In natural surface waters, which effect of sunlight is most directly beneficial for water quality?

Introduction:
Sunlight influences aquatic ecosystems through heating and by enabling photosynthesis. The water-quality parameter most directly and consistently improved by sunlight during daylight is dissolved oxygen (DO), generated by photosynthetic organisms.

Given Data / Assumptions:

• Options include effects on bacteria, algae, DO, and turbidity.
• We focus on the net, direct effect commonly leveraged in self-purification.

Concept / Approach:
Photosynthesis converts CO2 and water into organic matter and oxygen in the presence of light. Daytime algal and macrophyte activity raises DO, supporting aerobic decomposition and aquatic life. Sunlight does not reliably reduce turbidity; algal blooms may even increase it. UV has limited disinfection in natural waters due to shading and short exposure.

Step-by-Step Solution:
Identify the process most directly tied to sunlight: photosynthesis. Link photosynthesis to increased DO concentrations during the photoperiod. Select “Increases dissolved oxygen content”.

Verification / Alternative check:
Diurnal DO curves show daytime peaks and nighttime declines, reflecting photosynthesis and respiration cycles.

Why Other Options Are Wrong:

• Helps growth of bacteria: not a typical, direct benefit; many bacteria prefer low-light or are light-insensitive.
• Impedes growth of algae: the opposite—sunlight promotes algal growth.
• Reduces turbidity: not a direct effect; can be neutral or negative during blooms.

Common Pitfalls:

• Assuming sunlight disinfects bulk water; natural UV exposure is insufficient for consistent pathogen removal.

Final Answer:
Increases dissolved oxygen content.