Activity: Investigating Seasonal Cycles
Summary: Students investigate how the unique properties of water contribute to seasonal cycles in bodies of water like the Great Lakes. By creating a micro-climate within an aquarium, students will use temperature, water density and weight to recreate seasonal conditions.
- 50 minutes
- Colored ice (frozen with food coloring added)
- Aquarium (5 gallon) containing cold tap water
- Sieve or aquarium net
- Heat lamp
- Rubber bands
- Masking tape
- Food coloring
- 300 ml beaker or glass jar
- 7 thermometers
- 2 plastic rulers
- Small fan
- 300 ml flask
- Safety glasses (must be worn at all times during this activity!)
Part 1 – Working as a whole class:
- Set up the aquarium as in Figure 1. Add enough plain ice (without food coloring) to the aquarium, stirring occasionally, to lower the temperature of the water to 4 degrees Centigrade (C) near the bottom. You should still have a shallow layer of ice on the surface, where the temperature will be 0 degrees C. Allow the water to come to rest.
- Students should record the temperature of the seven thermometers in Column 1 of the water temperature data chart.
- Remove the rest of the ice with a sieve or aquarium net as gently as possible. Turn on the heat lamp. Note: Be very careful not to splash any water on the heat lamp because it could explode!
- Students should record the current time as the start time on data sheet as the beginning of the experiment.
- After five minutes, students should record the temperatures of each of the seven thermometers in Column 2 on the data chart.
While waiting for the five minutes to pass:
- Students should graph temperature (degrees C) versus depth (cm) on Graph 1 (using data from Column 1 on data chart). Label the graph with the time of year.
- Ask students: What time of year do these temperatures represent?
- Answer: These temperatures represent winter before the spring turnover. Temperatures at most depths should be about 4 degrees C, with the surface water at about 0 degrees C.
- Label: Winter.
- At the 12-minute mark, students should record the temperatures on the seven thermometers in Column 3 of the data chart. Record the water temperatures again at 30 minutes and 45 minutes in Columns 4 and 5 of the data chart, respectively.
- As you wait for time to pass, pour 250 ml of ice water into a 300 ml flask. Weigh the flask.
- Ask students: what is the temperature and weight of this water?
- Answer: The water should be about 0-4 C degrees C. Specific weight will vary depending on temperature.
- Empty the flask and refill it with 250 ml of very hot tap water. Weigh and record weight of the flask with warm water.
- Ask students: what is the temperature and weight of this water? Which temperature of water is denser (weighs more)?
- Answer: This water will probably be about 30 degrees C. It should weigh somewhat less than the cold water, depending on the exact water temperature. (Only a sensitive balance will detect the difference based on temperature.)
- Answer: The cold water is denser. Water is most dense at 4 degrees C. It becomes less dense as temperatures increase from 4 degrees C. It also becomes less dense as temperatures decrease from 4 degrees C to 0 degrees C, at which point it freezes. This is why warm layers of water rest on top of cold layers during the summer and why cold water sinks in the fall during fall turnover.
- Pour the warm water into a beaker or glass jar. Gently add a few colored ice cubes to the warm water.
- Ask students: what happens to the colored water from the ice as the ice melts? Why?
- Answer: Ice floats on water because it is less dense than water. As the ice melts, its colored water will sink because it is colder than the warm tap water. Eventually a cold layer of water will form at the bottom of the warm water, illustrating the stratification that occurs in lakes.
- Student groups should determine which column on the data chart best represents spring turnover in the model lake.
- Students should use the data from this column to graph temperature (degrees C) versus depth (cm) on Graph 2. Label this graph: Spring Turnover.
As you move through the experiment, ask students the following questions:
- Which column on the data chart best represents spring turnover in the model lake?
- Answer: Data in Column 2 likely best represent spring turnover. The temperatures should be the same over all depths, or slightly warmer at the surface.
- As summer approaches, the lake continues to warm. Why do surface temperatures increase during the summer, while water temperatures at lower depths remain cold?
- Answer: Surface temperatures increase from warming by the sun and from the surrounding air. Not as much sunlight reaches the lower depths, thus these areas are not warmed as much.
- Which temperature data column best represents the stratified summer lake? Students should use the data from this column to graph temperature (degrees Centigrade) versus depth (cm) on Graph 3. Students should label graph: Summer Stratification.
- Answer: Data in Column 5 best represent the stratified summer lake. Temperatures will be warmest at the surface, coldest at the bottom and may show a sharp drop at some depth in between. This sharp drop occurs at the metalimnion. This graph should be labeled: Summer Stratification.
- What is the temperature in the epilimnion? In the hypolimnion? How much temperature difference is there between these two layers?
- Answer: Temperatures in the eplimnion (surface) may be 20 degrees C or higher. Temperatures in the hypolimnion (bottom) may be about 5 degrees C. Temperature differences may be 15 degrees C or more.
- Wind at the surface of the lake creates waves and stirs up the water, adding oxygen to it in the same way a bubbler works in a fish tank. When temperature layers form in the lake in the summer, the epilimnion is separated from the hypolimnion by the metalimnion. The metalimnion acts as a barrier. It keeps oxygen in the surface waters from reaching the cold waters of the hypolimnion. Gradually the oxygen in the hypolimnion gets used up. What happens to the animals living at the bottom of the lake when there is no oxygen?
- Answer: These animals may die from lack of oxygen or may be forced to move to areas of the lake where there is more oxygen (surface or near shore waters).
- As autumn begins, the surface waters of the lake begin to cool. Why do surface waters of the lake cool off during the fall?
- Answer: Surface water temperatures cool during the fall as less sunlight reaches them. In addition, air over the lake is cooler and the lakes give up heat to the air.
- Fall turnover occurs when all the lake water has cooled and has been completely mixed by water movements and wind. How might fall turnover be good for animals living at the bottom of the lake?
- Answer: Fall turnover mixes the water in the lake. This mixing resupplies oxygen and other nutrients needed by animals to all depths in the lake.
- After you have recorded the 45-minute temperatures, turn off the heat lamp.
- Turn on the fan and use it to create a strong wind by blowing on the water from one direction. Keep blowing until all the water in the lake is completely mixed.
- Students should record the water temperatures in Column 6 of the data chart.
- Ask students: What seasonal change in lake temperatures has happened with the addition of the wind? Are there any temperature layers in the lake once the water is mixed?
- Answer: Fall turnover has occurred. There should be no temperature layers in the lake and temperatures should be fairly similar at all depths.
- Carefully add ice to the surface water to create an ice layer. Try not to mix or disturb the water.
- Students should record the water temperatures in Column 7 of the data chart.
- Questions: What time of year is represented by these temperature data in the last column?
- Answer: These temperature data represent late fall as water temperatures cool and ice starts to form on the lake.
- Students should use the data from the last column to graph temperature (degrees C) versus depth (cm) on Graph 4.
- Ask: What season does Graph 4 represent?
- Answer: This graph should be labeled: Fall Turnover.
All of the Great Lakes stratify to a degree during the summer.
- Discuss the depths of the Great Lakes, as well as their major bays, such as Green Bay or Saginaw Bay.
- In what kinds of areas would the thermocline go very deep?
- Hypothesize where the thermocline would be found in each of the Great Lakes and why it varies.