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The effects of phosphates on aquatic plants

The effects of phosphates on aquatic plants

Introduction: (Initial Observation)

Aquatic plants are an important component of lake systems. These plants may be totally submerged beneath the lake surface, floating, or growing along the shoreline. They provide food and shelter for bugs, fish and other organisms, prevent shoreline erosion, filter pollutants from adjacent shoreline activities, and provide oxygen to the surrounding environment.

Any conditions that harm such a balanced environment can potentially destroy thousands of organisms that live there. For this reason scientists constantly perform experiments to discover how different chemicals may affect aquatic plants and their ecosystem. Phosphates are among the chemicals found in run-offs and sewers because large amounts of phosphates are used in household and industrial detergents as well as fertilizers. In this project we investigate the effect of phosphates on aquatic plants.


This project guide contains information that you need in order to start your project. If you have any questions or need more support about this project, click on the “Ask Question” button on the top of this page to send me a message.

If you are new in doing science project, click on “How to Start” in the main page. There you will find helpful links that describe different types of science projects, scientific method, variables, hypothesis, graph, abstract and all other general basics that you need to know.  

Project advisor

Information Gathering:

Find out about what you want to investigate. Read books, magazines or ask professionals who might know in order to learn about the effect or area of study. Keep track of where you got your information from.

Search the Internet for keywords such as “Aquatic Plants”, “Aquatic plants + phosphate”, “Aquatic plants + phosphates”, “Aquatic plants + nutrients”, “Aquatic plants + effects”.

Following are some related links:


Phosphate If there is too much phosphate present, aquatic plants and algae can grow very quickly, called a bloom. Extensive algae growth can shade or smother aquatic vegetation. This can reduce or even kill many plants that live and grow under the water. Dead algae and other aquatic vegetation can use up oxygen in water that is required by fish and other aquatic organisms. Phosphate can come from human and animal wastes, as well as industrial wastes. It should not be present in amounts more than 1 mg/L. Visit the Source

Phosphorus is one of the key elements necessary for growth of plants and animals. Phosphorus in elemental form is very toxic and is subject to bioaccumulation. Phosphates PO4— are formed from this element. Phosphates exist in three forms: orthophosphate, met phosphate (or polyphosphate) and organically bound phosphate. Each compound contains phosphorous in a different chemical formula. Ortho forms are produced by natural processes and are found in sewage. Poly forms are used for treating boiler waters and in detergents. In water, they change into the ortho form. Organic phosphates are important in nature. Their occurrence may result from the breakdown of organic pesticides which contain phosphates. They may exist in solution, as particles, loose fragments, or in the bodies of aquatic organisms. Visit the Source

Sources of Phosphorus

Phosphorus comes from several sources: human wastes, animal wastes, industrial wastes, and human disturbance of the land and its vegetation. Sewage from wastewater treatment plants and septic tanks is one source of phosphorus in rivers. Animal waste containing phosphorus sometimes finds its way into rivers and lakes in the runoff from feedlots and barnyards. Soil erosion can also contribute phosphorus to rivers. The removal of natural vegetation for farming or construction, for example, exposes soil to the eroding action of rain and melting snow. Soil particles washed into waterways contribute more phosphorus and fertilizers used for crops, lawns, and home gardens usually contain phosphorus. When used in excess, much of the phosphorus in these fertilizers eventually finds its way into lakes and rivers. Draining swamps and marshes for farmland or shopping malls releases nutrients like phosphorus that have remained dormant in years of accumulated organic deposits. Also, drained wetlands no longer function as filters of silt and phosphorus, allowing more runoff and phosphorus to enter waterways. Visit the Source

Aquatic plants are a vital part of any lake or pond. They convert sunlight and chemical elements into living plant tissue. Fish, waterfowl, insects, mammals, and microscopic animals use the plants for food. Plants also replenish the aquatic environment with oxygen, which is essential to aquatic animals. Additionally, rooted plants create a varied aquatic environment in which fish food organisms reside. They also provide cover for spawning fish, nesting waterfowl, shoreline mammals, and their young.
Visit the source

Terms and Abbreviations and synonyms:

L or l : Liter (unit of volume in metric system, used for fluids)

mL or ml : milliliter = 1/1000th of liter = cc = Cubic Centimeters

g : gram (Unit of mass or weight in metric system)

mg : milligram = 1/1000th of gram

Question/ Purpose:

What do you want to find out? Write a statement that describes what you want to do. Use your observations and questions to write the statement.

Question/ Purpose:

What do you want to find out? Write a statement that describes what you want to do. Use your observations and questions to write the statement.

The purpose of this investigation is to see the effect of phosphates on aquatic plants. We will perform plant growth experiments with different concentrations of phosphate and compare the results.The purpose of this investigation is to see the effect of phosphates on aquatic plants. We will perform plant growth experiments with different concentrations of phosphate and compare the results.

