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Lessons on the Lake -> Chapter List -> Chapter 6 -> Lesson 6: Activity: Agricultural Runoff

The Effects of Agricultural Fertilizer Runoff
Upon Aquatic Ecosystems

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 aquarium.

  • 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”, Page 154).

Getting Ready:

  1. Place aquaria in their permanent locations (make sure they are not too sunny; sunlight 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.

  3. 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 of many different levels of pollution. 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 155).

  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.

By the end of this experiment, you should be able to answer these questions!

  • 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?

Young deer, called fawns, have white spots to help camoflage them from predators.
  1. Introduction
  2. Essential Questions
  3. Natural Resource Balance
  4. Activity: How We See the Puzzle
  5. What are our most precious Natural Resources?
  6. Activity: Agricultural Runoff
  7. Activity: Salt and Cypress
  8. Activity:  "I AM"