Nitrogen in the form of dissolved nitrate is a major nutrient required by vegetation. The very high importance of nitrogen in aquatic ecosystems rests upon its role in the synthesis and maintenance of protein that is along with carbohydrates and fats, a major constituent of living substance.
Derived originally from the atmosphere, nitrogen enters a complex cycle involving plants and animals in several forms of the element. Certain species of bacteria in soil, especially those living on roots of legumes, and the blue-green algae in the water and other microbiota occurring in water can extract nitrogen from air and convert it to nitrate.
Nitrogenous compounds in natural waters may be derived from outside sources (allochthonous) or may be fixed within the body of water (autochthonous).
The allochthonous forms or sources of nitrogen are:
1) precipitation carrying compounds in the form of nitrate and ammonia
2) surface runoff which contains terrestrial compounds of nitrogen including nitrate from the soil. Through decomposition of plants, some nitrate is lost from the land in drainage and runoff which later appears in river water.
3) inflow of ground water from springs and seepages which can be particularly influenced by agricultural practices.
Additionally, high nitrate concentrations in shallow ground water in some agricultural areas have been attributed to the leaching from livestock corrals by rainfall. This happens because farm animals produce considerable amounts of nitrogenous organic waste. This concentrates in places where large numbers of animals are confined.
A example of this from Colorado, where substantial contributions of both nitrogen and phosphorus reached the ground water beneath irrigated fields. Particularly large contributions were associated with feed lots.
In groundwater, nitrate usually appears to be the only form of nitrogen of significance, although nitrite bearing water may occur in reducing environments.
Autochthonous nitrogen compounds in water result from fixation processes carried on by certain bacteria and algae. The oxidation and reduction of aqueous nitrogen species are closely tied to biological activity. Both paths followed and the end products of such reactions depend very strongly on kinds and numbers of biota present.
Influence of biota on the reactions of the element and the extensive departure from equilibrium conditions that may be observed produce the problems of variation in studying the nitrogen cycle. The extent to which biologically fixed nitrogen contributes to the total supply is being studied. For instance, studies to determine how much of the nitrogen content of a given lake is fixed within the lake by its own organisms and how much is delivered to the lake from the outside are ongoing. Once in the aquatic system, a great proportion of these nitrogen compounds is caught up in a cycle of biological assimilation and decomposition and inorganic processes in the economy of the ecosystem.
The nitrogen cycle shown in Figure 18.104.22.168.1 illustrates the movement of nitrogen through its cycle of reactions. In the nitrogen cycle, proteins are broken down yielding ammonia (NH3) compounds. This is the course of the metabolic processes of all animals including the activities of the heterotrophic bacteria, filamentatious fungae and actenomycetes.
This process is called ammonification. Some of the ammonia is oxidized to form nitrites (N02) and nitrates (NO3) through the action of autotrophic bacteria. This process is called nitrification. Other types of bacteria act on ammonia in the process of denitrification by which nitrogen (N2) is liberated into the atmosphere. Nitrogen is removed from the air by nitrogen fixing bacteria which live either freely in the soil or as symbiotic in root nodules of legumes and other plants. Some blue green algae, fungae and yeast also fix nitrogen.
Nitrates and perhaps also the simpler nitrogen compounds are absorbed and used by the aquatic plants for the synthesis of amino acids and proteins.
Ammonia compounds, nitrates and other substances are added to the soil in small amounts by rainfall. These sources of nitrogen compounds are volcanic eruptions, terrestrial decomposition and atmospheric nitrogen fixation by lightning.
Depending upon the efficiency of the aquatic community, certain quantities of these and other compounds will be lost. Lake and stream communities can loose nutrient materials through their outflows, by incorporation of the substances into sediments and through processes of denitrification.
There are at least two possible sources for elemental or uncombined nitrogen in natural waters. One reservoir, and very likely the most important, is the atmosphere. The second reservoir of uncombined nitrogen, is that produced by bacteria nitrification of ammonia.
This uncombined nitrogen N2 is rather inert. The only organisms capable of using it are certain microorganisms such as nitrogen fixing bacteria and algae. The solubility of molecular nitrogen and fresh waters is related to temperature and pressure. The temperature relationship is an inverse one.
Very little study has been made of nitrogen in lakes, but it has been established that supersaturation can exist under certain pressure conditions at the air-water interface. This is the situation that exists with the spillways on some reservoirs that cause supersaturation of nitrogen. This supersaturation of nitrogen can produce concentrations of 130 to 140% nitrogen.
Furthermore, during summer stratification of the lake, the vertical distribution of uncombined nitrogen follows inversely that of temperature mixing during spring and fall overturn distributes the nitrogen throughout the lake.
The synthesis of inorganic substances into plant and animal tissues and the metabolic processes of protoplasm produce various compounds containing nitrogen. These inorganic nitrogen compounds include, for example, nitrogen in combination with carbon and other elements, animal and plant proteins, urea and ureic acid as animal metabolic waste. Of the total content of soluble nitrogen in filtered or centrifuged surface waters of lakes, it is probable that 50% or more is in the form of organic nitrogen. Some 60 to 80% of this organic nitrogen is composed of amino compounds such as free amino acids, polypeptids and proteins. These substances are contained primarily in living plants and animals. The presence of the compounds in water doubtless reflects metabolic processes of living organisms as well as decomposition of dead bodies.
With respect to the formation by living organisms, it has been established that many blue-green algae secrete extracellular nitrogenous materials including polypeptides, amides and amino acids. In the algal species tested, the plants were not able to utilize their own excreted products as nitrogen sources.
The concentration of organic nitrogen may be expected to vary seasonally. There is little evidence that much of the organic nitrogen is available as nutrient for plants and animals. A measure of the total organic nitrogen content is, however, a valuable indication of the productivity of the body of water. Certainly, most of the substance will ultimately be transferred into states that can enter into production of living matter.
