How can heat islands impair water quality?

• A. Rapid temperature changes in aquatic ecosystems resulting from warm storm water runoff can be particularly stressful, even fatal to aquatic life.
• B. Increased water salinity
• C. Decreased rainfall
• D. Lower water temperatures improving oxygen levels

Answer: (A)

Heat islands impair water quality by warming stormwater runoff from urban areas. When rain falls on city surfaces, the runoff carries this heat into nearby streams, rivers, and lakes, causing a rapid rise in water temperature. Warmer water holds less dissolved oxygen and speeds up metabolic rates of aquatic organisms, which increases their oxygen demands while the supply is reduced. This thermal pulse can stress or kill fish and invertebrates, disrupt spawning and habitat for many species, and can contribute to algal blooms that further degrade water quality.

Increased salinity doesn’t typically result from heat islands, rainfall patterns aren’t reliably decreased by them, and cooler water doesn’t occur due to heat islands (warmer runoff, not cooler, is the usual effect).