the biochemical oxygen demand (BOD) can indicate the amount of organic matter in the water, but the oxygen level itself is not a direct indicator.

3. **Evaluating Option 2 - Presence of Algae**:
- Algae growth can indicate nutrient pollution, particularly from fertilizers or sewage, which can lead to eutrophication.
- An excessive presence of algae suggests that there is a high level of nutrients in the water, which is a clear sign of pollution.
- Therefore, the presence of algae is a valid indicator of water pollution.

4. **Evaluating Option 3 - Presence of Decomposers**:
- Decomposers, such as bacteria, are naturally present in water bodies and play a role in breaking down organic matter.
- Their presence alone does not indicate pollution, as they are a normal part of the aquatic ecosystem.

5. **Evaluating Option 4 - Bacteria in the Feces of Mammals**:
- While the presence of certain bacteria can indicate contamination from fecal matter, it is not a direct indicator of overall water pollution.
- It can indicate specific types of pollution but does not provide a comprehensive view of water quality.

6. **Conclusion**:
- After evaluating all options, the best indicator of water pollution among the given choices is the **presence of algae**.

### Final Answer:
The correct answer is **Option 2: Presence of Algae**.

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