Eutrophication from nutrient pollution

Steps:

• Soluble fertilisers run off into lakes, adding excess nutrients like nitrates and phosphates.
• These nutrients cause rapid algal growth, forming blooms that block sunlight.
• Algae die and decompose, with bacteria using up dissolved oxygen in the process.
• Low oxygen levels lead to deoxygenation, harming aquatic life.

Why D is correct:

• Soluble fertilisers provide bioavailable nutrients that trigger eutrophication, where microbial decomposition depletes dissolved oxygen (DO) via aerobic respiration: organic matter + O₂ → CO₂ + H₂O + energy.

Why the others are wrong:

• A. Acid rain lowers pH and affects aquatic chemistry but does not directly reduce oxygen levels.
• B. Excess calcium hydroxide raises pH (alkalinity) without promoting algal blooms or oxygen consumption.
• C. Insoluble nitrates do not dissolve, so they fail to enrich water and cause eutrophication.

Final answer: D