I'm Trying to Breathe Here! Dissolved Oxygen vs. Temperature

Objective

Dissolved oxygen is an important measure of water quality for aquatic life. In this project you will use a test kit to measure the level of dissolved oxygen in water samples. This project has two goals:

1. to measure dissolved oxygen in water samples at different temperatures, and
2. to determine the saturating oxygen concentration for water samples at different temperatures.

Introduction

Dissolved oxygen is one of many measures of water quality, but an important one for aquatic life. Like land animals, fish and shellfish require oxygen to survive. When oxygen levels fall below 5 mg/l, fish are stressed. At oxygen levels of 1–2 mg/l, fish die.

The amount of oxygen that can dissolve in water (i.e., the saturating concentration of oxygen) depends on water temperature. Colder water can hold more oxygen than warmer water. You'll see for yourself just how much more in this project.

Where does dissolved oxygen come from?

There are two main sources of dissolved oxygen: air and photosynthesis. Consider photosynthesis first. You probably know that photosynthesis is the fundamental biological process that uses light energy to produce sugar from carbon dioxide and water. Oxygen is a by-product of photosynthesis. Both algae (phytoplankton, seaweeds) and plants can be found in natural bodies of water. These organisms are net producers of oxygen in the daytime, but at night become net consumers of oxygen.

Now consider oxygen from the air. At the surface of the water, oxygen from the air equilibrates with oxygen dissolved in the water. This is a dynamic equilibrium: the oxygen molecules are in constant motion. At any given moment, some are leaving the water for the air, and some are leaving the air to dissolve in water. At equilibrium, there is a balance. On average, an equal number of oxygen molecules are leaving and entering the water. If the water temperature increases, the water can't hold as much oxygen as before—the water is oversaturated with oxygen. For a time, there will be more oxygen molecules leaving the water than entering it from the air. Then a new equilibrium will be reached, with less oxygen in the water than before.

Moving water has a rougher surface than still water. With more surface area in contact with air, moving water will equilibrate with air more quickly. (You'll make use of this in your experiment.) In natural situations, water can also become stratified into different layers (see the Science Buddies project Can Water Float on Water?). For example, cold water is denser than warm water, and salt water is denser than fresh water. Can you think of ways that different layers of water might form in a lake or ocean? What do you think happens to the oxygen in a colder layer of water trapped under a warmer layer of water? (Remember that the warmer layer cannot hold as much dissolved oxygen as the colder layer. See the Variations section for a project idea on this topic.)

What causes dissolved oxygen levels to vary?

So far we've seen that dissolved oxygen can come from the air or from photosynthesis, and that when water warms up, there is a net loss of dissolved oxygen. Besides warming, how else can dissolved oxygen become depleted? The answer is another fundamental biological process: respiration. Respiration uses oxygen to break down molecules, in order to produce energy for cells. So the amount of dissolved oxygen will be determined by:

• how much oxygen the water can hold (temperature-dependent),
• how much surface area is available for diffusion from the air,
• how much oxygen is produced by photosynthesis, and
• how much oxygen is consumed by respiration.

Here is a real-world example of variations in dissolved oxygen levels from a continuous monitoring site in the Chesapeake Bay (Maryland DNR, 2006). All of the data were collected at the same location over the same time period. The first graph shows dissolved oxygen, the second graph shows temperature and the third graph shows chlorophyll concentration (a measure of how much algae is present in the water). Notice the daily fluctuations in oxygen level and water temperature. Notice also how the oxygen level and chlorophyll level both declined toward the end of the time period.

Figure 1. The three graphs show (from top to bottom) dissolved oxygen, water temperature, and chlorophyll concentration at a monitoring site in the Chesapeake Bay over a one-week period. (Maryland DNR, 2006)

Sometimes imbalances occur that lead to skyrocketing concentrations of algae. For a project that investigates water quality measures and algal blooms, see the Science Buddies project Harmful Algal Blooms. You can also check out the references in the Bibliography section.

How is dissolved oxygen measured?

Dissolved oxygen can be measured with an electronic metering device or with a chemical test. Dissolved oxygen meters cost hundreds of dollars, so this project will use the chemical testing method. You can buy a dissolved oxygen test kit for around $50. The kit will test 100 water samples. Commercial test kits are based on the "modified Winkler method." You can read more details on this method in the Bales and Gutmann reference in the Bibliography, but here is a basic outline of how the test works:

1. A water sample is collected and the sampling container is sealed under water. This prevents exposure of the sample to the atmosphere.
2. A chemical is added to the water sample to react with all of the dissolved oxygen in the sample. An insoluble precipitate is formed.
3. Additional chemicals are added to drive the first reaction to completion, and to prevent an unwanted reaction from occurring in the final step.
4. A third addition causes the precipitate to change color.
5. The oxygen is now "fixed" and can no longer react with the atmosphere.
6. In the final step, a titration is performed. In this step, a chemical is added in liquid form, one drop at a time. The added compound reacts with the colored precipitate, causing it to lose color. The water sample is mixed after the addition of each drop. When the color change is complete (sample is clear again), it means that the added compound has reacted with all of the fixed oxygen in the sample. By counting the number of drops that were added, the amount of oxygen in the sample can be calculated.

