Phosphorus (P) is a chemical element found in waters most commonly in one of several forms of phosphate (PO4). The inorganic dissolved form is orthophosphate ion. Orthophosphate (estimated analytically as soluble reactive phosphorus or SRP) is an essential nutrient for photosynthesis; not only do plants convert this form into organic cell matter, but P is also a crucial component of the energy-transfer molecule ATP. Organic P includes particulate forms such as living or dead cell matter and sewage solids, and several forms derived from them. Mineralization of organic matter by bacteria and fungi converts organic P into orthophosphate. Total P (TP) measures the sum of all forms. Settling of solids and of algal and bacterial cell matter, as well as uptake by rooted plants, removes P from the water. Sediment P can re-enter the water column as a result of chemical, biological, and physical processes. TP measurements show the maximum potential for algal growth and can be used to classify the trophic status of a lake. Orthophosphorus measurements show the amount of P immediately available for plant life. Concentrations of TP less than 0.02 milligram per liter (part per million) are generally desirable in lakes.

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