Tilapia Topic, Aquaculture Environment, Nitrogen, dissolved

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:

• Elemental nitrogen (N₂): Although inert biologically, N₂ is the most abundant form of nitrogen in water. The dissolved elemental nitrogen content of water is usually in equilibrium with the nitrogen gas in the surrounding atmosphere. Level of N₂ is 10 to 12 mg/L at saturation under usual conditions. The importance to fish health of N₂ gas is nitrogen gas supersaturation of culture water. In Hawaii, nitrogen gas bubble disease is a fairly common occurrence in tilapia culture settings which utilize recirculated water pumped under pressure. Because of the availability of organic and inorganic complex forms of nitrogen that are directly available for utilization by biota, N₂ fixation is probably not an important source of biologically accessible nitrogen in tilapia culture in Hawaii.
   
• Total ammonia nitrogen: Total ammonia nitrogen is the combined concentration of NH₄+ and NH₃. These two forms occur in a dynamic equilibrium in water. Although NH₃, or un-ionized ammonia, is toxic to fish, analyses for ammonia measure the total ammonia concentration in the water and the level of NH3 must be then estimated by calculation. Both forms of ammonia nitrogen can be directly assimilated by plants. In tilapia culture the main sources for ammonia nitrogen are excretion from fish and the breakdown of proteins by microbes in the detritus, a process called ammonification.
   
• Nitrite nitrogen (N0₂-N): Nitrite nitogen is an intermediate product of nitrification formed from the oxidation of ammonia by a specific group of nitrifying bacteria (Nitrosomonas spp.). Nitrite is important to fish health because it is toxic at modest concentrations. Nitrite is not directly assimilated as a nitrogen source by plants. For more information see: Nitrites
   
• Nitrate nitrogen (NO₃-N): The formation of nitrate is the second step in the nitrification of ammonia. Nitrate is largely non-toxic unless at extremely high levels. Nitrate is utilized by plants as a nitrogen source. Large amounts of nitrate may accumulate in recirculation biofiltration systems. Nitrate below 10mg/L should not be directly toxic to tilapia. In a closed system, low ammonia and nitrite and high nitrate indicates a proper functioning biofilter. In recirculation biofiltration systems, nitrate may accumulate unless removed by water exchange or is assimilated by aquatic plants. When nitrate exceeds 25 - 30 mg/L, removal is indicated. Water exchange is usually the means used to accomplish this.
   
• Organic nitrogen: Nitrogen is also present in many types or organic molecules common to aquaculture ponds.
   

The Nitrogen Cycle

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.

Also:

• Nutrients in the aquarium

  .