Water Quality - Unit 3

Dissolved Oxygen

Teacherís Instructions

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

Materials for the class:

    • 40 liter aquarium
    • Bag of ice
    • Microwave, stove or hotplate (each group will need to heat the water in one of the beakers)
    • 1- roll of plastic wrap

Materials for each group:

    • 3 goldfish OR 9 daphnia magna (large water fleas)
    • 3 - 1000mL beakers (Pyrex or other heat treated glass)
    • 1-Thermometer
    • 1-Large plastic tub (~ 30 cm long x 30 cm wide x 15 cm deep)
    • 1- Electric aquarium air bubbler
    • Dissolved oxygen measuring kit
    • Marker and masking tape

Note: Read all of the instructions before proceeding!! Depending upon the availability of goldfish, you may elect to use daphnia magna water fleas. If you decide to do so, substitute 3 water fleas for 1 fish in the experiment. Also, the students will be heating water in a beaker and placing it in a water bath to reduce the amount of dissolved oxygen that is present in the water. You may elect to boil the water ahead of time to reduce the possibility of the students being burned when they are handling boiling water. It is important to make sure that you use heat-resistant glass beakers (such as Pyrex) at least for those beakers that will be heated (1 per group).

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 13 requires that there be at least 4 members in a group (3 to observe and 1 to time.)

General Setup:

1. To begin with, have the students fill all three of the beakers with water from the aquarium that the goldfish are in. Instruct them to fill each one with 750 mL (milliliters) of water.

2. With masking tape and a marker, have the students label one beaker "High Dissolved Oxygen", one beaker "Low Dissolved Oxygen", and the third "Control".

3. Instruct the students to fill the plastic tub with ice, until it is almost full. Then add water to it until the water is about 2 centimeters below the rim of the tub. This will serve as the water bath that will be used to cool the Low Dissolved Oxygen beaker in step 7.

Preparation of the Beakers:

4. Have the students place the electric aquarium air bubbler in the beaker labeled High Dissolved Oxygen. Let it bubble for at least 10 minutes.

Note: Be sure not to remove the aquarium bubbler until immediately before you place the fish in the water.

5. Instruct the students to set the Control beaker aside. Use the thermometer to check the temperature of the water in this beaker.  Be careful not to agitate the water; disturbing the water will increase the amount of dissolved oxygen in the water.

6. Have the students heat the beaker that is labeled Low Dissolved Oxygen using the microwave, stove or hotplate. Heat the water until it boils and allow it to boil continuously for at least 5 minutes.

7. After boiling the water, have the students place the beaker (Low Dissolved Oxygen) into the ice-water bath.  The bath should be in a large container that can hold the beaker.  Put in as much ice as possible and add water, filling it to a level just below the height of the beaker.

8. Instruct the students to place a thermometer in the water and immediately cover the beaker with plastic wrap.  Explain to them that they need to push the plastic wrap down inside the beaker so that it is touching the surface of the water.  There should not be any air space between the water and plastic wrap. Have the students remove the beaker from the ice-water bath when the temperature has dropped to approximately the same temperature as the water in the Control beaker (~24-26 ļC).

Note:  Make sure they stir the water gently to get an accurate measurement of the temperature.  Stirring the water very gently will help to distribute the temperature evenly throughout the solution. If they stir the water too briskly, they will reintroduce oxygen into the water.

Determination of Dissolved Oxygen:

9. Help the students measure the level of dissolved oxygen of the water in each beaker using the measurement kit.

Note:  Be sure to carefully follow the directions included with the water quality kit.

Predictions:

10. Have the students make predictions on the behavior of each goldfish in the three different dissolved oxygen levels. The students should record their predictions in the Student Journal Activity.
 
 
Observations:
 

11. Have the students take three of the fish currently in the aquarium and place goldfish into each of the beakers.

Note:  While doing this, try to avoid agitating the water.  Keep the plastic wrap over the Low Dissolved Oxygen beaker at all times.  Make sure that the air bubbler in the High Dissolved Oxygen beaker is removed.

12. Allow the fish to be undisturbed for 5 minutes to acclimate to the different environments. Remind the students to be sure to note the time when the fish were placed into the beakers.

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

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

    • swishing of tail
    • flexing of mouth
    • flexing of gills
Each of these movements are normally performed by fish. Count the number of times the goldfish makes this movement over the period of 1 minute.
 
Have the students record their observations in the Student Journal Activity sheet.

Note: Counting the number of times that the fish swishes its tail is probably the easiest measurement to make.

Discussion:

14. Have students compare their results with what they predicted.  The results will be based on any behavioral changes observed.  Are the results the same as the predictions or are they different?  Is their behavior different than from the control group?

15. As a class, discuss the results.  If any effect was (or wasn't) seen, discuss what happened and how the fish were affected. If there was a difference in their behavior, did everyone's experiment provide the same results?  If there were no observed effects, why did this happen?  What could be done to change the experiment that would possibly produce results?