This lesson was developed by Heidi Mehl, USGS Hydrologist
Kansas State Standards can be downloaded from our school standards page
Materials: A plastic bottle with a cap, cold water, a dissolved-oxygen kit or water-quality sonde.
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:
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
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
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)
USDA Forest Service, Pacific Northwest Fisheries Program. Available: http://www.fs.fed.us/r6/fishing/regional/index.html.
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