What two water characteristics are inversely proportional to dissolved oxygen levels?

• A. Temperature and salinity
• B. Temperature and pH
• C. pH and turbidity
• D. Turbidity and salinity

Answer: (A)

Dissolved oxygen in water is the amount of oxygen gas that can remain dissolved under the prevailing conditions, and two main factors directly affect how much can stay dissolved: temperature and salinity. Temperature has a strong influence because gases become less soluble as water warms. When water heats up, its tendency to keep oxygen dissolved decreases, so DO levels fall as temperature rises. Salinity also lowers the solubility of oxygen; salty water holds less dissolved gas than freshwater at the same temperature, so increasing salinity reduces DO through the so-called salting-out effect. Put together, higher temperature and higher salinity both push DO downward, making them inversely related to dissolved oxygen.

While pH or turbidity can impact aquatic oxygen dynamics indirectly (through effects on photosynthesis, respiration, or decomposition), they do not show the same direct, consistent inverse relationship with dissolving oxygen as temperature and salinity do.