An oxygen sag curve is an area of low oxygen on a graph of oxygen vs. space that occurs because of oxygen depletion in a stream or river, explains Encyclopedia.com. These areas form when sewage or other pollutants with organic material are released into a river and plants, bacteria and other organisms gather and use all the oxygen in the water trying to digest the material.
When the water in a stream or river is polluted and bacteria decompose the excess organic material, they go through a process called bacterial respiration. During this process, bacteria take in organic material and oxygen and release carbon dioxide, water and energy. This process uses the oxygen in the water. As the organic material in the water is used up, the oxygen content in the water rises to normal levels.
Because of the lack of oxygen, aerobic or oxygen-using organisms cannot live in areas affected by sag curves. Instead, anaerobic bacteria remain in this area, producing toxic chemicals, such as methane and hydrogen sulphide.
Scientists divide the oxygen depleted area into four parts: a decomposition zone just after organic material has spiked and where oxygen is lowest, a septic zone where the level of organic waste decreases but oxygen still remains low, a recovery zone where organic matter remains at a stable level while oxygen increases and a clean zone where the levels of both oxygen and waste level out.