Which one of the following statements is not correct ?

Options

A. Eutrophication indicates that water body is polluted
B. The dissolved oxygen concentration below 6 ppm inhibits fish growth
C. Eutrophication leads to increase in the oxygen level in water
D. Eutrophication leads to anaerobic conditions

Answer

To solve the question regarding which statement about eutrophication is not correct, we will analyze the process of eutrophication and the options provided.

### Step-by-Step Solution:

1. **Understanding Eutrophication**:
- Eutrophication is a process that occurs when water bodies become enriched with nutrients, particularly nitrogen and phosphorus. This nutrient enrichment leads to excessive growth of algae, known as algal blooms.

2. **Effects of Algal Blooms**:
- The dense growth of algae can block sunlight from reaching aquatic plants, which can lead to their death.
- When the algae die, they are decomposed by bacteria, a process that consumes oxygen in the water. This reduction in oxygen levels can create hypoxic (low oxygen) or anoxic (no oxygen) conditions.

3. **Consequences of Low Oxygen Levels**:
- Aquatic animals, such as fish, rely on dissolved oxygen in the water to survive. When oxygen levels drop below certain thresholds (typically below 6 ppm), it can inhibit fish growth and lead to fish kills.

4. **Evaluating the Options**:
- We need to evaluate the statements provided in the options to identify which one is incorrect. The statements generally revolve around the implications of eutrophication and its effects on aquatic life and water quality.

5. **Identifying the Incorrect Statement**:
- If one of the options states that "the dissolved oxygen concentration below 6 ppm inhibits fish growth," this statement is actually correct. Therefore, it cannot be the incorrect statement we are looking for.
- Another option might claim that eutrophication leads to anaerobic conditions, which is also correct since the decomposition of organic matter in the absence of oxygen leads to such conditions.
- The statement that indicates eutrophication signifies that a water body is polluted due to the dense plant population is also correct.

6. **Conclusion**:
- After analyzing the statements, we find that the statement regarding the effects of dissolved oxygen concentration below 6 ppm inhibiting fish growth is indeed correct. Hence, the statement that is not correct must be identified from the remaining options.

### Final Answer:
The incorrect statement regarding eutrophication is **option number 3**.