Effects

• Since phosphorus is often scarce in freshwater ecosystems, it's typically a limiting nutrient, meaning that it limits the amount of life the system can sustain. When humans add phosphate-rich sewage or agricultural runoff, algae growth may no longer be limited by the scarcity of phosphorus in its environment and may grow out of control.
  
  

Identification

• Phosphate in water is often identified by adding a reagent, a chemical that will react with the phosphate ions so that the solution changes color. One common reagent-based test involves two chemicals called ammonium molybdate and ascorbic acid (better known as vitamin C); other chemicals are also used but are basically variations on the same theme.
  
  

Function

• Once the molybdate reagent is added to the water sample, it will react with any phosphate ions in the water to form a chemical that in turn reacts with the ascorbic acid. The resulting chemical complex tints the water sample blue.
  
  

Features

• While the color change denotes the presence of phosphate ions, determining precisely what levels of phosphate are present requires a technique called spectrophotometry. Researchers pass a beam of light with a specific wavelength through the sample; the more light absorbed, the more of the phosphate is present.
  
  

Significance

• In order to control algae growth, the EPA recommends that phosphate levels not exceed .05 milligrams per liter for streams discharging into lakes or reservoirs, .1 milligrams per liter for lakes and reservoirs, and .1 milligrams per liter for other streams and rivers.