Terms, Concepts and Questions to Start Background Research

To do this project, you should do research that enables you to understand the following terms and concepts:

• dissolved oxygen,
• titration,
• oxygen saturation,
• hypoxia,
• harmful algal blooms,
• Gulf of Mexico "dead zone,"
• photosynthesis,
• respiration.

More advanced students will want to study the chemistry used in the test kits (modified Winkler method). The reference by Bales and Gutmann in the Bibliography is a good place to start.

Questions

• What concentration of dissolved oxygen is required to support aquatic life?
• What are some of the processes that increase dissolved oxygen concentration in natural bodies of water?
• What are some of the processes that decrease dissolved oxygen concentration in natural bodies of water?

Bibliography

• For more background information on water quality measures, including dissolved oxygen, see:
  Munson, B.H. et al., 2005. "Water on the Web: Understanding: Water Quality: Parameters: Dissolved Oxygen," University of Minnesota Duluth and Lake Superior College [accessed May 15, 2006] http://waterontheweb.org/under/waterquality/oxygen.html.
• The dissolved oxygen test kits can seem rather mysterious. The instructions that come with the kits explain the procedure step-by-step, but they do not explain how the test works. The following webpage is a good resource for understanding what is going on with each step. (For advanced students, there is also a separate page with more detailed information on the chemical reactions involved.):
  Bales, R. and C. Gutmann, date unknown. "The Chemistry Section: Dissolved Oxygen," Department of Hydrology and Water Resources, University of Arizona [accessed May 15, 2006] http://www.hwr.arizona.edu/globe/Hydro/kit_chem/DOchem.html.
• Archived data (including dissolved oxygen) from Maryland DNR continuous monitoring stations in Chesapeake Bay can be found at:
  Maryland DNR, 2006. "Eyes on the Bay," Maryland Department of Natural Resources [accessed May 15, 2006] http://mddnr.chesapeakebay.net/eyesonthebay/index.cfm.
• These websites have background information on harmful algal blooms, which can deplete water of dissolved oxygen:
  
  • Anderson, D.A., 2006. "Red Tide and Harmful Algal Blooms," [accessed May 1, 2006] http://www.whoi.edu/redtide/.
  • NOAA, 2006a. "Harmful Algal Blooms," NOAA's National Ocean Service [accessed May 1, 2006] http://www.oceanservice.noaa.gov/topics/coasts/hab/welcome.html.
• These sites have background information on the Gulf of Mexico "Dead Zone," a massive area of hypoxic water that appears every summer near the mouth of the Mississippi River:
  
  • Science Museum of Minnesota, date unknown. "Dead Zone Home," Science Museum of Minnesota [accessed May 15, 2006] http://www.smm.org/deadzone/.
  • Bruckner, M., 2006. "The Gulf of Mexico Dead Zone," Science Education Resource Center, Carleton College [accessed May 15, 2006] http://serc.carleton.edu/microbelife/topics/deadzone/index.html.
• Roach, J., 2005. "Gulf of Mexico 'Dead Zone' Is Size of New Jersey," National Geographic News, May 25, 2005 [accessed May 15, 2006] http://news.nationalgeographic.com/news/2005/05/0525_050525_deadzone.html.

Materials and Equipment

To do this experiment you will need the following materials and equipment:

• dissolved oxygen test kit;
  Note: kits are available from two manufacturers, Hach and LaMotte. Both use essentially the same chemistry (modified Winkler method, see Bales and Gutmann reference in the Bibliography). Both are available from multiple online suppliers; a web search on the brand name + dissolved oxygen test kit will locate them.
  • Hach OX-2P, single-parameter test kit for dissolved oxygen, will test 100 samples,
  • LaMotte 7414 or 5860 dissolved oxygen test kit, will test 50 samples.
• thermometer (range 0°C–100°C),
• gloves,
• safety glasses/goggles,
• at least 3 containers (1 L or more) for water, one to be heated on stove,
• spray bottle (for rinsing test container in the field),
• sealable container (at least 500 ml, for test waste in the field).
• Optional equipment for aerating the water samples (if not readily available, see the Experimental Procedure section for an alternative aeration method):
  • aquarium aerator pump,
  • 2.5 cm or larger fine-mist air stone,
  • plastic aquarium tubing.

Experimental Procedure

Safety notes: Read and follow all of the instructions in your test kit, including all safety precautions. Wear safety goggles and gloves when using test kit reagents. Avoid skin contact with test kit reagents.

In this experiment, you will measure how dissolved oxygen changes in water samples at different temperatures. You will test both aerated and non-aerated water samples at each temperature.

