Terms to know by the end of the lesson: Dissolved oxygen Turnover Atmospheric diffusion Thermal stratification Hypolimnion, metalimnion and epilimnion Productivity Hypoxic zone Anoxic zone Dead zone Bacterial decomposition Background From late spring through early fall, some lakes in temperate climates experience thermal stratification, a phenomenon wherein lakes separate into three distinct thermal layers Figure 1.
Identified tropic levels are: Oligotrophic olig-oh-trof-ik — An oligotrophic lake has low nutrient concentrations and low plant growth e. It is usually considered to have low productivity. Eutrophic yoo-trof-ik — A eutrophic lake has high nutrient concentrations and high plant growth. It is considered to have high productivity. Mesotrophic meso-trof-ik — Mesotrophic lakes fall somewhere in between eutrophic and oligotrophic lakes. They are considered to have average productivity. Figure 2.
Dead zones in Lake Erie from Begin by asking if any of the students have swum in a lake or pond during the summer and felt cold water at their feet. If so, they may have felt thermal stratification. Ask students if they can define thermal stratification and then clarify what it is by using the background information above.
Encourage students to ask questions about why water stratifies. Educators can relate thermal stratification to the layering that happens with oil and vinegar. Oil and vinegar have different densities, thus one floats on the other.
This is similar to water at different temperatures. Cold water is denser than warm water. Denser water will sink and warmer water will float, thus creating layers.
This is a good opportunity to present Figure 1 and provide an opportunity for students to ask questions. Ask if students know about dissolved oxygen. To help them understand the idea, ask if they ever have seen a bubble stone in an aquarium.
If so, ask why they are used. Some answers might be: Bubble stones circulate water and increase oxygen levels in aquariums by directly inputting oxygen into the system and by increasing the amount of water coming into contact with the air. This promotes atmospheric diffusion of oxygen into the water.
Now discuss air diffusion at the lake scale. In what ways might a lake receive oxygen? Discuss the background information provided above so students know about the three methods of oxygen diffusion.
Most students know that plants produce oxygen, and educators can connect this idea to the aquatic environment. Ask students why they may think dissolved oxygen is important in a lake. Make sure students understand that, like terrestrial animals, aquatic animals require oxygen. Describe how much oxygen cold water and warm water fish need.
Then explain how oxygen levels can become very low at certain times of year due to thermal stratification. Introduce the concept of dead zones. This is a good opportunity to display Figure 2. Elaborate In this section students are provided with additional resources on hypoxia. Unlike almost all other liquids, water is most dense at 39 degrees Fahrenheit 4 degrees Celsius , and is lighter at both warmer and colder temperatures.
In other words, when water reaches the critical temperature of 39 degrees Fahrenheit, further cooling causes the water molecules to become less dense and rise to the surface.
This unusual characteristic allows water to form distinct layers within an otherwise uniform liquid. This phenomenon explains why ice forms at the surface and does not sink. There are two ways that help visualize this phenomenon. First, think of diving down into a lake in the summer.
The top layer of water is very warm. As you swim deeper you often feel a distinct and sudden drop in temperature. Now imagine it is winter and the lake is frozen over. Solid ice floats and stays on top of the lake because it is less dense than liquid water, despite the ice being colder than the water it floats on. The weather keeps the water near the surface cool, making it less dense than the warmer water deep in the lake.
Ice also acts as an insulating blanket, preventing deeper lakes from freezing completely solid. Due to seasonal changes in sunlight intensity, surface water temperature begins to transition in the spring and fall. In the spring, the water surface warms. This causes the temperature of the top and bottom layers of the lake to equalize.
U niversity of W isconsin —Madison. Lake Mendota blowing in the wind — in March. Photo: A. Previous post: Water We Talking About? Next post: Water We Talking About? Toothy Fish and a Mussel Mystery.
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