Sewage Pollution Effects on Lake Water

Steps:

• Sewage introduces organic matter and nutrients like nitrates, increasing their levels in the water.
• Elevated nitrates promote algal blooms, leading to more bacterial growth during decomposition.
• Bacterial activity consumes dissolved oxygen, reducing its availability.
• Compared to unpolluted water, the lake has higher bacteria and nitrates but lower oxygen.

Why A is correct:

• Sewage pollution causes eutrophication, defined as nutrient enrichment (e.g., nitrates from waste) that boosts bacteria and depletes oxygen via respiration and decay.

Why the others are wrong:

• B: Nitrates increase, not decrease, due to sewage input; oxygen decreases from bacterial demand.
• C: Nitrates increase from nutrient pollution, not decrease.
• D: Bacteria increase from organic matter; oxygen decreases, not increases.

Final answer: A