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:
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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.
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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.
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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
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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.
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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.
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