The following is from the U.S. EPA website on water quality monitoring
Why is phosphorus important?
Both phosphorus and nitrogen are essential nutrients for the plants and animals that make up the aquatic food web. Since phosphorus is the nutrient in short supply in most fresh waters, even a modest increase in phosphorus can, under the right conditions, set off a whole chain of undesirable events in a stream including accelerated plant growth, algae blooms, low dissolved oxygen, and the death of certain fish, invertebrates, and other aquatic animals.
There are many sources of phosphorus, both natural and human. These include soil and rocks, wastewater treatment plants, runoff from fertilized lawns and cropland, failing septic systems, runoff from animal manure storage areas, disturbed land areas, drained wetlands, water treatment, and commercial cleaning preparations.
Forms of phosphorus
Phosphorus has a complicated story. Pure, "elemental" phosphorus (P) is rare. In nature, phosphorus usually exists as part of a phosphate molecule (PO4). Phosphorus in aquatic systems occurs as organic phosphate and inorganic phosphate. Organic phosphate consists of a phosphate molecule associated with a carbon-based molecule, as in plant or animal tissue. Phosphate that is not associated with organic material is inorganic. Inorganic phosphorus is the form required by plants. Animals can use either organic or inorganic phosphate.
Both organic and inorganic phosphorus can either be dissolved in the water or suspended (attached to particles in the water column).
The phosphorus cycle
Phosphorus cycles through the environment, changing form as it does so (Fig. 5.12). Aquatic plants take in dissolved inorganic phosphorus and convert it to organic phosphorus as it becomes part of their tissues. Animals get the organic phosphorus they need by eating either aquatic plants, other animals, or decomposing plant and animal material.
As plants and animals excrete wastes or die, the organic phosphorus they contain sinks to the bottom, where bacterial decomposition converts it back to inorganic phosphorus, both dissolved and attached to particles. This inorganic phosphorus gets back into the water column when the bottom is stirred up by animals, human activity, chemical interactions, or water currents. Then it is taken up by plants and the cycle begins again.
In a stream system, the phosphorus cycle tends to move phosphorus downstream as the current carries decomposing plant and animal tissue and dissolved phosphorus. It becomes stationary only when it is taken up by plants or is bound to particles that settle to the bottom of pools.
In the field of water quality chemistry, phosphorus is described using several terms. Some of these terms are chemistry based (referring to chemically based compounds), and others are methods-based (they describe what is measured by a particular method).
The term "orthophosphate" is a chemistry-based term that refers to the phosphate molecule all by itself. "Reactive phosphorus" is a corresponding method-based term that describes what you are actually measuring when you perform the test for orthophosphate. Because the lab procedure isn't quite perfect, you get mostly orthophosphate but you also get a small fraction of some other forms.
More complex inorganic phosphate compounds are referred to as "condensed phosphates" or "polyphosphates." The method-based term for these forms is "acid hydrolyzable."
Monitoring phosphorus is challenging because it involves measuring very low concentrations down to 0.01 milligram per liter (mg/L) or even lower. Even such very low concentrations of phosphorus can have a dramatic impact on streams. Less sensitive methods should be used only to identify serious problem areas.
While there are many tests for phosphorus, only four are likely to be performed by volunteer monitors.
All these tests have one thing in common they all depend on measuring orthophosphate. The total orthophosphate test measures the orthophosphate that is already present in the sample. The others measure that which is already present and that which is formed when the other forms of phosphorus are converted to orthophosphate by digestion.
The chart above is from a Bridgewater State College nutrient study on the upper Taunton River in 2000. This chart shows phosphorus and nitrogen levels increase dramatically downstream of the outfall of the Brockton Sewer Plant. The black center is nitrogen the lighter outside is phosphorus. The last column (ASTP) is the sample site above the Brockton Sewer Plant. The other three sample sights are below the sewer plant.
The chart below is from the same 2000 study, it shows the nutrient loadings on the Town River and Nemasket both of which receive water from sewer treatment plants (Town River, Bridgewater Treatment Plant, Nemasket, Middleboro Treatment Plant) . Between the Town and Nemasket is the loadings for the Matfield which is downstream of the Brockton Sewer Plant. It also shows loadings on the Winnetuxet which does not receive sewer water. The last column is nutrient loadings at Titicut St Bridgewater on the Taunton River.
The chart below displays the criteria set forth by the EPA and DEP for determining whether or not a waterbody is impaired. In the case of Total Phosphorus the standard as written below refers to micro grams per liter (23.75 ug/L). In table 6g and 6o of the sampling results phosphorus is measured in milligrams per liter mg/L. To convert 23.75 micrograms per liter to milligrams per liter you divide 23.75 micrograms by the conversion factor of 1000. This gives you the standard in milligrams per liter (mg/L) which would be 0.02375 mg/L or rounded off 0.02 mg/L.
As you can see in the tables below Total Phosphorus was well above the EPA suggested standard of 0.02 mg/l on all samples taken on the Matfield River and the Salisbury Plain River below the Brockton Sewer Plant. As was Nitrogen.
The table below shows sampling results from the Salisbury Plain River. Sample site SPR1 is below the sewer plant at Belmont St West Bridgewater. Site's SPR2 and SPR3 are above the plant in the City of Brockton. Note the higher levels of both Phosphorus and Nitrogen below the plant at site SPR1.
Below are the sampling results from the Satucket River at Rt 106 East Bridgewater for comparison. Satucket is a tributary to the Matfield which does not receive effluent from sewer plants.