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 regarding the consequences of overgrowth of algae, we can break it down step by step:

### Step 1: Understanding Algal Overgrowth
Algal overgrowth, often referred to as algal blooms, occurs when there is an excessive growth of algae in water bodies. This can be caused by various factors, including nutrient pollution (especially nitrogen and phosphorus), warm temperatures, and sunlight.

**Hint:** Consider what conditions lead to algal blooms and their impact on aquatic ecosystems.

### Step 2: Effects on Dissolved Oxygen
As algae grow, they undergo photosynthesis during the day, which increases the levels of dissolved oxygen temporarily. However, when the algae die or when they are decomposed by bacteria, the decomposition process consumes a significant amount of oxygen. This leads to a decrease in the overall dissolved oxygen levels in the water.

**Hint:** Think about the balance between photosynthesis and respiration/decomposition in aquatic environments.

### Step 3: Biological Oxygen Demand (BOD)
Biological Oxygen Demand (BOD) is a measure of the amount of oxygen that microorganisms will consume while decomposing organic matter in water. With the overgrowth of algae, there is an increase in organic matter when the algae die. Consequently, the BOD increases because more oxygen is required for the decomposition of this organic matter.

**Hint:** Remember that higher BOD indicates more organic matter that needs to be broken down, which requires more oxygen.

### Step 4: Impact on Aquatic Life
As the dissolved oxygen levels decrease due to the high BOD, aquatic organisms, particularly fish, struggle to survive. Fish and other aquatic life depend on dissolved oxygen for respiration. A significant drop in oxygen levels can lead to the mortality of these organisms.

**Hint:** Consider how oxygen levels directly affect the survival of aquatic species.

### Conclusion
From the analysis, we can conclude that the overgrowth of algae results in:
1. Increased Biological Oxygen Demand (BOD)
2. Decreased levels of dissolved oxygen
3. Mortality of fishes

Thus, the correct answer to the question "Overgrowth of algae results in:" is **mortality of fishes**.

### Final Answer
**Mortality of fishes.**