Phosphorus is a nutrient required by all organisms for the basic processes of life. It is a natural element found in rocks, soils and organic activity.
Rainfall can cause varying amounts of phosphates to wash from the soil into waterways. Phosphates stimulate the growth of plankton and aquatic plants which provide food for fish. This may cause an increase in the fish population and improve the quality of the water system. However, if there is too much, the algae and water plants grow wildly, choke up the waterway and use up large amounts of oxygen. This condition is known as EUTROPHICATION or over-fertilization of receiving waters. When these algae die, bacteria decompose the algae and use up oxygen, leading to low levels of dissolved oxygen and death of aquatic life. Phosphates are not toxic to people or animals unless they are present in very high levels, in which case it leads to digestive problems.
Forms of Phosphorus
Phosphorus has a rather complex story. Pure phosphorus is rare. In nature, phosphorus usually exists as part of a phosphate molecule (PO4). In aquatic systems, it occurs as organic and inorganic phosphate. Organic phosphate consists of a phosphate molecule associated with a carbon-based molecule. Phosphate that is not associated with organic material is inorganic. Inorganic phosphorus is the form required by plants. Both types can be dissolved in the water or suspended. Phosphorus cycles through the environment, changing form as it does so (illustrated in diagram above). Aquatic plants take in dissolved inorganic phosphorus and convert it to organic phosphorus as it becomes part of their tissues. Animals get the organic phosphorus they need by eating either aquatic plants, other animals, or decomposing plant and animal material.
As plants and animals excrete wastes or die, the organic phosphorus they contain sinks to the bottom, where bacterial decomposition converts it back to inorganic phosphorus, both dissolved and attached to particles. This inorganic phosphorus gets back into the water column when the bottom is stirred up by animals, human activity, chemical interactions, or water currents. Then it is taken up by plants and the cycle begins again.
In a stream system, the phosphorus cycle tends to move phosphorus downstream as the current carries decomposing plant and animal tissue and dissolved phosphorus. It becomes stationary only when it is taken up by plants or is bound to particles that settle to the bottom of pools.
In the field of water quality chemistry, phosphorus is described using several terms. Some of these terms are chemistry based (referring to chemically based compounds), and others are methods-based (they describe what is measured by a particular method). They can be measured using a Comparator or a Photometer.
The term “orthophosphate” is a chemistry-based term that refers to the phosphate molecule all by itself. “Reactive phosphorus” is a corresponding method-based term that describes what you are actually measuring when you perform the test for orthophosphate. Because the lab procedure isn’t quite perfect, you get mostly orthophosphate but you also get a small fraction of some other forms.
More complex inorganic phosphate compounds are referred to as “condensed phosphates” or “polyphosphates.” The method-based term for these forms is “acid hydrolyzable.”
Factors affecting Phosphorus Concentrations
- Domestic / industrial effluent – found in sewage in the form of organic phosphates. Ortho- and polyphosphates are major constituents of many cleaning materials.
- Fertilizers that are carried into surface waters.
- Animal Waste – run-off from farms, dairies.
- Soil erosion – from new development, forest fires
- Water treatment
Criteria in South Africa:
Phosphate (Soluble Ortho-Phosphate) < 1.0 mg / litre for discharge of effluent water to rivers – for discharge of effluent water to other areas