Water Quality - Unit 5 

Nitrates

Teacherís Instructions

Purpose: The purpose of this experiment is to help the students appreciate the response of goldfish to different levels of nitrates in the water.

Materials for the entire class:

    • 40 liter aquarium
    • Nitrate source (fertilizer pellets)
    • Nitrate nitrogen measurement kit
    • Dissolved oxygen measurement kit
    • Scale for weighing small quantities of fertilizer

Materials for each group:

    • 3 goldfish 
    • 3 - 1000 mL beakers
    • Marker and masking tape

Note: This experiment is intended to show how the goldfish are impacted by varying the level of nitrates in the water. It is expected that you will not get a measurable difference in the activity of the goldfish if the experiment is conducted without "conditioning" the water.  It would be ideal if the beakers could be set up 3-4 weeks before introducing the goldfish into the beakers.  This would more accurately show the effects of excess nitrates in the water.  It is expected that you would then see a more dramatic difference between the fish in different nitrate levels.  These differences could be linked to the effects of algae on dissolved oxygen.

Suggestion: When designing this experiment we had in mind that you, the teachers, would divide the class into groups. This experiment is designed to be flexible given each teacherís available supply of fish, beakers, etc. So, you may modify our setup given your number of students and available resources. However, step 10 requires that there be at least 4 members in a group; 3 to serve as observers and 1 to serve as a timer.

Note:  It is suggested that you perform steps 1-6 at least 14-21 days before you have the class perform the experiment.  The beakers should sit undisturbed in direct sunlight during this time period.  This will promote algae growth and should help your students observe more dramatic experimental results.    

Teacher Setup:

1.Fill all three of the beakers with water from the aquarium that the goldfish are in. Fill each one with 1000 mL (milliliters) of aquarium water.

2. Label the beakers with masking tape and a marker. Have them label one beaker High Nitrates, which will represent a high level of excess nitrates in the water.  Have them label a second beaker Low Nitrates, which will represent a low level of excess nitrates in the water and a third beaker, Control, which will represent approximately normal conditions.

Preparation of the Beakers:

3. Instruct the students to set the Control beaker aside.  No fertilizer pellets should be added to this beaker.

4. Have the students add an amount of fertilizer pellets that will yield 2 parts per million of total nitrate nitrogen to the beaker that is labeled Low Nitrates.

5. Have the students add an amount of fertilizer pellets that will yield 10 parts per million of total nitrate nitrogen to the beaker that is labeled High Nitrates

As a first approximation, weigh out the appropriate amounts of fertilizer pellets to make the 2 and 10 ppm nitrate nitrogen solutions.  2 parts per million is the same as 2 milligrams in 1000 grams (1000 milliliters) of water.  Since you may not have access to a precision scale that weighs in milligrams, you can achieve the same result by weighing out 0.2 grams of nitrate fertilizer and putting this into 100 grams (100 milliliters) of water.  Take 1 milliliter of this solution (contains 0.002 grams of nitrate) and add this to the 1000 milliliters of aquarium water.  Add 5 ml of the nitrate solution to the other 1000 milliliters of aquarium water to achieve a 10 ppm nitrate nitrogen solution.  This will be an interesting math exercise for the students to work through.

Determination of Nitrate Nitrogen Level:

6. Have students determine the nitrate concentration of each beaker. To do this, take the nitrate nitrogen test kit from the Water Quality test kit and follow the directions carefully.  The test kit will yield a measurement in parts per million (milligrams of nitrate nitrogen / 1000 mL of water).

Predictions:

7. Have the students make predictions on the behavior of each goldfish in the three different nitrogen nitrate level solutions. Record these predictions in the Student Journal Activity.
 
Wait ~ 3weeks before introducing the fish into these beakers. 
 
Observations:

8. Have the students take three of the fish currently in the aquarium and place one in each of the beakers.

9. Allow the fish to be undisturbed for 5 minutes in each beaker. This period allows the fish time to acclimate to their new environment. Remind the students to be sure to note the time when the fish were placed into the beakers.

10.  Select one of the following physical movements to monitor in every goldfish:

    • swishing of tail
    • opening/closing of mouth
    • flexing of gills
Each of these movements are normally performed by fish.  

11. After 5 minutes have elapsed, have the students make observations on the behavior of the fish.

      Count the number of times each goldfish makes the selected movement during a 1 minute observation         period.  Also, record any differences in the speed that the fish move, their physical appearance, or their general behavior.

      Have the students record their observations in the Student Journal Activity sheet.

Note: It has been found that counting the number of times that the fish swishes its tail is probably the easiest measurement to make.

Discussion:

12. Have students compare their results with what they predicted.  The results will be based on measurements of either swishing of the tail, opening/closing of the mouth, or flexing of the gills.  Are the results the same as their predictions or are their predictions different?  Is the general behavior different or appearance of the experimental fish different from the control group?

13. As a class, discuss the results.  Discuss what happened and how the fish were affected. If there was no observed effect, discuss why no effect was observed and what could be done next time so that an effect could be observed (e.g. more extreme algae growth or more sensitive organism).

Additional Measurements:

14. You may now want to have the students take measurements of the amount of dissolved oxygen that is present in each of the beakers.  Then, as a class, you can discuss how the algae bloom affects the dissolved oxygen in the water.  To observe the most dramatic results, store the beakers in the dark for 2-3 days prior to the experiment.  Without light some of the algae will die and aerobic bacteria will proliferate and consume some of the oxygen in the beaker.