Minimum dissolved oxygen concentration for warmwater fish: 5.5 mg/L
Minimum dissolved oxygen concentration for coldwater fish is 6.5 mg/L
All animals need oxygen to survive. Dissolved oxygen is what makes aquatic life possible. Changes in oxygen concentration may affect species dependent on oxygen-rich water, like many macroinvertebrate species. Without sufficient oxygen they may die, disrupting the food chain.
Natural factors that influence dissolved oxygen:
- Aquatic life-animals living in water use up dissolved oxygen. Bacteria take up oxygen as they decompose materials. Dissolved oxygen levels drop in a water body that contains a lot of dead, decomposing material.
- Elevation-the amount of oxygen in the atmosphere decreases as elevation increases. Since streams get much of their oxygen from the atmosphere streams at higher elevations will generally have less oxygen.
- Salinity (saltiness)-Salty water holds less oxygen than fresh water.
- Temperature-cold water holds more dissolved oxygen than warmer water.
- Turbulence-more turbulence creates more opportunities for oxygen to enter streams.
- Vegetation-riparian vegetation directly affects dissolved oxygen by releasing oxygen into the water during photosynthesis. It indirectly affects dissolved oxygen concentrations because vegetation shading a stream may decrease water temperatures and as temperature decreases dissolved oxygen increases.
Human factors that influence dissolved oxygen:
- Clearing land (e.g., construction, logging)-may send excess organic matter into streams. Organic matter is decomposed by microorganisms, which use up oxygen in this process. Therefore, if there is a lot of organic waste in the stream the microorganisms use more oxygen than can be replaced in the stream.
- Destruction of riparian areas (e.g., development or overgrazing) decreases the amount of shade and increases the water temperature. Warmer water holds less DO than colder water.
Why care about dissolved oxygen?
All terrestrial and aquatic animals need oxygen to survive. Many aquatic macroinvertebrate species depend on oxygen-rich water. Without sufficient oxygen they may disappear. Even a small change in dissolved oxygen concentration can affect the composition of aquatic communities. Many fish require a certain dissolved oxygen range in order to survive. Oxygen concentration can also affect other chemicals in the water. For example, cadmium stays in a solid form in the presence of oxygen and sinks to the bottom of the water. However, if the water goes anoxic (without oxygen) cadmium may dissolve into the water. This is a problem because cadmium (as with other hard metals) is poisonous to animals.For more information see Understanding Your Watershed: Dissolved Oxygen.