Phosphate (P) is a macronutrient essential for the survival and growth of organisms in aquatic systems. High concentrations of phosphate are found in agricultural fertilizers, manure and organic wastes in sewage effluent. In freshwater phosphate is generally considered the main 'limiting nutrient' and thus excessive inputs often leads to eutrophication. Therefore, the Water Framework Directive stipulates that phosphate inputs, from both diffuse and point sources, need to be tightly controlled.

Total Phosphate is most often found in natural waters. Excess phosphorus in the water causes extensive algal growth otherwise known as blooms, which causes decreased oxygen levels in the water. This results in plants dying off faster than they can decompose. This dead plant builds up together with sediment until the water becomes shallower making it inhabitable for plants and aquatic wildlife. Bodies of water are being aged at a much faster rate than geological forces can create new ones. In these circumstances one would expect to find scum on the water along with a fishy smell as well as low dissolved oxygen levels. Don't expect a high phosphate reading in the water if the algae is already blooming as it will no longer be in the water, but in the algae.

There are many different types of phosphate which can be found in the water, but when monitoring a total phosphate-phosphorus reading is all that is needed for water quality. For uncontaminated waters, the water sample level should be between 0.01-0.03 mg/L.