Total Nitrogen/Total Phosphorus and Chlorophyll a

Expanded Monitoring for Total Nitrogen/Total Phosphorus (TN/TP) and Chlorophyll a Concentration

Overview of Process

To measure total nitrogen/total phosphorus (TN/TP) and chlorophyll a concentration, you will take an integrated water sample from the lake every month throughout the growing season.  The water sample is “integrated” because it represents a sample of the water column from the surface to a depth of 6 feet.  The integrated sample allows us to examine the water column where phytoplankton lives (i.e. the part of the water column with enough sunlight for photosynthesis to occur).  The sample is partitioned in a sample bottle and a specific volume of water is filtered.  All of the algae (and other suspended particles) in the water will collect on the filter, which is then analyzed in the laboratory for chlorophyll a concentration.

Sampling checklist

Before going out on the lake to make your Secchi disk reading, make sure that you have everything you need and the weather conditions are okay for sampling! Please confirm everything on this checklist:


  • Sunny / Partly Sunny / Partly Cloudy
  • Winds calm to breezy (NO WHITECAPS!!)

Date and time of day:

  • Between 10 a.m. and 4 p.m.

Do you have:

  • Secchi disk?
  • Boat anchor?
  • Sampling instructions?
  • Data forms?
  • Pen/pencil

Equipment for Total Nitrogen/Total Phosphorus and Chlorophyll a Sample Collection:

  • Filtering apparatus ~ cap, upper chamber, filter support plate & receiver
  • Pitcher
  • 250 milliliter graduated cylinder
  • 7 cm filter paper (in plastic case) with Tweezers
  • Hand-operated pump with clear tubing
  • PVC pipe (for sample collection)
  • Sample bottles ~ 1--6 ml opaque (chlorophyll a ) & 1-- 125 ml clear (total nitrogen/total phosphorus)
  • Styrofoam mailer
  • Sharpie pen

Download paper checklist

TN/TP and Chlorophyll a Sampling Gear

How to Collect the Samples

  1. Use the map of your lake and its marked sampling site and proceed to the site. Always take your water samples from this same general location.
  2. Anchor the boat at the sampling site. Remove your sunglasses and hat and collect Secchi depth transparency measurement . Record data.
  3. Rinse the pitcher with lake water twice by simply dipping the pitcher into the lake.
  4. Rinse the integrated sampler by slowly lowering the end of the pipe into the water so that the 6 foot mark on the pipe is 2 feet below the lake surface. The ball valve needs to be open to allow water to move freely through the sampler.
  5. Slowly pull the sampling pipe back up and out of the water. Repeat.
  6. Refer to Table 1 and determine the amount of water recommended for chlorophyll-a filtration. For example, if your Secchi depth was 10 feet you would need to filter at least 1000 ml of water for chlorophyll a, plus collect 125 ml for your total phosphorus bottle. Each integrated sample delivers about 1000 ml so you would need to collect 2 samples of lake water in the pitcher before filtering or filling any bottles.
  7. Collect the number of integrated water samples needed by slowly and evenly lowering the end of the pipe into the water until the 6 foot water mark on the pipe is even with the surface of the water.
  8. Pull the rope to close the pipe (if having difficulty closing the pipe you can purchase a stopper at the hardware store that you can retrofit your sampler with to function like holding a finger over a straw.)
  9. Hold the open end out of the water, keeping the pipe perpendicular to the water’s surface.
  10. Hold the end of the pipe over the pitcher. In order to prevent contamination, be careful not to let the coupling on the end of the pipe touch anything (your hands, the pitcher, the water that you will empty into the pitcher).
  11. Slowly release the valve in the pipe and allow the entire sample to drain from the pipe before continuing. *Repeat water collection if needed.

  1. Swirl the pitcher to thoroughly mix the water.
  2. Carefully pour the water from the pitcher into the TN/TP bottle (clear bottle). (The bottle has been specially washed so do not rinse it out prior to filling). Be careful not to let the mouth of the TN/TP bottle touch the pitcher or anything else.
  3. Fill the bottle up to the bottom of the neck in order to allow for expansion of the water when you freeze it.
  4. Securely screw the cap onto the bottle.
  5. Write the date sampled directly on the label of the TN/TP bottle, making sure to use the bottle with the right month. (Bottle should have lake name, date, and TN/TP noted)
  6. Keep sample cool until you are able to freeze.
  7. Freeze samples in mailer or loose in freezer.

NOTE: The chlorophyll a filtration procedure should be conducted out of direct sunlight.  Exposure to direct sunlight promotes the degradation of chlorophyll aTry to keep the filtration apparatus out of direct sunlight whenever possible.

See Illustration A.