1. Do your background research and make sure that you are knowledgeable about the terms, concepts, and questions, above.
2. Read the instructions that came with your dissolved oxygen test kit so that you know how to perform the test. The Bales and Gutmann reference in the Bibliography has an explanation of what is happening with each of the steps, plus a separate page with a more detailed explanation of the chemistry involved (for more advanced students).
3. Collect your water sample (4 l minimum). The sample can be from a natural body of water, such as an estuary, ocean, lake, pond, or stream. You can also use plain old tap water.
4. Take a baseline dissolved oxygen measurement.
   1. When you collect your water sample, bring along your dissolved oxygen test kit, thermometer, spray bottle and sealable waste container.
   2. Measure the temperature of the water at the collection site.
   3. Test the dissolved oxygen content of the water at the collection site. This is your baseline measurement of dissolved oxygen.
   4. When your measurement is complete, discard the test sample down a drain; do not throw it back in the body of water you sampled. Do the same with the rinse water when you clean the sampling container. If need be, bring the test waste back home in a sealable container and flush it down the drain at home.
   5. To be sure that your results are consistent, you should repeat the test at least three times, using a fresh sample each time. Use the spray bottle to rinse your test container. Discard rinse water down a drain or into your waste container for disposal at home.
   6. Be sure to record the temperature of the water.
5. At home, divide your water sample equally into three separate containers:
   1. container 1 will be cooled with ice,
   2. container 2 will be allowed to equilibrate to room temperature, and
   3. container 3 will be heated slightly.
6. Add enough ice to container 1 to bring the water to about 4–8°C. When the water has cooled, record the temperature and measure the dissolved oxygen concentration. As before, you should run the test at least three times, to be sure that your results are consistent.
7. Next, aerate the sample and re-test. The point of aeration is to saturate the water with oxygen (i.e., dissolve as much oxygen as the water can hold). You can aerate the water with an aquarium aeration pump and airstone. Lots of small bubbles work best. Allow 5–10 minutes for equilibration. Alternatively, you can pour the water back and forth between two large buckets for 5–10 minutes to aerate the water. In either case, check the temperature periodically and add more ice if needed to maintain the temperature.
8. When the water has been aerated, repeat the dissolved oxygen test. Make sure to record the temperature. As before, you should run the test at least three times to be sure that your results are consistent.
9. Run similar tests (aerated and non-aerated) for container 2, the water sample at room temperature (it may take a few hours to equilibrate, depending on how cold the sample was to start).
10. Run similar tests (aerated and non-aerated) for container 3, the water sample that you heat. You can warm it on the stove, or in the microwave. Mix the sample gently and check the temperature frequently. Aim for a temperature from 35–40°C. You don't want to scald yourself when testing the dissolved oxygen concentration.
11. Summarize your results in a table. For example:

    Sample	Aerated?	Temp #1	DO #1	Temp #2	DO #2	Temp #3	DO #3
    Baseline	N
    Chilled	N
    Chilled	Y
    Room temp	N
    Room temp	Y
    Warmed	N
    Warmed	Y

12. Make a graph of your results. You can plot dissolved oxygen vs. temperature. Use separate symbols for:
    1. your baseline sample,
    2. your non-aerated samples,
    3. your aerated samples.
13. From your graph, do you think your original baseline sample was saturated with oxygen? Why or why not?

Variations

• Does seawater hold as much dissolved oxygen as freshwater at the same temperature? Compare aerated fresh- and salt-water samples at different temperatures. If the ocean is too far away, make your own saltwater by adding between 30 and 35 g of table salt for each liter of water. Use a double bath for cooling the saltwater sample down without diluting it (saltwater sample in the inner container, surrounded by ice water in the outer container).
• How could you modify the experiment to test whether the initial baseline sample is saturated with oxygen?
• Measure dissolved oxygen in water with and without aquatic plants. If you have access to a planted aquarium, it would be interesting to monitor oxygen levels both in daytime and at night. As in the experiment described above, this experiment could be done both with and without aeration. If fish are present, monitor the nighttime oxygen level frequently. Also, monitor the fish for signs of oxygen stress (e.g., increased gill beat rate, gulping at the surface).
• If you live near an estuary or other natural body of water, you could monitor dissolved oxygen from one or more sites over time. For example, you could sample multiple times during a 24-hour period to track the daily fluctuation of oxygen. Alternatively, you could sample over a longer time period, and look for changes correlated with weather systems. What effect would you predict for cloudy weather?
• Compare dissolved oxygen in a still portion of a stream vs. a rapidly flowing portion. Or compare oxygen levels in water sampled at different depths.
• For a more advanced project that uses archived water quality data from monitoring stations in the Chesapeake Bay, see the Science Buddies project Harmful Algal Blooms in the Chesapeake Bay.

Credits

Andrew Olson, Ph.D., Science Buddies

Sources

• Maryland DNR, 2005. "Eyes on Dissolved Oxygen," Maryland Department of Natural Resources, Eyes on the Bay Lesson Plan Series [accessed May 15, 2006] http://mddnr.chesapeakebay.net/eyesonthebay/lesson_plans/do_lesson_plan.pdf.

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