The concentration of oxygen held in solution in water. Usually it is measured in mg/l (sometimes in μg/m3) or expressed as a percentage of the saturation value for a given water temperature. The solubility of oxygen varies inversely with temperature; this is important, because the warmer the water the larger the proportion of dissolved oxygen that is used by poikilotherms. The dissolved oxygen level is an important first indicator of water quality. In general, oxygen levels decline as pollution increases.
http://www.lenntech.com/why_the_oxygen_dissolved_is_important.htm. The importance of dissolved oxygen explained in detail.
Subjects: Ecology and Conservation.