As a soluble constituent, nitrogen is of major importance in aquaculture water. Nitrogen is required for amino acid synthesis and molecular forms are utilized by some types of microorganisms in energy metabolism. Nitrogen in aquaculture water occurs either as the biologically inert dissolved nitrogen gas, whose source is the atmosphere, or in the several forms of combined nitrogen. Processed feed is the principal source of the biologically available, combined nitrogen which enters the water mainly as soluble ammonia excreted by tilapia or as a byproduct of microbial breakdown of protein and amino acids. In the nitrogen cycle, the major nitrogen forms are the following:
In tilapia culture waters the main source of nitrogen is the protein in feed. Ammonia is the waste product of protein metabolism in fish and more than 90% of the nitrogen that enters the culture water is ammonia excreted by the tilapia. Protein is also dominated in the environment through biological activity in a process called ammonification. If the source of the nitrogen in the organisms was nitrogen fixation, the process is referred to as mineralization. In tilapia culture systems mineralization is a much less important process than ammonification.
Another process in the nitrogen cycle is denitrification which involves the reduction of nitrate to nitrite and nitrite to the following gases: N2, NH3 and N2O. Denitrification occurs in areas lacking oxygen. Heterotrophic bacteria utilize NO3 and NO2 as terminal electron acceptors in anaerobic respiration. In this way nitrate and nitrite replace the function of oxygen for these organisms. In ponds, culture tanks or biofiltration systems that contain an appreciable accumulation of organic debris, denitrification can contribute substantial quantities of nitrite.
Ammonia and nitrate are immobilized when taken up by aquatic plants and assimilated into proteins. Immobilization means that the nitrogen is for the time being sequested in the plant tissues. Assimilation is a major means for removal of ammonia from aquaculture ponds that contain appreciable quantities of aquatic plants (e.g. phytoplankton). This explains why when fish standing crop and feeding rates are high, ammonia level will rise rapidly following a phytoplankton crash.
During periods of high pH volatilization or diffusion of un-ionized ammonia to the atmosphere can cause loss of NH3 from aquaculture ponds.
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