Minimum concentration warmwater fish: 5.5 mg/L
Minimum concentration for coldwater fish is 6.5 mg/L
Dissolved oxygen is a molecule of O2 that is dissolved into the water. It is invisible to our naked eye. It is not the bubbles in water, nor the oxygen component of the water molecule H2O. Dissolved oxygen can get into the water two ways, through atmospheric oxygen mixing into a stream in turbulent areas or by the release of oxygen from aquatic plants during photosynthesis.
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.
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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.