Identify Variables:

When you think you know what variables may be involved, think about ways to change one at a time. If you change more than one at a time, you will not know what variable is causing your observation. Sometimes variables are linked and work together to cause something. At first, try to choose variables that you think act independently of each other.

Independent variable for our experiment is the rate of phosphate in water. Dependent variable is plant growth.


Based on your gathered information, make an educated guess about what types of things affect the system you are working with. Identifying variables is necessary before you can make a hypothesis.

I think small amounts of phosphate can help the growth of aquatic plants. I also think very small change in the concentration of phosphate can cause a big change in the growth rate. However, excessive use of phosphate may harm the plant and completely stop it’s growth.

Experiment Design:

Design an experiment to test each hypothesis. Make a step-by-step list of what you will do to answer each question. This list is called an experimental procedure. For an experiment to give answers you can trust, it must have a “control.” A control is an additional experimental trial or run. It is a separate experiment, done exactly like the others. The only difference is that no experimental variables are changed. A control is a neutral “reference point” for comparison that allows you to see what changing a variable does by comparing it to not changing anything. Dependable controls are sometimes very hard to develop. They can be the hardest part of a project. Without a control you cannot be sure that changing the variable causes your observations. A series of experiments that includes a control is called a “controlled experiment.”

Experiment: Testing the effects of phosphate on the growth of aquatic plants.


Gathered information indicated that the amount of phosphate in water must not be more than one milligram per liter. So we will test the values between one milligram up to one gram per liter.


Before starting your experiments you need to get some type of phosphate. Your choices are organic phosphates such as crushed bone, Inorganic water insoluble phosphates such as calcium phosphate and finally water soluble phosphate such as sodium phosphate.

For best results I recommend using a water soluble phosphate such as sodium phosphate or ammonium phosphate. In this way you will be sure that 100% of the phosphate that you use is available to the plant immediately.

Sodium phosphate for example is a general purpose cleaner and degreaser that you can purchase from hardware stores and paint stores. It is also known as Tri-sodium Phosphate or T.S.P. (Its formula is Na3PO4 . 12 H2O). From it’s formula you can calculate that every 380 grams of TSP contains 95 grams phosphate ion in the form of PO4—. The rest is sodium and water.

Where did you get the number 380?

380 is the molecular weight of T.S.P.. Molecular weight is the sum of atomic weight of all elements in one molecule.

Atomic weight of Na is 23, atomic weight of Oxygen is 16. Atomic weight of Phosphorus is 31 and atomic weight of Hydrogen is 1. So the molecular weight of Na3PO4 . 12 H2O is = 23 x 3 + 31 + 16 x 4 + 12 x (1 x 2 + 16) = 380

From the above numbers we estimate that each 40 grams of TSP contains 10 grams of phosphorus. Or each 4 grams of TSP contains 1 gram (1000 mg) phosphorous. So we can use these information to prepare our phosphate solution.

For example you can dissolve 4 grams of TSP in about 50 milliliter water and then dilute it to make it exactly 100 milliliters. Each 1 ml of this solution contains 10 mg TSP. You can use a 1ml pipette to extract 0.1 ml of solution that contains exactly 1 mg phosphate ion.

If you don’t have a pipette, a beaker or a graduated cylinder, you can still divide TSP powder to get to one milligram.

Weight 4 grams of TSP powder that is equivalent to one gram phosphate ion. Place the powder on a flat table (with glass on top if possible) and start to divide the powder. Use a card to move the pile of powder to left and write until it forms a long line. Then visually divide it into two. This gives you two approximately equal 500 mg sections. Divide the 500 mg pile into two parts of 250 mg each. Continue the same way until you get to 1 mg.

This method usually has about 90% accuracy and serves our needs perfectly.


Fill ten, quart-size canning jars with aged water to within an inch of their tops.

Place one 4- to 5-inch long sprig of Elodea in each jar.

Add different amounts of phosphate in the first 9 jars and screw on the lids, (Optionally place two or three guppies, or one water snail, in each jar) then label each jar with the amount of phosphate in that jar.

Put these mini-aquariums in a place that is out of direct sunlight, and where you will be able to observe them easily. Maintain the water temperature at approximately 70 degrees Fahrenheit (about room temperature). Try to keep the water temperature in each jar the same so you can determine the effect of phosphate on each jar without introducing another variable.

You do not add any phosphate to the 10th jar and label that as “control”.

Make daily observations of all 10 aquariums and record your observations for about 3 weeks. (Depending on the temperature and light conditions, you will need 2 to 4 weeks to see measurable results)

While observing plants in aquariums, in addition to recording the measurable data, such as animal population, and plant growth, you should also look for subtle changes in the aquariums. Are the fish active? Are they gasping for breath? Are the plants green and healthy? Is the water clear or cloudy? Does it have an odor? In this experiment you will not add any animals; however, some animals may be accidentally transferred with your plants.

