A primary determinant of algal growth is phosphorus. Orthophosphate (or just phosphate) is the form that is readily used by algae and is soluble in water. In an unpolluted stream, phopshate is usually found in low to undetectible levels because it is easily assimilated by algae or bacteria and is usually in short supply in nature (5 to 50 µg/L). When the stream becomes polluted with an external source of phosphate (e.g. from sewage, or runoff from farms or lawns), it can be found in high concentrations. Therefore, a measurement of greater than 50 µg/L of phosphate may be an indicator of a polluted stream. One of the most common methods to measure orthophosphate in water is to let it react with chemicals containing molybdate to form a blue complex—the intensity of the blue colour is directly proportional to the concentration of phosphate in the sample. The method is said to measure Soluble Reactive Phosphorus (SRP), since the phosphate is soluble and reactive to molybdenum.
For research purposes, we measure the absorbance of a sample using a colorimeter or a spectrophotometer, and then use a standard curve that relates the absorbance to a known concentration of orthophosphate. Using this technique, we can measure phsophate accurately to less than 5 µg/L. We have also used the less expensive Nix sensor to record the red colour of the sample, and relate that value to the concentration of a water sample. Although this is less accurate, it is acceptable for screening sites that have unacceptably high concentrations of phosphate (i.e >25 µg/L).