  1. Separate the chlorophyll a filtration apparatus by unscrewing the upper chamber from the receiver.
  2. Pick up one of the 4.7 cm filters with tweezers and place the filter on the filter support plate on top of the receiver.
  3. Carefully place the upper chamber back on top of the filter support and receiving flask.
  4. While holding the upper chamber piece stationary, tighten the locking ring until the upper chamber is firmly seated on the receiver. (Do not over tighten the locking ring or allow the upper chamber to rotate while tightening because this may tear the filter paper.)
  5. Connect the tubing from the hand pump to one of the two side-arms on the side of the receiver. (The other side-arm must have a cap on it in order for a vacuum to form).
  6. Using your previously measured Secchi depth, refer again to Table 1 to determine the amount of water to filter. [Example: if the Secchi depth was 7 feet, then you would measure out 800 ml (3 x 250 ml + an additional 50 ml contained in graduated cylinder) of water from the integrated sample in the pitcher].

    ** Remember – Table 1 gives you a starting point. Look for a solid green color. Too much turbid water will clog the filter and then you will have to start over. Too little water can lead to inaccurate results.

  7. Pour water from graduated cylinder into upper chamber. Squeeze the vacuum pump until the pressure on the dial reaches 7 on the outer scale of the dial. Do not exceed 7 psi or the pressure may damage the filter. Over time the pressure will decrease so you will need to periodically squeeze the hand pump to maintain pressure.
  8. Note that the upper chamber and receiver only hold 500/1000 ml each. If you are required to filter more than 500/1000 ml, you must disassemble the filtering apparatus and empty out the receiver. 
    1. After all water is off the filter, squeeze the vacuum pump several extra times to remove as much water as possible from the filter.
    2. Release the pressure of the vacuum pump by rotating the vacuum release valve to the right or pulling gently on the trigger.
    3. Hold the upper chamber stationary while unscrewing the locking ring.
    4. Carefully remove the upper chamber by lifting directly upwards. The filter support plate should now be exposed on the top of the receiver.
    5. Grasp the filter support plate by its edges and lift directly upwards. The filter support plate should snap out of the receiver.
    6. Carefully empty the water in the receiver into the lake. Remember, we are only interested in what is left on the filter paper, not the filtered water. Be sure not to dump water down the plastic tubing that connects the hand pump to the receiver.
    7. After emptying the flask, return the filter support plate with the “green” filter containing algae and particulate matter to the receiver by snapping it back on the top of the receiver. (Be sure not to touch the filter).
    8. Reconnect the upper chamber as instructed in step (3) and (4) and proceed filtering the remainder of your sample as indicated by the Secchi depth. (Remember you already filtered 500/1000 mls!)
  9. When you are done filtering the recommended amount of lake water, release the pressure on the hand pump and remove the upper chamber.
  10. Using the tweezers, carefully pick up the edge of the filter and fold it in half on top of the filter support plate. (All of the algae and other particles are trapped on top of the filter paper, therefore this “green,” top side of the filter must always be on the inside of the fold). Make sure that all of your folds are crisp to ensure that the “green” stays inside and is not lost during transfer to or from the bottle.
  11. Place the folded filter paper into the opaque amber chlorophyll sample bottle and write the date sampled and volume of water filtered on the chlorophyll bottle’s label with a “Sharpie” pen. Record this information on your data sheet.
  12. Keep sample cool until you are able to freeze.


  1. Place both the Chlorophyll a and the TN/TP bottles in the styrofoam mailer and put the mailer in the freezer. Samples must be frozen immediately. If you will be out on the lake for an extended time place samples inside of a cooler. If the styrofoam mailer will not fit in your freezer, put the loose sample bottles in the freezer.
  2. Once you have collected two TN/TP samples and two Chlorophyll a samples you are ready to ship.
  3. Pack styrofoam cooler as shown in Illustration B. Substitute the ice bottle for the freezer pack. (Do not remove your mailing labels from the mailer.) Seal with tape. Place sealed cooler in mailing bag and affix label to the outside
  4. Drop at the nearest USPS location. drop off closest to the express mail outgoing shipment so it arrives cold.

** Samples must be cool when we receive them so please time your shipment such that the mailer does not sit in a warm
Post Office room or truck for too long. **

  1. Rinse all of the sampling equipment [pitcher, hose, filter apparatus (be sure to remove hand pump), graduated cylinder, tweezers] with tap water. IN ORDER TO PREVENT CONTAMINATION, DO NOT USE ANY TYPE OF DETERGENT ON THE EQUIPMENT.
  2. Let the equipment air dry.
  3. Once equipment is dry, reassemble the filter apparatus and reconnect the hose ends in order to prevent any contamination from entering the hose.
  4. Return the equipment to the storage bucket. Place the bucket in a safe, dry place until next month.

Table 1

Secchi DepthMinimum Volume of Water to Filter
< 1 ft50 mls
1-1.5 ft100 mls
>1.5-2.5 ft200 mls
>2.5-3.5 ft300 mls
>3.5-6 ft.500 mls
>6-9 ft800 mls
>9-16 ft1000 mls
>16 ft1500 mls

">" means "greater than"

Illustration A: Chlorophyll-a filtration apparatus
Illustration B: How to pack the styrofoam cooler for shipping.