Record the results in a table like this:


Phosphate  Plant condition Plant height Plant color Water color Water odor
1 mg/L
2 mg/L
5 mg/L
10 mg/L
20 mg/L
50 mg/L
100 mg/L
500 mg/L
1000 mg/L

Start to make daily observations and fill up one table like this for each day. Skip the days that you do not see any noticeable change from your previous report.

Dispose dead plants or animals and continue with the rest of your samples.

Note: Tri-sodium phosphate is a strong base with a high pH. It is better to use Di-sodium phosphate or Monosodium phosphate for this experiment, but I am not sure that these two are easily available. If you have access to a pH meter or pH paper, it is good if you prepare a TSP solution outside, neutralize it with vinegar and then add the neutralized solution to the jar.

How do I make a 1000 mg/L solution?

1000mg is one gram. Add one gram TSP to a beaker and then add water to bring the volume to 1 liter. Now you have 1 liter of 1000mg/L T.S.P. solution.

Make a 1:2 dilution of 1000mg/L solution to make a 500mg/L solution. To make 200 ml of 1:2 dilution, mix 100ml of this with 100ml of pure water.

make a 1:10 dilution of 1000mg/L solution to get a 100mg/L solution. To make a 1:10 dilution, mix one cup of this solution with 9 cups of water.

By diluting a 100mg/L solution, you can make all other concentrations.

Materials and Equipment:

  • 3 one-liter mason or canning jars with lids
  • aged water to fill the jars
  • 3 identical sprigs of Elodea or Anacharis about 3 to 5 inches long
  • Sodium phosphate
  • Other material described in the experiment design

Results of Experiment (Observation):

Experiments are often done in series. A series of experiments can be done by changing one variable a different amount each time. A series of experiments is made up of separate experimental “runs.” During each run you make a measurement of how much the variable affected the system under study. For each run, a different amount of change in the variable is used. This produces a different amount of response in the system. You measure this response, or record data, in a table for this purpose. This is considered “raw data” since it has not been processed or interpreted yet. When raw data gets processed mathematically, for example, it becomes results.

Record the results of your daily observation in a table like this:

Date: 02/02/2003

Phosphate  Plant condition Plant height Plant color Water color Water odor
None/control  Normal 4″ green clear none
1 mg/L  Normal 4″ green cloudy
2 mg/L  Normal 4″ light green algae odor
5 mg/L  Normal
10 mg/L  Normal
20 mg/L  Normal yellow
50 mg/L  Normal white
100 mg/L  Normal dark green
500 mg/L  rotted black
1000 mg/L  dead

You will use your results tables for your final analysis and report.


You need to calculate the rate of phosphate ion in the compound that you use. What matters here is the amount of phosphate ion, not the amount of phosphate salt.

To calculate the growth rate of each plant, record the plant height at the beginning and at the end of your experiment. The difference between these two values is the growth rate of the plant for the period of your experiment. You can divide this by the number of days in your experiment period to calculate the daily growth rate.

Summery of Results:

Summarize what happened. This can be in the form of a table of processed numerical data, or graphs. It could also be a written statement of what occurred during experiments.

It is from calculations using recorded data that tables and graphs are made. Studying tables and graphs, we can see trends that tell us how different variables cause our observations. Based on these trends, we can draw conclusions about the system under study. These conclusions help us confirm or deny our original hypothesis. Often, mathematical equations can be made from graphs. These equations allow us to predict how a change will affect the system without the need to do additional experiments. Advanced levels of experimental science rely heavily on graphical and mathematical analysis of data. At this level, science becomes even more interesting and powerful.


Using the trends in your experimental data and your experimental observations, try to answer your original questions. Is your hypothesis correct? Now is the time to pull together what happened, and assess the experiments you did.

Related Questions & Answers:

What you have learned may allow you to answer other questions. Many questions are related. Several new questions may have occurred to you while doing experiments. You may now be able to understand or verify things that you discovered when gathering information for the project. Questions lead to more questions, which lead to additional hypothesis that need to be tested.

Possible Errors:

If you did not observe anything different than what happened with your control, the variable you changed may not affect the system you are investigating. If you did not observe a consistent, reproducible trend in your series of experimental runs there may be experimental errors affecting your results. The first thing to check is how you are making your measurements. Is the measurement method questionable or unreliable? Maybe you are reading a scale incorrectly, or maybe the measuring instrument is working erratically.

If you determine that experimental errors are influencing your results, carefully rethink the design of your experiments. Review each step of the procedure to find sources of potential errors. If possible, have a scientist review the procedure with you. Sometimes the designer of an experiment can miss the obvious.


List your references.