The availability of oxygen is one of the most important indicators of the condition of a water body, because dissolved oxygen, or DO, (the amount of oxygen dissolved in water) is necessary for most aquatic organisms, including fish and invertebrates. Some species have very defined lower limits of DO that they can tolerate. Increases in DO can indicate improvements in water quality, such as has occurred in many parts of the world in the last 30 years.
Over the two decades, rivers in Europe and Australasia have shown a significant statistical reduction in biological oxygen demand concentrations, (an indicator of the organic pollution of freshwater), suggesting positive trends. There was no change in the assessed results for North America, although there was a tighter data distribution, indicating the data available for 1991-2000 is less variable than for previous periods.