Dissolved oxygen (DO)

The amount of oxygen dissolved in water or sewage. Concentrations of less than 5 ppm can limit aquatic life or cause offensive odors. Excessive organic matter present in water because of inadequate waste treatment and runoff from agricultural or urban land generally causes low DO.

The amount of oxygen dissolved in a stream in an indication of the degree of health of the stream and its ability to support a balanced aquatic ecosystem.

Source: Handbook of water and wastewater treatment plant operations