Thermal pollution — in aquatic systems, an increase in water temperature chiefly reduces which critical water-quality parameter, leading to stress on aquatic life?

Introduction:
When warm effluents enter rivers or lakes, the physical chemistry of water changes. The key ecological concern is the effect on oxygen availability to aquatic organisms. The question asks which parameter is reduced primarily by temperature rise.

Given Data / Assumptions:

• Parameter candidates: dissolved oxygen (DO), biological oxygen demand (BOD), vapor pressure.
• Effect considered: consequence of increased water temperature.
• Aquatic ecology: fish and invertebrates require adequate DO.

Concept / Approach:
Solubility of gases in water decreases as temperature increases. Thus DO saturation concentration falls with higher temperature, reducing available oxygen even if pollution loads are unchanged.

Step-by-Step Solution:

Verification / Alternative check:
DO saturation tables show a monotonic decrease of DO with increasing temperature, confirming the conclusion.

Why Other Options Are Wrong:

• BOD: represents oxygen demand; temperature often accelerates biochemical activity, effectively increasing oxygen consumption rather than reducing BOD numerically by heating alone.
• Vapor pressure: rises with temperature.
• All of the above: invalid since items (b) and (c) are not reduced.

Common Pitfalls:
Confusing BOD increase in reaction rate with a decrease in its numerical value; the central physical effect is reduced oxygen solubility.

Final Answer:
Water temperature rise primarily reduces dissolved oxygen content.