What leads to eutrophication in rivers?

Options

A. Excessive use of fertilizers
B. Excessive production of crops
C. Decrease in carbon dioxide in water
D. Increase in the temperature of water

Explanation

### Step-by-Step Solution:

1. **Understanding Eutrophication**: Eutrophication is a process that occurs in water bodies, such as rivers and lakes, where an increase in nutrients, particularly phosphorus and nitrogen, leads to excessive growth of algae.

2. **Role of Nutrients**: The primary nutrients that contribute to eutrophication are phosphorus and nitrogen. These nutrients can come from various sources, including agricultural runoff, wastewater, and fertilizers.

3. **Excessive Use of Fertilizers**: When farmers use fertilizers in their fields, not all of the nutrients are absorbed by the crops. The excess fertilizers can wash off into nearby rivers and lakes, especially during rainfall.

4. **Algal Bloom**: The influx of nutrients into the water body stimulates the rapid growth of algae, known as an algal bloom. This bloom can cover the surface of the water, blocking sunlight from reaching aquatic plants below.

5. **Impact on Aquatic Life**: The lack of sunlight prevents underwater plants from photosynthesizing, which can lead to their death. When the algae eventually die, they are decomposed by bacteria, a process that consumes oxygen in the water.

6. **Decline in Oxygen Levels**: The decomposition of dead algae leads to a significant decrease in oxygen levels in the water, which can create a hypoxic environment (low oxygen) that is harmful or even deadly to fish and other aquatic organisms.

7. **Conclusion**: Therefore, the excessive use of fertilizers is the primary cause of eutrophication in rivers, leading to a series of ecological problems.

### Final Answer:
**The excessive use of fertilizers (Option A) leads to eutrophication in rivers.**

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