Air Underwater Lesson

This lesson was developed by Heidi Mehl, USGS Hydrologist

Overview: Students will learn about dissolved oxygen in water and its importance to aquatic life and the health of our streams and rivers.

Suggested grades: 5-7 and 8-12

Kansas State Standards can be downloaded from our school standards page

Objective: Develop the concept that there is oxygen in the water just as in the air we breathe, and aquatic plants and animals depend on it to "breathe" as well. Discuss the types of aquatic organisms that live in low dissolved oxygen vs. high dissolved oxygen environments. Discuss factors that may affect the level of dissolved oxygen in water, such as temperature, water movement, and pollution. Learn methods to measure dissolved oxygen, and how to tell whether it is normal, high, or low.  
 

Materials: A plastic bottle with a cap, cold water, a dissolved-oxygen kit or water-quality sonde.

Lecture: Oxygen is essential to almost all living things, including aquatic plants and animals. The level of oxygen in the water is called “dissolved oxygen.”  Dissolved oxygen (DO) is important for healthy streams, rivers, and lakes. If the dissolved oxygen levels are too low, many fish and other species will have trouble breathing and could even die (can use this point to discuss how fish “breathe” through their gills, aquatic plants respirate, etc). Some aquatic creatures need more oxygen to survive than others. Fish species are classified as either “warm-water” or “cold-water” fish. All fish species living in Kansas streams, rivers, and lakes are warm-water fish.  
 

Warm-water fish include:  walleye, largemouth and smallmouth bass, perch, crappie, bluegill, and channel catfish.  
 

Cold-water fish include:  Rainbow Trout, Coastal Cutthroat Trout, Brook Trout, Dolly Varden, and Salmon.
 

Other species may indicate when dissolved oxygen levels in a stream or lake are too low. For example, macroinvertebrates such as mayfly nymphs, stonefly nymphs, and caddisfly larvae need high levels of dissolved oxygen to survive. If these species begin disappearing and species such as leeches, sludge worms, and blackfly larvae begin showing up, it could indicate that DO levels are too low. (This portion can be combined with a macroinvertebrates lecture/field trip. Resource:  http://www.dnr.state.md.us/streams/pubs/freshwater.html).  
 

The level of dissolved oxygen is affected by many different factors:

1. Temperature – Cold water holds more dissolved oxygen than warmer water. Do you think water would have more dissolved oxygen in the morning, or in the afternoon? In the summer, or in the winter?
    

Activity: Fill a plastic bottle with cold water and tighten the cap. Describe to students that the water is cold, and ask if it will get colder or warmer sitting in the sun, and what will happen to the oxygen levels as it warms up. Place the bottle in the sun, and check it again at the end of the activities. After it warms in the sun, there should be bubbles clinging to the sides of the bottle. This is dissolved oxygen being released from the water as it warms up.  

2. Water movement – As water flows in a stream or river, it can pick up oxygen from the air. The more turbulent the stream, the more oxygen it will be able to trap. The trapped oxygen will dissolve into the water.  
    

Activity: If a dissolved oxygen meter is available (such as a multi-parameter water quality sonde from U.S. Geological Survey), you may have the students measure the dissolved oxygen in a stream, comparing the water from a stagnant pool (low DO) to the water from a riffle or other fast-moving portion (high DO).  If no sonde is available, the same measurements may be done with a dissolved oxygen test kit. These can be found on a number of websites providing water-testing kits. With either one of these measuring tools, you may also have students measure the DO in still water, then shake the water in a closed container for about 15 seconds and measure the DO again. It should be higher after agitating the water.  

3. Pollution – Certain types of pollution may lower the dissolved oxygen levels in water, making it difficult for aquatic organisms to survive. This can happen in a number of ways.  

Sometimes the effluent (water flowing into a natural body of water from a natural or man-made source) itself has low levels of dissolved oxygen. When it is added to the natural body of water (such as a stream), it can lower the dissolved oxygen levels in the stream.

Some types of power plants use water to cool their equipment, which is then discharged back into the stream or river.  It may be clean water, but it is hot from being used as a cooling agent in the power plant. The hot water raises the temperature of the river, which as we know decreases the dissolved oxygen.  
 

Pollution often contains chemical or biological agents (ex. Bacteria) which have high oxygen demand.  These agents, which aren’t present naturally, can gobble up the available oxygen.  The aquatic organisms then won’t have enough available to survive.  

Effluent from wastewater plants and runoff from farmland often contain large amounts of nutrients. Nutrients are usually a good thing, and are present naturally. But too many nutrients in a stream can cause algal blooms. Algae often outcompetes other aquatic plants, and can grow very rapidly (ask students if they’ve ever seen a pond filled with green “goo,” or if outside point out some algae).  As the algae population explodes, other plant species begin to die and decompose. The bacteria which help the plants decompose use up quite a bit of dissolved oxygen. Once the supply of nutrients is used up, the algal bloom may also die and decompose, further depleting the dissolved oxygen. Algae blooms can be very harmful to aquatic life.  
 

Measuring Dissolved Oxygen: Dissolved oxygen may be measured with a sensor on a water-quality sonde, or there are many different kinds of kits which use chemical reactions to measure DO.  
 

Dissolved oxygen can be measured in ppm (parts per million) or Percent Saturation. If you know the atmospheric pressure and temperature, a chart can tell you the maximum amount of oxygen that water can hold under those conditions. This water would be 100% saturated.  However, natural water can vary widely and is not always 100% saturated. Generally, DO content between 8-10 ppm is considered very good. If the DO is below 4ppm, certain species will have trouble surviving.  
 

The DO test must be performed as soon as possible, preferably at the source of the water.  DO concentrations are affected by many factors, and may change in a short amount of time.
    

Activity: You may purchase a DO test kit from a number of different scientific supply companies, such as LaMotte Industries (http://www.lamotte.com/pages/edu/5886.html).    Allow students to test different sources of water and determine whether the source is healthy or oxygen-depleted.  
 

Some U.S. Geological Survey offices do outreach projects. You may contact your local office and ask them to bring a multi-parameter water-quality sonde to demonstrate to the students.  (https://www.ysi.com/ysi/Products/Product_Family/Product?productID=X_OBS_199_6600)

References

U.S. Environmental Protection Agency ,1986, Quality criteria for water, 1986: U.S. Environmental Agency Report 440/5-86-001, unnumbered pages.

USDA Forest Service, Pacific Northwest Fisheries Program. Available: http://www.fs.fed.us/r6/fishing/regional/index.html.