There is a sharp decline in dissolved oxygen downstream from the point of sewage discharge. Why? What are its adverse effects ?

Explanation

### Step-by-Step Solution:

1. **Understanding the Problem**: The question asks why there is a sharp decline in dissolved oxygen downstream from a sewage discharge point and what adverse effects this decline has on aquatic life.

2. **Identifying the Cause**: The sharp decline in dissolved oxygen is primarily due to the presence of microorganisms that are involved in the biodegradation of sewage. When sewage is discharged into water bodies, it introduces a large amount of organic matter.

3. **Microbial Activity**: These microorganisms break down the organic matter in the sewage. This process of biodegradation requires oxygen, leading to an increased consumption of dissolved oxygen in the water.

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