Activity: The Effects of Agricultural Fertilizer Runoff Upon Aquatic Ecosystems

The following experiment along with the next one illustrate the adverse effects of only two of the many changes that have been made in the Lake Pontchartrain Basin. As your students perform these exercises, they will develop an appreciation of just how dramatically certain environmental changes can affect our precious natural resources.

The watershed of the Lake Pontchartrain Basin is fed by streams and ditches which drain the crop and pasture lands north of the lake. Many of these streams contain extremely high levels of organic nutrients, chemical fertilizers, and pesticides. Most of this pollution is directly attributable to runoff from dairy and crop farms which have not installed holding ponds. Holding ponds allow much of the pollution to become incorporated into plant tissues or to be biodegraded into less harmful components while at the same time allowing associated bacteria to be rendered harmless. This experiment allows students to determine for themselves exactly what effect these compounds can have upon aquatic and wetland ecosystems. A dramatic comparison between polluted and unpolluted sites can be made using aquaria in which stable "ecosystems" have already been established.

Teaching Materials:

An even number of "aquariums". These can be actual aquaria, or large, clean glass jars. The size of the jar will necessarily limit the number of plants and animals as well as the types of animals which can be incorporated into the "ecosystem".

Cleaned sand, gravel, oyster shells or other substrate for the aquaria.

A supply of pond water, enough to make up at least 1/4 of the total in each

An assortment of pond animals and plants which can be evenly distributed among all aquaria. Examples can include snails, small fish, crawfish, duckweed, common aquarium plants like Elodea, etc. NOTE: While using animals better approximates a real ecosystem, this experiment can be successfully conducted by using only pond water in an aquarium or jar.

A moderately sunny windowsill or some artificial light source such as bright

aquarium lights.

Assorted pollutants. One of the best is a well-balanced, basic garden fertilizer, something like 13-13-13.

A small chart with numbers or text in decreasing font sizes, much like an optometrist's eye chart, which can be placed behind the aquarium to measure turbidity or water cloudiness (Refer to "Data Analysis and Collection",

Getting Ready:

  1. Place aquaria in their permanent locations (make sure they are not too sunny; nlight for half of the day should be about right). Allow students to select from available substrate materials to construct their ecosystems. Fill aquaria 1/2 to 3/4 full with tap water and arrange plants in the gravel, etc. Let the system stand overnight. Add fish, snails, etc. and finish filling with pond water (or aged tap water).

  2. After aquaria have 'set-up' for a few days, you will be ready to apply fertilizer treatments. The number of different fertilizer levels (amounts of fertilizer) will depend upon how many aquaria you have available. This exercise can, of course, be done with as little as two aquaria...more allows for replication or many different levels of pollution.

  3. Fertilizer solutions can be tested against non-fertilizer solutions (controls) as well as each other (increasing levels of pollution).


  1. Add 100mg of fertilizer to each 10 gallon aquarium (you may have to adjust this amount depending upon the size of the aquarium), or vary the amounts in each tank (i.e., 100mg, 200mg, 300mg, etc.).

  2. Remember to keep one tank "clean" (no fertilizer) to use as a control.

  3. Have your students measure and record the turbidity levels in each tank every morning using the charts they have constructed. (Refer to "Data Collection Sheet" Page 114)

  4. Continue to monitor the pollution levels and help students formulate hypotheses concerning agricultural runoff.

  5. PLEASE!! remember to feed your "critters" if any live animals have been used in the tanks.

Does runoff affect aquatic ecosystems? How?

Are animals affected by runoff? Do you think that greater amounts of runoff would change this?

Would mixing other runoff (pesticides, manure, motor oil, gasoline) affect the animals more?

Why or why not?

How can we help improve the health of Lake Pontchartrain?

How can we reduce the amount of runoff entering Lake Pontchartrain?

Data Analysis and Collection:

Use reproductions of the Turbidity Indicator and the Data Collection Sheet to order to estimate the degree of turbidity in a particular aquarium and to record data.

Turbidity Measurements:

  • Make photocopies of the Turbidity Indicator and tape them to the rear of your aquaria.

  • Record on your data sheets the turbidity level indicated by the smallest legible word that you can read.

  • You may wish to construct your own indicator using the scientific names of wetland plants and animals in decreasing font sizes...this has the additional benefits of disallowing "cheating" onto the next, more turbid, level; particularly if the word for the next level is not known, and also encourages your students to learn these scientific names.

Go back

[an error occurred while processing this directive]