Relative dissolved oxygen (DO): Compared with fresh river water at the same temperature and pressure, seawater typically contains how much dissolved oxygen?

Introduction / Context:
Dissolved oxygen (DO) solubility guides aeration, ecology, and outfall design. Salinity reduces oxygen solubility relative to freshwater, which matters when estimating reaeration and oxygen sag in marine discharges.

Given Data / Assumptions:

• Equal temperature and barometric pressure for comparison.
• Typical ocean salinity (~35 g/L).
• Focus on equilibrium DO solubility.

Concept / Approach:

Gas solubility decreases with increasing salinity (the “salting out” effect). Empirical correlations and tables show seawater DO solubility is roughly 15–25% lower than that of freshwater over common environmental temperatures, making “about 20% less” a practical rule of thumb.

Step-by-Step Solution:

Verification / Alternative check:

Standard DO tables (fresh vs. sea) confirm the reduction near 20% across common temperatures.

Why Other Options Are Wrong:

10% less underestimates the salinity effect; “more DO” contradicts the salting-out principle; 5% less is too small.

Common Pitfalls:

Comparing DO concentration measurements influenced by biological demand rather than equilibrium solubility; ignoring temperature/pressure dependence.

Final Answer:

About 20% less DO