Definition - What does Dissolved Oxygen mean?
Dissolved oxygen is the oxygen (O2) that is in a liquid (e.g., water) as measured in milligrams per liter.
Aquatic life, such as fish or amphibians, require a certain minimum level of dissolved oxygen to survive. Stagnant water with too much fresh organic matter might have an insufficient amount of dissolved oxygen and therefore be unable to support life.
MaximumYield explains Dissolved Oxygen
The amount of dissolved oxygen varies greatly based upon a number of factors. Most commonly for natural bodies of water, the water's motion or flow influences the dissolved oxygen levels. A rushing mountain stream working its way down a ridge contains more dissolved oxygen than a stagnant pool.
The amount of organic matter contained in a river or waterway can also affect the dissolved oxygen levels. During the decomposition process the organic matter uses oxygen, but releases it again after it is fully decomposed.
Pollutants such as pesticides and herbicides can reduce the amount of dissolved oxygen, as can the presence of a too much nitrogen.
In cases of nitrogen contamination, the levels of nitrogen are so high that the river, lake, or body of water will be so filled with algae that the dissolved oxygen levels are again too low to support life.