Question

Which one of the following statements is NOT correct ?

• A. The dissolved oxygen concentration below 6 ppm inhibits fish growth
• B. Eutrophication indicates that water body is polluted
• C. Eutrophication leads to increase in the oxygen level in water
• D. Eutrophication leads to anaerobic conditions

Hint:

The key consequence of eutrophication is not the algal bloom itself, but the subsequent decomposition of the dead algae. This bacterial decomposition consumes dissolved oxygen, leading to the death of fish and other aquatic life.

Answer 1

The Correct Option is C

Let's analyze the process of eutrophication and each statement. Eutrophication is the enrichment of a water body with nutrients, primarily nitrates and phosphates.
Statement (A): The dissolved oxygen (DO) concentration below 6 ppm inhibits fish growth. This is a correct statement. Healthy aquatic ecosystems require a DO level of 6-8 ppm or higher. Levels below this cause stress and can be lethal to most fish species.
Statement (B): Eutrophication indicates that a water body is polluted. This is correct. The excess nutrients are a form of pollution, often from agricultural runoff or sewage, which disrupts the natural ecosystem.
Statement (D): Eutrophication leads to anaerobic conditions. This is correct. The decomposition of dead algae by aerobic bacteria consumes large amounts of dissolved oxygen, leading to hypoxic (low oxygen) or anaerobic (no oxygen) conditions.
Statement (C): Eutrophication leads to an increase in the oxygen level in water. This statement is incorrect. While the initial dense growth of algae (algal bloom) produces oxygen through photosynthesis, this is a temporary effect. When these algae die and sink, their decomposition by aerobic bacteria consumes a massive amount of dissolved oxygen from the water, leading to a severe net decrease in oxygen levels.
Therefore, the incorrect statement is that eutrophication leads to an increase in the oxygen level.