Eutrophication from nitrogen fertilizer runoff

Steps:

• Nitrogen fertilizers wash into rivers, providing nutrients for excessive growth of water plants and algae.
• This leads to a bloom of plants that eventually die off.
• Bacteria decompose the dead plant matter, consuming large amounts of dissolved oxygen in the process.
• Reduced oxygen levels suffocate fish, causing deaths.

Why D is correct:

• D accurately describes eutrophication, where nutrient enrichment boosts plant growth, followed by bacterial decomposition that depletes dissolved oxygen via aerobic respiration.

Why the others are wrong:

• A: Incorrect sequence; oxygen depletion occurs after plant growth and decomposition, not before.
• B: Incomplete and reversed; decomposition follows growth and causes oxygen loss, not the reverse.
• C: Opposite effect; more plant growth increases decomposition, reducing oxygen, not increasing it.

Final answer: D