Total phosphorus

Total phosphorus is a measure of all the various forms of phosphorus that are found in a water sample. Phosphorus is an element that, in its different forms, stimulates the growth of aquatic plants and algae in waterbodies. The chemical symbol for the element phosphorus is P and the symbol for total phosphorus is TP. Some phosphorus compounds are necessary nutrients for the growth of aquatic plants and algae, and some are found naturally in many types of rocks. Mines in Florida and throughout the world provide phosphorus for numerous agricultural and industrial uses.

The Role of Phosphorus in Waterbodies
Like nitrogen, phosphorus is an essential nutrient for the growth of all plants, including aquatic plants and algae. Phosphorus in waterbodies takes several forms, and the way it changes from one form to another (called cycling ) is complex. Because phosphorus changes form so rapidly, many aquatic scientists generally assess its availability by measuring the concentration of total phosphorus rather than the concentration of any single form.

In some waterbodies, phosphorus may be at low levels that limit further growth of aquatic plants and/or algae. In this case, scientists say phosphorus is the limiting nutrient. For example, in waterbodies having TP concentrations less than 10 micrograms per liter, waters will be nutrient poor and will not support large quantities of algae and aquatic plants.

There are many ways in which phosphorus compounds enter waterbodies. The more common ones are described below:

Some areas of Florida and other parts of the world have extensive phosphate deposits. In these areas, rivers and water seeping or flowing underground can become phosphorus enriched and may carry significant amounts of phosphorus into waterbodies.
Sometimes phosphorus is added intentionally to waterbodies to increase fish production by fertilizing aquatic macrophyte and algal growth.
Phosphorus can enter waterbodies inadvertently as a result of human activities like landscape fertilization, crop fertilization, wastewater disposal, and stormwater run-off from residential developments, roads, and commercial areas.

In Florida
Waterbodies in the Florida LAKEWATCH database have total phosphorus concentrations ranging from less than 1 to over 1000 micrograms/L. Using these average concentrations of total phosphorus from this same database, Florida lakes were found to be distributed into the four trophic states as follows:

Approximately 42% of these lakes (those with TP values less than 15 micrograms/L) would be classified as oligotrophic;
About 20% of these lakes (those with TP values between 15 and 24 micrograms/L) would be classified as mesotrophic;
30% of these lakes (those with TP values between 25 and 100 micrograms/L) would be classified as eutrophic;
nearly 8% of these lakes (those with TP values greater than 100 micrograms/L) would be classified as hypereutrophic.

(This distribution of trophic state is based solely on total phosphorus values without utilizing information on total nitrogen, chlorophyll, water clarity, or aquatic macrophyte abundance.)

The location of a waterbody has a strong influence on its total phosphorus concentration. For example, lakes in the New Hope Ridge/Greenhead Slope lake region in northwestern Florida (Washington, Bay, Calhoun, and Jackson counties) tend to have total phosphorus values below 5 micrograms/L, while lakes in the Lakeland/Bone Valley Upland lake region in central Florida (Polk and Hillsborough counties) tend to have values above 120 micrograms/L.

Health Concerns
There is no known level of total phosphorus in waterbodies that poses a direct threat to human health.

A bum rap?
Since waterbodies with low concentrations of total phosphorus will have relatively clear water, the public may think their water quality is better than waterbodies with higher TP. It's actually a misconception that clearer water is intrinsically better than water of a less clear quality. Unfortunately, the association of clear water with low phosphorus levels have given the public the mistaken notion that phosphorus is a pollutant.

See also:
Water clarity
Water quality
Limiting nutrient
Phosphorus
Total nitrogen
Trophic state and each of its categories:
Oligotrophic, Mesotrophic, Eutrophic, and Hypereutrophic

The information on this page was derived in part from the following publication of
Florida Lakewatch of the University of Florida Department of Fisheries and Aquatic Sciences.
This booklet may be downloaded in its entirety as a PDF file by clicking here.
Scroll down till you see the Circular 101 icon.

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A collaboration of the Center for Aquatic and Invasive Plants, University of Florida, and the Invasive Plant Management Section of the Florida Fish and Wildlife Conservation Commission.