The source of nitrogen compounds of biological importance is molecular nitrogen (N2) in air. We have already referred to the composition of air and solubility of atmospheric gases in water (see DO).
The nitrogen cycle involves ammonia fixing and nitrifying reactions in organisms and denitrification, which is the same process in reverse. The nitrification process involces oxidation of ammonia to nitrite and nitrite to nitrate which is an energy yielding process utilized by nitrifying bacteria. The reduction of nitrate to N2 is brought about by denitrifying bacteria.
The different forms of nitrogen present in natural waters include: molecular nitrogen (N2) in solution, organic compounds, protein and their breakdown products (amino-acids, urea and methylamines); ammonia as NH3, NH4 and NH4OH; nitrite as NO2 mainly and fractions as HNO2; and nitrate as NO3.
We have referred to the solubility of N2 and trace argon in water (Table II). Nitrogen is a biological inert gas and the problem of excess N2 in water is that of supersaturations. Often supersaturations exist in waters pumped up fro subsoil water and also in cases where rapid waring takes place. If air saturation increases over 110%, this supersaturation can cause problems in several fishes, by causing “gas embolism” or the gas bubble disease. It is important to recognize that with increase in depth the contents of dissolved gases in increase (Table II) and this has implications in using water pumped up from depths for aquaculture purposes. Atmospheric nitrogen is fixed by heterocyst bearing blue green algae (Nostoc, Anabaena) and possibly also by other blue greens
Unlike nitrogen and oxygen, ammonia is highly soluble in water. In natural waters where decomposition of organic matter takes place ammonia level will be high. Ammonia increase is often concommittant with decrease in DO and increase in CO2. The level of ammonia will also depend on the activity of the nitrifying and denitrifying bacteria, referred to earlier. Ammonia is important as the predominant excretory product of aquatic animals and in high density culture high ammonia levels can develop, through NH3 excreted directly and also by degradation of faecel matter and uneaten feed.
In natural waters, where extensive fish culture can be practised, ammonia levels can be high at the bottom. A depth profile of distribution of nitrogenous compounds namely NH3, NO2 and NO3 along with temperature and redox potential, discussed earlier, in a lake is given in Fig. 18. It is seen in the hypolimnetic waters that ammonia is high. NO2, a transcient form, is low and NO3 is higher in the middle depth waters. See also the distribution of various forms of nitrogen in a fish pond (Fig. 15).
Ammonia is highly toxic at levels less than 0.1 mg/l even, they cause toxic effects in several fishes. Tropical species can withstand higher toxicities and tropical fish ponds develop ammonia. Levels as high over 3 – 4 mg/l are sustained by carps and tilapias.
The toxicity of ammonia is mainly caused by the unionised ammonia (UIA) (NH3). Mead (1985) observes that NH3 (UIA) is 300 – 400 times more toxic than NH4. The effect of ammonia toxicity is high at higher pH, the proportion of unionised ammonia being higher at higher pH. This aspect has been indeed well studied recently especially owing to its importance in intensive culture systems (Tiews, 1981).
The amount of UIA at different levels of pH can be calculated from the dissociation constant (pKa) values for ammonia, (Smart, 1972) shown bellow:
Using the formula:
Fig. 18. Distribution of ammonia, nitrite, nitrate and redox potential and temperature in a temperate lake. (After Hutchinson, 1957).
The levels of UIA can be estimated:
e.g. UIA at 30°C and pH 7.5
The % UIA in aqueous solution at different pH and temperature has been tabulated by several workers (eg. Wickins, 1979); EIFAC, (1986) recommends total unionized ammonia contents in fresh water, taken from Emerson et al (1975), provided dissolved solid content is neglifible. These values are reproduced in Table V, where % UIA values for 0 – 30°C, at pH 6 – 10 are given. % UIA values for salt waters (8 – 22 %o S) and 23 – 27%) are given in Table VI and VIII respectively. % UIA values for S = 32 – 40%, are given in Table VII. % UIA in fresh water vary differently at different levels of dissolved solids (TDS). Values calculated for values pH values (6.0 – 10.0), for TDS values of range 0 – 1600 mg/l are given in Table IX.