Decomposition of algae and macrophytes

Correct option is D

The correct answer is (D) Reduces dissolved oxygen, leads to fish mortality and impairs water quality

Explanation:
• When algae and macrophytes (aquatic plants) die, they sink to the bottom of the water body where they are broken down by aerobic bacteria.
• This decomposition process requires significant amounts of oxygen. As the bacterial population explodes to consume the dead organic matter, they deplete the Dissolved Oxygen (DO) in the water.
• This leads to a state of hypoxia (low oxygen) or anoxia (no oxygen), which is fatal for fish and other aquatic organisms that rely on dissolved oxygen for respiration.
• This is the final and most destructive stage of Eutrophication, resulting in "dead zones" where most aquatic life cannot survive.

Information Booster:
• Biological Oxygen Demand (BOD) is a measure of how much oxygen is needed by microbes to decompose organic matter; high BOD indicates polluted or eutrophic water.
• Summer kills: Higher water temperatures further reduce the oxygen-holding capacity of water, worsening the impact of decomposition.
• Nutrient loading (Nitrogen and Phosphorus) is the primary trigger for the initial overgrowth of algae.

Additional Knowledge:
• Option B: Photosynthesis by living algae increases DO during the day, but their decomposition after death strictly decreases it.
• Option C: Quality is impaired, not enriched, due to the production of toxins and foul odors (like Hydrogen Sulfide) during anaerobic decomposition.