Oxygen is critical for fueling cellular respiration, which in turn enables cells to carry out functions like growth and repair. Similar to carbon dioxide, atmospheric pressure causes molecular oxygen to dissolve in water. This dissolved oxygen is measured in mg/L.

The amount of dissolved oxygen in water determines which organisms can survive in a particular environment. Foul-smelling bacteria that use nitrate or sulfate to accept electrons and provide fuel can survive in environments with very little oxygen. A higher content of dissolved oxygen in water relates to purer water quality and the ability for more diverse life forms to survive. Brown trout adults and their eggs thrive in water that contains 9-12 mg/L of dissolved oxygen.

Water temperature, stream velocity, altitude, and level of organic matter content all affect the amount of dissolved oxygen that water can hold. As mentioned above, cold water contains more dissolved oxygen than warm water. Standing water often contains less dissolved oxygen than rapidly flowing water, and water at high elevations contains less dissolved oxygen than water at low elevations due to decreased air pressure. Bacteria consume dissolved oxygen during decomposition of organic matter and therefore lower the oxygen content of water. As temperatures increase downstream, dissolved oxygen values will decrease because warmer water holds less oxygen. The state standard for dissolved oxygen is a minimum of 6 mg/L, with a 7 mg/L minimum during fish spawning periods.

Dissolved Oxygen Information Sheet

Influences on Dissolved Oxygen Amounts:
1. Air pressure (elevation) -- greater pressure at sea level than in mountains; Water in Basalt, Colorado (6,600 feet) can hold only 79% of the oxygen as water at sea level (0 feet).

2. Temperature -- an increase in temperature decreases the amount of dissolved oxygen.
3. Photosynthesis/Respiration -- plants produce oxygen during photosynthesis, peaking around 3 pm, and use oxygen when not photosynthesizing (respiration) that peaks about 4 am.

Other factors:
Turbulance/ripples/velocity
Decomposition (as part of the nitrogen cycle) lowers dissolved oxygen levels