Dissolved oxygen (DO) is the amount of gaseous oxygen dissolved in water. Oxygen diffuses into water from the surrounding air, by aeration and as a by-product of photosynthesis. Dissolved oxygen levels are used as an indicator of water quality.

Table with two columns titled dissolved oxygen and characteristic. 0 to 2 mg per L has a characteristic of not enough to support life. 2 to 4 mg per L has only a few fish and aquatic insects can survive. 4 to 7 mg per L is good for many aquatic animals. 7 to 11 mg per L is very good for most stream fish.

Figure 1. Values of oxygen and their importance in life support