Eutrophication Sequence in Rivers

Steps:

• High nitrates stimulate rapid growth of algae (single-celled plants), turning water green (4).
• Algae die off and sink to the bottom (1).
• Bacteria decompose the dead algae, multiplying quickly (2).
• Bacterial decomposition consumes dissolved oxygen, causing rapid decrease (3).

Why D is correct:

• D matches the eutrophication process: nutrient enrichment leads to algal bloom (4), followed by die-off (1), microbial decomposition (2), and oxygen depletion (3) as defined in aquatic ecology.

Why the others are wrong:

• A: Starts with plant death (1), but algal growth (4) precedes die-off.
• B: Begins with bacterial multiplication (2), which occurs after algal death.
• C: Opens with oxygen decrease (3), a late-stage effect, not the initial change.

Final answer: D