Why is the amount of oxygen in a body of water important?
For the survival aquatic organisms.
Tha amount of dissolved oxygen is essential for the survival of aquatic organisms and which mainta the quality of water. Some submerged aquatic animals such as fish respire by gills and the source of oxygen for such is dissolved in the water bodies.
The microornasms purify water and maintain the quality. They survive in the water bodies where sufficient amount of dissolved oxygen is found. It is calculated by BOD and COD.