Oxygen is inseparable from the survival, growth and reproduction of all oxygen - consuming organisms. High and stable oxygen content in air, accounting for about 21%, so the land biological threat and rarely hypoxia; dissolved oxygen in water is less volatile; saturated dissolved oxygen is only equivalent to the amount of oxygen in the air 1/20 general fresh water, sea water less, so the amount of dissolved oxygen in water to aquatic animal life the process of life phenomenon and a limiting factor, is one of the most important factors in aquaculture.
However, in the breeding practice, for a long time due to the lack of timely and effective monitoring of dissolved oxygen in the water, the potential harm to the dissolved oxygen changes of lack of knowledge, many breeding groups regard oxygen cost, the aquaculture is floating phenomenon as the criterion for judging the adequacy of the dissolved oxygen in the water, see the floating head after aeration measures taken, this is actually the oxygen as a "help" measures; some farming groups worry about the impact of water hypoxia on cultured organisms, without taking into account the actual situation of dissolved oxygen, oxygen increasing continuously in the aquaculture water environment, the water oxygen concentration has been at a high state. These are unscientific methods of management, which often lead to unnecessary losses or reduce potential benefits.
The dissolved oxygen in a water body is dissolved in water in a molecular state. Dissolved oxygen in water (dissolved) and analytical (emission) is a dynamic and reversible process, when dissolved and analytical rates are equal, i.e. to achieve dynamic balance of dissolved oxygen, the concentration of oxygen in the water is under the condition of saturated dissolved oxygen content, saturated dissolved oxygen. The amount of saturated dissolved oxygen in water is affected by the partial pressure of atmospheric oxygen, the temperature of water and other factors such as other gases, organic matter or inorganic matter. The saturated dissolved oxygen in water is positively correlated with the partial pressure of atmospheric oxygen. Under the natural conditions, the partial pressure of atmospheric oxygen will not change significantly, so the effect of saturated dissolved oxygen can be ignored. Dissolved oxygen increases with the increase of water temperature, and the amount of saturated dissolved oxygen decreases. Salinity also has a direct and obvious effect on dissolved oxygen. With the increase of water salinity, the amount of saturated dissolved oxygen decreases.
In most cases, the actual content of dissolved oxygen in aquaculture water is lower than that of saturated dissolved oxygen. Its value depends on the dynamic balance effect of aeration and oxygen consumption in the water at that time. When the oxygen is larger than the oxygen consumption, oxygen saturation, sometimes "saturation" phenomenon, which generally occur in the sunny afternoon, algae density, strong photosynthesis in the pond; when the oxygen consumption dominates, the oxygen in the water to continue to decline, the results will appear at this time may even anaerobic Hypoxia Zone. Animal breeding appeared "floating head", even "Pan Tang" phenomenon. In pond culture, the increase of oxygen in water is mainly caused by phytoplankton photosynthesis, oxygen release, artificial aeration (mechanical aeration, chemical aeration, etc.) and the natural dissolution of oxygen in the atmosphere, but under different conditions, the proportion of these oxygen increasing effects is also different. In the eutrophic still water pond, photosynthesis is the main way to increase oxygen. The high density intensive pond is mainly aerated by artificial oxygen, and the poor nutrient water and flowing water contribute greatly to atmospheric dissolved oxygen.
Oxygen consumption in water can be divided into biological, chemical and physical sources of oxygen consumption. Biological oxygen consumption includes the dissolved oxygen consumed by the respiration of animals, plants and microorganisms. In most cases, oxygen consumption of plankton and benthos in the water occupies most of the pond's oxygen consumption. Chemical oxygen consumption includes the oxidation decomposition of organic matter and the oxidation and reduction of inorganic substances in the environment. Physical oxygen consumption mainly refers to the oxygen in the water to the air in the oven, only occupy a small part of this process is only carried out at the air-water interface.