The main pollutants that lead to the decrease of dissolved oxygen content in water are

The main pollutant causing the decrease of dissolved oxygen content in water body is organic matter.

Under normal circumstances, in clear rivers, lakes, reservoirs and other water bodies, the dissolved oxygen content in water is between 6-8 mg/L, and in deep water, the dissolved oxygen content in water will increase due to pressure. However, in seriously polluted water bodies, such as the downstream of rivers and the effluent from urban sewage treatment plants, the dissolved oxygen content in water is usually low, generally between 2-4 mg/L.

The weather has an effect on the content of dissolved oxygen in water. With the increase of temperature, the dissolved oxygen content in water will decrease; When the air pressure decreases, the dissolved oxygen content in water will also decrease. In addition, water quality is also a key factor affecting the dissolved oxygen content in water.

Water bodies with high content of pollutants such as organic matter, nitrogen and phosphorus often have low dissolved oxygen content. In addition, plants also have an effect on the dissolved oxygen content in water. Oxygen produced by photosynthesis will increase the dissolved oxygen content in water; Organic matter produced by plant death and decay will consume dissolved oxygen in water.

Dissolved oxygen supersaturation

Due to severe aeration and other reasons, the content of molecular oxygen dissolved in water in the air increases obviously, which makes the dissolved oxygen in water supersaturated. The content of dissolved oxygen in water is closely related to the partial pressure of oxygen in the air and the water temperature. Under natural circumstances, the oxygen content in the air does not change much, so the water temperature is the main factor. The lower the water temperature, the higher the dissolved oxygen content in the water.

However, water conservancy projects will lead to supersaturation of dissolved oxygen. For example, when the discharge building overflows under the condition of high dam and large reservoir, or when the dam discharges through the spillway, the process of water flow drop is accompanied by strong water-gas exchange, and the content of dissolved gas in the downstream water body is often significantly increased due to strong aeration, which leads to the downstream area being far away, which has adverse effects and injuries to aquatic organisms, especially fish.