In addition to nitrogen occurrence in the uncombined state and in organic compounds, nitrogen is also present in natural waters in the form of inorganic nitrogen compounds, such as ammonia nitrite and nitrate. In most fresh waters, the concentrations of these inorganic compounds are relatively low, but nevertheless very important in determining the productivity of a given community. All of the inorganic forms can be used by most green plants and particularly by various algae in their role of primary producers of energy containing mass that can enter the aquatic food chain.
Nitrogen that is locked up in organic compounds is returned to the environment through decomposition and, to a lesser extent, by excretions of nitrogenous wastes of animals. The end product of the first stage of oxidation and degradation of animal and plant proteins is mainly free ammonia. Lesser amounts of ammonia compounds such as the base ammonium hydroxide (NH40H) and the salt ammonium carbonate (NH4)2CO3 are also released.
The agents of the decomposition process are microbial organisms, such as certain fungi and bacteria. The free ammonia content of natural waters is derived in part from this bacterial decomposition of proteins and in part from the deamination (the removal of an amino group (NH2) from an amino acid) also involving bacteria.
One of the prime reasons nitrogen is so biologically important is that in elemental reactions, nitrogen can occur at all oxidation states ranging from valence of -3 to valence of +5. Although not all these states are represented by species that are important in natural water, they are of biological significance.
In ammonia, the oxidation state is nitrogen -3 and aqueous forms include NH4+ and NH40H (aq). Amino or organic nitrogen forms such as proteins or amino acids may occur in water that contains organic waste. Urea (NH2CONH2) is a common constituent of nitrogenous wastes containing the NH2 or amino group.
Hydroxyl amine (NH2OH) contains nitrogen in the -1 oxidation state. On oxidation, the reduced nitrogen species may be converted to N2 gas or to nitrite N02- in which nitrogen is in the +3 state, and finally to nitrate N03- where nitrogen is in the +5 form.
In addition to these species, nitrogen forms certain complex inorganic ions that are probably not significant in natural systems, but they may enter water supplies through industrial disposal. An important example is the cyanide ion. The oxidation state of nitrogen in this ion is uncertain because it depends on the arbitrary assignment of the oxidation number to carbon, but in any event, the ion is stable over a considerable range of chemical conditions and forms strong complexes with many metal cations.
The ranges of nitrogen concentration in natural waters depend upon the oxidative state of the nitrogen within each compound. In unpolluted waters, ammonia and ammonium compounds occur in relatively small quantities, usually on the order of 1 mg/L or less because of abundant supply of oxygen. With the uptake of oxygen, however, the concentration of ammonium may increase in extreme cases to 12 mg/L or more.
The relationships between ammonia and its basic compound, the undisassociated ammonium hydroxide seem to rest upon the pH of the water. At a temperature of 18° C. and pH 6, the proportion of ammonia+ to ammonium hydroxide is apparently 3,000 to 1, while at pH 8, the proportion is nearer to 30 to 1. Under highly alkaline conditions, the concentration of ammonium hydroxide may reach toxic levels.
Free ammonia concentrations over 2.5 mg/L in neutral or alkaline waters are apt to be harmful to many freshwater species. The common ammonium salt of fresh waters, ammonium carbonate, is usually present in small amounts, but in concentrations of about 20 mg/L or more under alkaline conditions. It is also toxic to certain animals.
Nitrate nitrogen usually occurs in relatively small concentrations in unpolluted fresh waters. The world nitrate average is around .3 mg/L. This form of nitrogen may be entirely absent at times. Under normal conditions, however, the amount of dissolved nitrate in a body of water at any given time is determined by metabolic processes, i.e. production and decomposition of organic matter. Nitrate is also contributed to the ecosystem as a byproduct of bacterial nitrification.
Seasonal fluctuations of nitrate, nitrogen and other forms of nitrogen are observed in many lakes. The annual cycles may be expected to vary as to time of maximum nitrogen production depending upon latitude, basin morphology and chemical nature of the drainage area geologic substrate and productivity.
Vertical distribution of nitrate is apparently related to lake productivity.
In some oligotrophic lakes that have been studied, there is little evidence of nitrate stratification. In others, the nitrate content of the upper zone decreases during summer from the amount present at spring overturn with little change in the deep waters.
In eutrophic lakes, the nitrate concentration is typically decreased in the upper zones by plankton utilization and in the deepest regions by bacterial reduction. The result is a high content near the lower limits of the trophogenic zone.
For algae, crustacean and other fish food organisms, total concentration of nitrogen is not as important as the form in which it exists. Organic nitrogen, amino acids and ammonia may inhibit biological growths whereas nitrates stimulate phytoplankton. Some green plants, again apparently do not need to depend upon complete nitrification, but can utilize nitrogen from ammonia or nitrates. The critical concentration of nitrogen below which algal growths were not troublesome is about 0.3 mg/L provided that phosphorus was kept below 0.015 mg/L.
Fish production is highest in ponds and streams containing the most organic nitrogen. For some algae, the optimum nitrogen to phosphorus ratio appeared to be about 30 to 1. For other algae, ratios of 15 to 18 to 1 were evident. The presence of 0.01 mg/L of phosphorus and .3 mg/L of inorganic nitrogen in ponds or lakes at the time of spring overturn would probably foster the production of nuisance algal blooms.
Some researchers suggest that in many instances, nitrogen rather than phosphorus may be the limiting element in the growth of algae. Others, however, point out that enormous growth of plants in streams and lakes does not occur if the nitrate as nitrogen is kept below .3 mg/L and that total nitrogen as in is below 0.6 mg/L.