Total Phosphorus

Background Information

Why is phosphorus important to lake health?
Phosphorus is an essential nutrient for algae and aquatic plants (which in turn are food for micro-fauna and larger animals). Therefore phosphorus is an important element of the food chain within a lake. Phosphorus is usually present in very small amounts in a lake and is considered a ‘limiting factor' for algae and plant growth; i.e., even if there are plenty of other nutrients such as nitrates and carbonates, algae and plants will not grow if there is not enough phosphorus. Lake with low nutrient levels are said to have a low trophic level–we could say these lakes are ‘lean' and not very productive; scientists call that trophic level oligotrophic (little food). Lakes with more, but not excessive, amount of nutrients are called mesotrophic (middle food), and lakes overenriched with nutrients are called eutrophic (true or much food).

lake photo

Oligotrophic lakes are generally clear, deep and free of weeds or large algae blooms. Though beautiful, they are low in nutrients and do not support large fish populations, However, oligotrophic lakes often develop a good food chain capable of sustaining a very desirable fishery of large game fish.

Virtual New Zealand

Eutrophic lakes are high in nutrients and support a large biomass (all the plants and animals living in a lake). They are usually either weedy or subject to frequent algae blooms, or both. Eutrophic lakes often support large fish populations, but are also susceptible to oxygen depletion. Small, shallow, eutrophic lakes are especially vulnerable to winterkill which can reduce the number and variety of fish. Rough fish are commonly found in eutrophic lakes. Devoid of oxygen in late summer, the hypolimnions (lower lake layer) of deeper eutrophic lakes limit cold water fish and cause phosphorus cycling from sediments.

Mesotrophic lakes lie between the oligotrophic and eutrophic stages.

What is the trophic level of my lake?
Researchers use various methods to calculate the trophic state of lakes. Common characteristics used to make the determination in lakes without excessive weeds are:

  • total phosphorus concentration (important for algae growth)
  • chlorophyll a concentration (a measure of the amount of algae present)
  • Secchi disk reading (an indicator of water clarity)." (Adapted from Understanding Lake Data, Shaw et al University of Wisconsin-Extension, 1994)

The Carlson's Index shown here includes all three parameters (use a straight edge to match your TP level to the Trophic State).

Lake Sampling Method

(demonstrated on our Lake Sampling Techniques video)

Surface Procedure


  • One 250 ml or 125 ml Total Phosphorus bottle
  • Field data sheet and pencils
  • Cooler and Koolits or ice
  • Zip-loc bag (1 gallon size) to put ice and sample in


  • Rinse the TP bottle and cap 3 times with lake water at the surface. Be sure not to put your fingers inside the bottle or the cap, and to empty the rinse water on the other side of the boat.
  • Then dip the bottle upside down in the water to elbow length, turn it upside right, and wait until there are no more air bubbles coming out of the bottle before removing it from the water.
  • Simultaneously squeeze and cap the bottle.
  • Place sample in cooler with ice, or, if your cooler contains more than just samples, in a zip-loc bag filled with ice, the whole thing inside your cooler.
  • On field sheet , write sample ID and depth taken and write 'TP' in Nutrients column.

Lake Bottom Procedure


  • Wisconsin sampler
  • Calibrated line
  • One 125 ml Total Phosphorus bottle
  • Distilled water
  • Cooler and Koolits or ice
  • Zip-loc bag (1 gallon size)


  • Rinse inside the Wisconsin sampler and outside the 125 ml bottle three times with distilled water.
  • Place the TP bottle inside the Wisconsin sampler, with the long tube inserted in the TP bottle.
  • Lower the Wisconsin sampler to 0.3 m (1 ft) above lake bottom and pull ropes to fill TP bottle.
  • Retrieve sampler back into the boat. Use extreme care not to touch the inside of the TP bottle or its cap (if you do, rinse again and repeat procedure as described above).
  • Place sample in cooler with ice, or, if your cooler contains more than just samples, in a zip-loc bag filled with ice, the whole thing inside your cooler.
  • On field sheet , write sample ID and depth taken and write 'TP' in Nutrients column.

River Sampling Method


  • One 250ml acid washed Total Phosophorus bottle, 250 ml
  • Field data sheet and pencils
  • Cooler
  • Ice
  • Frozen koolit
  • Zip-loc bag (1-gallon size)

Sampling Protocol

Use pre-labeled sample bottle (obtained from program coordinator or laboratory).

Sample should be taken from flowing water. The water must be deeper than the sample bottle and free of surface scum and debris. If the water is not deep enough at your regular sampling site, look for another location nearby which is equally representative of the site but deeper. If there is none, do not collect a sample and indicate on your field sheet that water level is too low. Note that sampling from the streambank, as opposed to wading in, is discouraged, as it can result in non-representative samples.

Carefully wade into the stream, walking upstream to avoid stirring up bottom sediment. Stop and wait for pre-disturbance (from wading in) conditions to return before taking sample. If you are in a canoe, have your partner steady it at the desired location for a sufficient time to take the sample. Take sample in mid-stream, if possible. If not, get as far out from shore as is safe. Caution! Do not wade into swift and deep streams, and be careful at all times.

Always sample upstream of your body, and point the bottle opening upstream. Keep the bottle closed until just ready to sample.

Remove the cap from the bottle with one hand, without touching the inside of the cap or the inside of the bottle. With the other hand, hold the bottle near its base and plunge it, neck downward, below the surface.

Turn bottle until neck points slightly upward and mouth is directed toward the current. If there is no current, create a current artificially by pushing bottle forward horizontally in a direction away from the hand. Be sure not to collect any sediment you may have suspended by walking on the streambed. Also avoid collecting any water from the surface layer of the water as this is often uncharacteristic of the water underneath.

Replace cap on bottle and tighten.

On river field sheet, record sample ID and check the ‘TP’ column


  • Label the bottle with lake name, site, date, and analyses requested (TP).
  • Store in cooler until sample can be frozen.
  • Back home, place sample in freezer.
  • When the sample is frozen, pack it in a styrofoam cooler with frozen Koolits and insulation.
  • Deliver frozen sample to lab. Holding time for frozen samples is 12 months.