In a stable body of water with no stratification, DO will remain at 100% air saturation. The 100% air saturation means that the water is holding as many dissolved gas molecules at it can in equilibrium. At equilibrium, the percentage of each gas in the water would be equivalent to the percentage of that gas in the atmosphere, which is known as its partial pressure.
The water will slowly absorb oxygen and other gases from the atmosphere until it reaches equilibrium at complete saturation. This process is sped up by aeration. It is possible for DO to exceed 100% air saturation in water by biological means.
What contributes to the concentration of Dissolved Oxygen in water:
- Atmospheric pressure: Higher atmospheric pressure allows bodies of water to retain more dissolved oxygen. The pressure from above allows the water to hold more oxygen molecules. Therefore, DO concentrations are normally lower at higher elevations due to lower atmospheric pressure.
- Temperature: A body of water with lower temperature can contain more dissolved oxygen because oxygen molecules have less movement. The increased movement of oxygen molecules in warmer water allows them to escape out of the water into the air.
- Depth of the water: The shallower the water, the higher the concentration of DO because wind creating waves on the surface increases DO, as well as aquatic plants living in shallower light-filled waters create DO as a by-product of photosynthesis.
- Salinity: Lower salinity contributes to a higher concentration of DO because salts affect the solubility of gases, essentially driving them out of the water.
- Bioactivity: Lower bioactivity of microorganisms in water leads to a higher concentration of DO, because microorganisms feeding on organics and decaying matter use oxygen in their respiration.
