Overgrowth of algae results in:

Options

A. Increased level of dissolved oxygen
B. Decreased biological oxygen demand
C. Mortality of fishes
D. All of the above

Answer

To solve the question "Overgrowth of algae results in:", we will analyze the options provided and understand the implications of algal bloom.

### Step-by-Step Solution:

1. **Understanding Algal Bloom**:
- Algal bloom refers to the rapid increase or accumulation of algae in water bodies. This phenomenon often occurs when there are excess nutrients (like nitrogen and phosphorus) in the water.

**Hint**: Remember that algal bloom is a condition characterized by excessive growth of algae.

2. **Analyzing Option 1 - Increase the level of dissolved oxygen**:
- During algal bloom, algae consume oxygen for their respiration. As a result, the overall level of dissolved oxygen in the water decreases, not increases.

**Hint**: Think about how respiration affects oxygen levels in water.

3. **Analyzing Option 2 - Decreased biological oxygen demand**:
- Biological Oxygen Demand (BOD) refers to the amount of oxygen required by microorganisms to decompose organic matter. With algal bloom, the organic matter increases, leading to a higher demand for oxygen, not a decrease.

**Hint**: Consider how the presence of more organic matter influences oxygen consumption.

4. **Analyzing Option 3 - Mortality of fishes**:
- The overgrowth of algae blocks sunlight from reaching aquatic plants, which can lead to their death. Additionally, as algae consume oxygen, fish and other aquatic organisms may suffocate, leading to increased mortality rates among these organisms.

**Hint**: Reflect on the relationship between oxygen levels and fish survival.

5. **Analyzing Option 4 - All of these**:
- Since options 1 and 2 are incorrect, option 4 cannot be correct either.

**Conclusion**: The correct answer is option 3 - mortality of fishes.

### Final Answer:
The overgrowth of algae results in **mortality of fishes** (Option 3).