Dissolved oxygen is the concentration of free molecular oxygen dissolved in water, usually expressed in milligrams per liter, parts per million, or percent of saturation.
Fish, bugs and other aquatic organisms need dissolved oxygen. If there is too little in the water, they will suffocate and die. It’s best if dissolved oxygen levels are above five milligrams of oxygen per litre of water (mgO2/L). Most fish cannot survive if levels are below three milligrams per litre for an extended period. When levels drop below one milligram per litre, the water is called hypoxic. When there is no oxygen, it is anoxic or anaerobic.
The amount of dissolved oxygen in the river water can be affected by several factors and processes going on in the river. Aquatic plants have a big impact on oxygen levels. Plants produce dissolved oxygen during the day and consume oxygen overnight so there can be swings in oxygen levels over the course of the day.
When there are too many plants in a river or stream, oxygen levels can drop so low overnight that aquatic creatures leave the area.