Which of the following statements is not correct regarding algal blooms ?

Options

A. Algal blooms are formed by blue-green algae.
B. Growth of Eichhornia crassipes causes colouration of water during during algal blooms.
C. Increased growth of algae causes depletion of `O_(2)` in water.
D. Algal blooms cause deterioration of water quality and fish mortality.

Explanation

### Step-by-Step Solution:

1. **Understanding Algal Blooms**:
- Algal blooms occur when nutrient-rich organic waste is dumped into water bodies. This leads to a rapid increase in the growth of algae, particularly planktonic algae, which can cause significant changes in the water's appearance and quality.

2. **Identifying the Statements**:
- We need to evaluate the provided statements regarding algal blooms to determine which one is incorrect.

3. **Evaluating Statement 1**:
- The first statement claims that algal blooms are formed by blue-green algae. This is correct, as blue-green algae (also known as cyanobacteria) are indeed responsible for many algal blooms.

4. **Evaluating Statement 2**:
- The second statement suggests that the growth of *Achaiaurea crecipis* causes coloration of the water during algal blooms. This is incorrect because it is the growth of planktonic algae that causes the characteristic coloration of the water, not *Achaiaurea crecipis*.

5. **Evaluating Statement 3**:
- The third statement states that increased growth of algae causes depletion of oxygen in the water. This is correct, as dense algal growth can lead to oxygen depletion, which negatively affects aquatic life.

6. **Evaluating Statement 4**:
- The fourth statement mentions that algal blooms cause deterioration of water quality and fish mortality. This is also correct, as algal blooms can lead to various environmental issues, including the death of fish due to low oxygen levels.

7. **Conclusion**:
- After evaluating all the statements, the only incorrect statement is the second one, which incorrectly attributes the coloration of water during algal blooms to *Achaiaurea crecipis* instead of planktonic algae.

### Final Answer:
- The statement that is not correct regarding algal blooms is: **The growth of *Achaiaurea crecipis* causes coloration of the water during the algal bloom.**

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