Phosphorus Cycle in Water
Phosphorus is often considered the most critical single factor in the maintenance of bio-geo-chemical cycles in ecological reactions.
This extreme importance comes from the fact that phosphorus is virtually necessary in the operation of energy transfer systems of every cell.
Extensive studies have been made of the effects of phosphorus in lakes because of nuisance of odors caused by algal blooms.
In nature, it normally occurs in very small amounts. It is an essential nutrient for plant and animal growth and like nitrogen passes through cycles of decomposition and photosynthesis. It combines directly with oxygen, sulfur, hydrogen, the halides and many metals.
The normally observed low concentrations of phosphorus mean that there is a deficiency of the nutrient. This in turn could lead to inhibition of phytoplankton increases resulting ultimately in decreased productivity of the aquatic system.
The concentration of Total Exchangeable Phosphorus in natural waters is determined primarily by:
(1) Basin morphometry as it relates to volume and dilution and to stratification or water movements
(2) Chemical composition of the geological formations of the area as they contribute dissolved phosphate
(3) Drainage area features in relation to introduction of organic matter, and
(4) Organic metabolism within the body of water and the rate at which phosphorus is lost to the sediments.
With respect to geologic influence, waters in local regions of highly phosphetic substrate contain considerable quantities of the ion.
Ground waters and flowing surface waters are typically richer in inorganic phosphate than are surface waters of open lakes, due mainly to less biological demand in proportion to water volume.
Reports that more than 0.2 mg/L of phosphorus in ground or surface waters indicates that some phosphorus of sewage origin is present.
In most open lakes, assimilation by phytoplankton and bacteria serves to reduce the inorganic phosphate content. In closed basins of arid regions, evaporation may result in very high concentrations of total phosphate.
The most common rock mineral in which phosphorus is a major component is apatite, a general name for mineral species that are principally calcium orthophosphate. These minerals are widespread both in igneous rocks and in marine sediments.
Phosphate is also made available for solution in water from several kinds of cultural applications. Since the element is essential in metabolism, it is always present in animal metabolic wastes.
These pollution sources probably are the most important causes of high concentrations of phosphorus in surface water.
Phosphorus, with a valence of +5, can exist in three types of phosphoric acids;
HP03 – metaphosphoric acid
H4P2O7; pyrophosphoric acid
H3PO4; orthophosphoric acid.
In water solution, the meta and pyro forms tend to change to the more stable ortho condition.
Orthophosphoric acid hereinafter referred to as phosphoric acid is of interest in water because it disassociates in three steps: with corresponding releases of hydrogen ions.
The orthophosphate ion (P04+3) is the final disassociation product of phosphoric acid (H3PO4).
The pH of water determines to a great extent the nature of the phosphate compound. This concept of disassociation is important for understanding what occurs when soluble phosphates are added to water.
It also illustrates the effect of phosphates in adding to the buffering capacity of water. For instance, in the process of titrating alkalinity, all the HPO4-2 will be converted to H2PO4 and that fraction would appear in the alkalinity value as an equivalent quantity of bicarbonate.
In water, phosphate may enter into combination with a number of ions. This happens most conspicuously with iron and calcium.
Under about neutral and moderately alkaline conditions, calcium phosphate is probably prevalent while extremely high pH results in the formation of sodium phosphate.
In acid waters, phosphate attraction swings toward iron to form ferric phosphate. Other probable species of phosphorus would appear to be phosphate ions, complexes with metal ions and colloidal particulate material.
The seasonal distribution of phosphorus is variable. The distribution is determined to a great degree by drainage basin chemical composition of the surrounding watershed, land use, and in the particular lakes and the annual cycles of mixing.
In relatively open basins, receiving considerable influx of surface water, phosphorus concentration is often regulated by stream discharge, particularly following high rainfall or spring snowmelt.
In lakes more self-contained, seasonal variations in total content and vertical distribution of phosphorus are trophic lakes. During summer stratification the phosphorus content in the hypolinmeon increases significantly following oxygen depletion.
The factors involved in this increase show that this phosphorus is apparently released from the iron (ferric) phosphorus complex that is insoluble in the presence of oxygen. In the absence of oxygen, the ferric compound is reduced to a soluble ferrous form, thus liberating phosphorus.
With lake overturn and the reintroduction of oxygen, the insoluble ferric phosphate is again formed and distributed throughout the lake.
Another factor is that in the presence of oxygen, phosphorus is absorbed on basis iron compounds in the oxidized microzone of the mud. Removal of oxygen brings about a reaction in which the ferric ion is reduced and the phosphorus is released into the water. While it is possible that both mechanisms operate, the presence or absence of oxygen is a critical factor.
Soluble inorganic phosphorus in the upper waters of eutrophic lakes is usually low throughout the year becoming depleted at times during high aquatic vegetative growth during the summer. The concentration of total phosphorus, mainly the organic form, may increase in late summer due to decomposition.
The discharge of excessive amounts of phosphorus to streams or lakes may result in an overabundant growth of algae with concomitant odors and detriment to fish. Phosphates seldom exhibit toxic effects upon fish and other aquatic life. It may be beneficial to fish culture by increasing algae growth and zooplankton population.
Finally, some of the Organic Pesticides which are used extensively may exhibit selective toxicity to many forms of aquatic life.
Both phosphorus and nitrogen are essential nutrients for plant growth.
Like nitrogen, a wide range of oxidation states is possible for phosphorus, but there is no strong resemblance in aqueous chemical behavior between the two elements.
Concentrations of phosphate normally present in natural waters are far less than those of nitrate. It is safe to state that all bodies of water that support some plant populations contain a quantity of phosphate even though it is small.
In some cases, it is less than 0.001 mg/L. Usually, the mean total phosphorus content for most lakes ranges from about 0.010 to 0.030 mg/L. It usually suffices to report total phosphorus and phosphate.
End of Phosphorus Cycle in WaterMore about Primary Nutrients and Eutrophication…
Significance of the Primary Nutrients