Detailed Explanation of Nitrogen Transformation and Balance in Aquaculture Water

Nitrogen is one of the important constituent elements of animals and plants in water bodies. The content and distribution state of nitrogen in water bodies has a profound impact on the yield and quality of fishery aquaculture. Therefore, studying the transformation and balance of nitrogen in fishery water bodies is of great significance, as it can help improve the economic benefits of aquaculture, improve fishery water quality, and at the same time reduce the pollution level of fishery wastewater.

Generally, in fishery water bodies,

the main source of nitrogen is feed. Part of this nitrogen is converted into aquatic products. Improving feed formulas and feeding techniques can increase the nitrogen conversion rate and also improve water quality. Fish excrete molecular ammonia through their gills, while microorganisms in the water body convert nitrogen oxides into molecular ammonia by decomposing residual feed and fish excrement. Molecular ammonia reacts with water to form ammonium ions (NH4+) and hydroxide ions (OH-). Therefore, an equilibrium exists between molecular ammonia and ammonium ions in the water body. The sum of molecular ammonia and ammonium ions is called total ammonia nitrogen (TAN). The input of large amounts of feed can cause the concentration of total ammonia nitrogen in the water body to rise, thereby making the concentration of molecular ammonia too high, to the extent that it harms fish health. The nitrogen content in fishery water bodies is less than the difference between feed nitrogen and fish body nitrogen, because molecular ammonia can volatilize through the water surface and can also be converted by nitrifying bacteria into nitrate (NO3-), and nitrate (NO3-) can in turn be partly converted by denitrifying bacteria into nitrogen gas and nitrogen oxides that volatilize into the air. Nitrogen generally exists in water body sediment in the form of nitrogen oxides and ammonium ions.

Research shows that about 90% of the nitrogen source in fishery water bodies comes from feed, with the rest coming from air and other sources. About one third of the nitrogen is converted into aquatic products, while another 30% or so of the nitrogen volatilizes into the atmosphere in the form of molecular ammonia or nitrogen gas or nitrogen oxides, and another 23% of the nitrogen enters the sludge. Using aerators and water circulation methods can accelerate the conversion of toxic ammonia into nitrate, thereby reducing the generation and volatilization of molecular ammonia. In addition, the injection of water is conducive to the suspension of organic matter in the sludge, thereby reducing the formation of sludge. Adding quicklime to acidic water bodies can prevent the water quality from becoming acidic, thereby facilitating the nitrification and denitrification of nitrogen. In addition, adding a small amount of bacteria or enzymes may help the beneficial transformation of nitrogen.