The nutrients nitrogen and phosphorus, essential to plant growth, move through the environment in cycles. Fertilizers and animal waste both contain nitrogen and phosphorus. When these elements are applied to the land in excess of plant needs, they can aversely affect water quality.
Nitrogen is present in aquatic ecosystems in several forms. The great reservoir of nitrogen, the air, is available to aquatic organisms only through the activity of a relatively few species of bacteria and blue-green algae that are capable of fixing this source into a form available to the remainder of the biota. The nitrogen cycle is a double cycle, that is, one cycle of oxidation and reduction of nitrogen by plants, animals and decomposers and the larger cycle, which is coupled to the smaller by the action of nitrogen-fixing organisms and denitrifying bacteria. Nitrogen is occasionally limiting to plant production and is probably second only to phosphorus in this role for most aquatic ecosystems.
In water, the forms of nitrogen are:
- Nitrate, which generally occurs in trace quantities in surface water. It is an essential nutrient for photosynthesis and in some cases has been identified as the growth-limiting nutrient.
- Nitrite, which is an intermediate form of nitrogen.
- Ammonium and ammonia are found in equilibrium in water. The equilibrium is governed by pH; above pH 9.5, ammonia may be predominant. Most waters are below 9.5.
- Organic nitrogen
All these forms of nitrogen are biochemically convertible and are components of the nitrogen cycle.
Total inorganic nitrogen is the sum of ammonium, nitrate and nitrite concentrations.
Phosphorus occurs in natural waters and in wastewaters almost solely as phosphates. There are several classifications of phosphates:
- Condensed phosphates
- Organically bound phosphates
They occur in solution, in detritus or in bodies of aquatic organisms.
The forms of phosphate come from a variety of sources. Small amounts are added to some water supplies during treatment. Phosphates are used extensively in treatment of boiler waters. They are applied to agricultural or residential cultivated land as fertilizers and are carried into surface waters with storm run off and melting snow. Organic phosphates are formed primarily by biological processes. They are contributed to sewage by body waste and food residues.
Phosphorus is the least abundant of the major nutritional and structural components of biotic organisms, making it the primary nutrient limiting biological productivity in aquatic systems. The cycling of phosphorus is very complex, with the majority being bound up in the particulate phase as living biota (i.e. algae and bacteria). Labile compounds are excreted by the algae and bacteria. The compounds, algae, and bacteria combine with each other to form a snot-like (colloidal) material. Some of this colloidal material is lost from the productive zone by sedimentation. Additionally, another portion is lost through hydrolyzation to soluble orthophosphate. Orthophosphate is quickly assimilated by macrophytes and algae, and represents only a small fraction of the phosphorus in the productive zone at any given time.
Colloidal and particulate phosphorus are replaced by regeneration of solubilized phosphorus from decomposition, precipitation, and lake/stream influents. The primary source of natural phosphorus is igneous rock that contains apatite. Phosphorus is found in the Earth's rocks primarily as the ion orthophosphate (PO
43-), which is the most significant form of inorganic phosphorus in aquatic systems. Reduced, gaseous phosphine (PH
3) oxidizes very quickly and is present only briefly in the water. It has been suggested that the will-o-the-wisps frequently observed in marshy areas could be a result of oxidizing phosphine which burns on contact with the air.
The best method to quantify phosphorus in aquatic systems is by measuring total phosphorus, because phosphorus is so rapidly transported between the biota and inorganic state. Total phosphorus includes both suspended and dissolved values. The phosphorus analysis consists of two steps:
- The conversion of the phosphorus present in the sample to orthophosphate.
- The colorimetric determination of dissolved orthophosphate.
Filtration through a 0.45 mm pore membrane filter further separates dissolved (or soluble) forms of phosphorus from suspended (or particulate) forms.