Although water is made of H2O, the oxygen is not available
for organisms. Dissolved oxygen in water is required for most organisms.
Dissolved oxygen (DO) refers to the volume of oxygen that is contained
in water. Oxygen enters the water by photosynthesis of aquatic plants
and by the transfer of oxygen across the air-water interface. The amount
of oxygen that can be held by the water depends on the water
temperature, salinity, and pressure. Gas solubility increases with
decreasing temperature (colder water holds more oxygen)
Flowing water is more likely to have higher dissolved oxygen levels
than is stagnant water because of the water movement at the air-water
interface. In flowing water, oxygen-rich water at the surface is
constantly being replaced by water containing less oxygen as a result of
turbulence. Because stagnant water undergoes less internal mixing, the
upper layer of oxygen-rich water tends to stay at the surface, resulting
in lower dissolved oxygen levels throughout the water column.
Oxygen rich water