Excessive nutrients in a fresh water body cause fish mortality give two reasons .

Explanation

### Step-by-Step Solution:

1. **Understanding Excessive Nutrients**: Excessive nutrients in freshwater bodies, such as nitrogen and phosphorus, often come from agricultural runoff, sewage discharge, or industrial waste. These nutrients promote the growth of algae.

2. **Algal Bloom Formation**: The increase in nutrients leads to a phenomenon known as algal bloom. This is characterized by a rapid increase in the population of algae in the water body.

3. **Oxygen Depletion**: As the algae grow and multiply, they eventually die off. The decomposition of this dead algae consumes a significant amount of dissolved oxygen in the water, leading to oxygen depletion.

...