A lake near a village suffered heavy mortality of fishes within a few days Consider the following reasons for this?
(a) Lots of urea and phosphate fertilizers were used in the crops in the vicinity.
(b) The area was sprayed with DDT by an aircraft.
(c) The lake water turned green and stinky.
(d) Phytoplankton population in the lake declined initially thereby greatly reducing photosynthesis. Which two of the above were the main causes of fish mortality in the lake?

Options

A. (a), (c)
B. (a), (b)
C. (b), (c)
D. (c), (d)

Answer

To solve the problem of fish mortality in the lake near the village, we need to analyze the provided reasons step by step:

### Step 1: Analyze the use of fertilizers
- **Reason (a)**: Lots of urea and phosphate fertilizers were used in the crops in the vicinity.
- **Explanation**: The use of fertilizers can lead to nutrient runoff into the lake, causing nutrient enrichment, which is a primary cause of eutrophication. Eutrophication leads to excessive growth of algae (algal bloom).

### Step 2: Consider the impact of pesticides
- **Reason (b)**: The area was sprayed with DDT by an aircraft.
- **Explanation**: DDT is a pesticide that can be toxic to aquatic life. However, its immediate effect on fish mortality is less direct compared to the consequences of eutrophication.

### Step 3: Observe the condition of the lake water
- **Reason (c)**: The lake water turned green and stinky.
- **Explanation**: A green and stinky lake indicates the presence of algal blooms, which occur due to excessive nutrients (like those from fertilizers). When these algae die, they decompose, leading to oxygen depletion in the water, which is detrimental to fish.

### Step 4: Assess the phytoplankton population
- **Reason (d)**: Phytoplankton population in the lake declined initially, thereby greatly reducing photosynthesis.
- **Explanation**: A decline in phytoplankton can lead to reduced oxygen production, but this is often a consequence of the algal bloom rather than a direct cause of fish mortality.

### Conclusion:
Based on the analysis, the two main causes of fish mortality in the lake are:
- **(a)** Lots of urea and phosphate fertilizers used in the crops (leading to eutrophication).
- **(c)** The lake water turned green and stinky (indicating algal bloom and subsequent oxygen depletion).

### Final Answer:
The correct options are (a) and (c).

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