Understanding Fish Kills and Water Deoxygenation

The question asks us to identify which factor from the given options doesnot cause fish kills by deoxygenating water. Deoxygenation refers to the reduction of dissolved oxygen (DO) levels in water, which is crucial for fish survival. When DO levels drop too low, fish cannot respire and die.

Analyzing Factors Causing Deoxygenation

Let's examine how each option affects dissolved oxygen levels in water:

• Sewage dumped in the river: Sewage contains a high amount of organic matter. When this organic matter enters a water body, bacteria decompose it. This decomposition process requires a significant amount of dissolved oxygen, leading to deoxygenation and potentially killing fish. Therefore, sewage fits the description.
• Increased BOD: BOD stands for Biochemical Oxygen Demand. It measures the amount of oxygen required by microorganisms to break down organic matter in water. An increased BOD level directly indicates that more oxygen is being consumed by microbes, resulting in lower dissolved oxygen levels and harming aquatic life, including fish. Thus, increased BOD fits the description.
• Eutrophication: This process involves excessive nutrient enrichment (like nitrogen and phosphorus) in a water body, leading to rapid growth of algae and plants (algal blooms). When these algae and plants die, their decomposition by bacteria consumes large amounts of dissolved oxygen, causing severe deoxygenation and fish kills. Therefore, eutrophication fits the description.
• Dust in the air: Dust particles are typically solids suspended in the atmosphere. While widespread air pollution can have indirect effects on water quality (e.g., through acid rain), dust itself does not directly consume dissolved oxygen in water through decomposition processes. It doesn't introduce organic matter or nutrients that fuel oxygen-depleting microbial activity in the same way as sewage or eutrophication. Therefore, dust in the air does not fit the description of causing fish kills by deoxygenating water directly.

Conclusion on Non-fitting Factor

Based on the analysis, sewage, increased BOD, and eutrophication all contribute to the deoxygenation of water, leading to fish kills. Dust in the air, however, is not a direct cause of water deoxygenation in this context.