Why is thermal pollution harmful?

• A. It cools the water to the point of freezing.
• B. It reduces nutrient availability and suppresses algae.
• C. It increases oxygen solubility and improves water quality.
• D. It lowers oxygen, stresses aquatic organisms, and can cause fish kills.

Answer: (D)

Thermal pollution harms aquatic life mainly by reducing dissolved oxygen and increasing stress on organisms. Warmer water holds less dissolved oxygen, so there’s less oxygen available for fish and other aquatic creatures. At the same time, higher temperatures boost metabolic rates, meaning these organisms use more oxygen. The combination can create low-oxygen conditions and trigger fish kills, especially in warm, slow-moving, or poorly mixed waters near heat discharges from industrial processes. Warm water can also promote algal blooms if nutrients are present; when those algae die and decompose, oxygen demand rises even further, worsening the problem.

The other ideas don’t fit how heat in water behaves. Cooling water to near freezing isn’t typical of thermal pollution, and suggesting that nutrients become less available or that oxygen solubility increases with heat contradicts the basic physics—oxygen solubility actually decreases as temperature rises.