This classroom connector addresses Instructional Objective 2.02.

Now that we have learned some techniques to study land ecosystems, we will find some ways to study lake ecosystems. Our state has several man-made as well as natural lakes. Which lake are you most familiar with? (response) What do you know about this lake? (response) Do you know how it was formed? (response) We will study the characteristics of a lake and see if some of these conditions we have already studied affect lakes.

As an example of an ecosystem, we shall examine a lake in a little more detail, considering the physical characteristics of the lake and the community of organisms within it.

All ecosystems have structure, in horizontal and vertical dimensions but also in time, reflecting changes at different hours or seasons. Lakes have sharp boundaries with surrounding land and air, so they provide clear examples of ecosystem structure.

As light passes down through waters of a lake, it is gradually absorbed and scattered. Phytoplankton near the surface use the light in photosynthesis. Here the color of the light becomes green, partly because the chlorophyll of the algae selectively absorbs blue and red wavelengths. Further down there occurs a certain depth (depends on the clarity of the water) at which oxygen production by photosynthesis equals the consumption of oxygen by all organisms. This is known as the compensation depth. The region above this depth is known as the eutropic zone, in which more organic matter is being made during photosynthesis than is being consumed. In the region below the compensation depth, decomposition is taking place.

Donation of a lake according to light intensity has a profound influence on the distribution of organisms. Shallow parts of the eutropic zone, referred to as the littoral zone, contains various rooted aquatic plants such as water lilies, rushes and sedges. In the open surfaces of the lake, called the limnetic zone, the dominant organisms are floating algae, herbivorous rotifers, small arthropods and carnivorous animals.

Beneath the limnetic zone, the chief input of energy is dead organic matter that falls from above. Various kinds of decomposers, fish, and invertebrates consume this dead matter or one another. On the lake bottom, the decomposers use up dissolved oxygen as they convert dead organic matter back into inorganic nutrients. In highly productive lakes, so much dead matter falls to the bottom ooze, or benthic zone, that little dissolved oxygen is left in the water near the bottom. Under these conditions the dominant decomposers are anaerobic bacteria. Where there is more oxygen in the benthic region, a variety of flatworms, protists, clams, crustaceans, and insect larvae may live. Most of these species can tolerate severe oxygen deficiency.

More aquatic life is present in the littoral zone around the lake edges where sunlight penetrates to the bottom and supports rooted aquatic plants. The shallow waters are sometime covered with duckweeds and a variety of animals such as insect larvae, rotifers, crustaceans, hydras, snails, and flatworms. Closer to the shore are plants such as pond lilies, bullrushes, and cattails. Dragonflies and damselflies lay their eggs on stems of these plants just below the water line, and the surrounding waters teem at certain seasons with tadpoles, fish fry, leeches, and insects such as diving beetle and water boatmen. Pickerel, sunfish, and other fish find plenty of food and shelter among the plants.

Temperature has a great influence on the seasonal activities in a lake. Water is the most dense at 4C. As a result, water at 4C sinks beneath water that is warmer or colder. In winter water cooled below 4C rises above water at 4C and at 0C the surface water freezes. Under the ice the water remains at 0C to 4C with a slight rise in temperature at the bottom due to heat coming from the underlying bedrock; plants and animals survive in the water under the ice. In spring, the ice melts and the sun warms the surface waters, which sink as they approach 4C, forcing colder water below to rise to the surface. This spring overturn is important because it brings nutrients from the bottom to the surface and carries oxygen down to deeper waters.

In the summer you may have been swimming in a lake where the surface layer is warm, but at your toes you feel much colder water. This is because the sun-warmed surface waters stay on top and do not mix with the denser colder water underneath. As a result the lake becomes thermally stratified, with an upper layer of warm water, epilimnion, and a deeper layer hypolimnion which never warms much above 4C. Between these two layers there is a narrow region of very rapid temperature drop called the thermocline. During the summer the depth of the epilimnion increases to as much as 20 meters. In the fall, the thermocline begins to rise again as the air and surface water temperatures fall, eventually leading to the fall overturn. Once again the nutrients are brought to the surface and oxygen is mixed in the deep waters. Trout fishermen know that spring and fall are good times to fish because trout can be found in cold rising surface waters. Trout need oxygen-rich water, and warm water holds less oxygen than cold water. Lakes in temperate regions go through two overturns each year. In colder regions there is only one overturn.

Lakes may also be divided into categories based on their production of organic matter. Eutrophic (good food) lakes are shallow and rich in organic matter and nutrients. Oxygen depletion occurs in the hypolimnion during summer due to a heavy accumulation of dead organic matter and the rapid rate of oxygen used by aerobic decomposers. These lakes have rich production of organic matter. Oligotrophic (very little food) lakes are usually deeper with steep sides and a poorly developed littoral zone. These lakes are poor in nutrients such as phosphorus, calcium, and nitrogen. They have few organisms and very little organic matter. Their water is usually very clear, with very deep water, always containing oxygen. Through time in the normal course of events, lakes usually become eutrophic becoming filled in with sediment and organic matter. It will eventually turn into a bog or marsh and finally dry land. For a deep lake, the process may take millions of years.

Problem: How can you make a secchi disk and use it to test light penetration?

Procedure:


1. The secchi disk is made from the top of a 10 tin can.

2. Drill or pound a hole in the middle, insert the bolt, and secure the nut.

3. Paint the disk in alternating panels of black and white using an oil paint.

4. Attach a heavy cord or rope that has been knotted in regular intervals of three to six inches. If needed, be sure to sand the edges of the metal so they are smooth and safe. (you may make a larger disk, if you have facilities to cut metal)

5. Black and white are standard colors for secchi disks, and if you use these colors, then you can compare your measurements to those obtained with the standard purchased disk.

(When near water, exert safety precautions with students at all times. If they are to go out in a boat to take readings, be sure they wear life jackets, even if they can swim. When standing on a dock or bank, be sure they are supervised and watched.)

6. Demonstrate the proper use of the secchi disk before students begin their experiments.

7. Shade the water with an umbrella or drop the disk over the shaded side of the boat.

8. The observer's eye should be one meter away from the surface of the water. Measure with a tape or meter stick. You may also count the knots on the rope. This will get the disk at the correct depth.

9. Lower the disk into the water until it cannot be seen. Record the depth by counting the knots on the rope.

10. Raise the disk slowly until it is visible again.

11. Record this measurement. Take an average of the two readings, and this average is the limit of visibility measurement

12. Carefully study the conditions where the reading was taken. Compare the testing conditions to the following standard conditions:

a. clear sky
b. sun directly overhead
c. shaded area
d. minimum ripple
e. distance of one meter from the observer's eye to surface of water.

Data:

Observer______ time of day______ condition of water surface______sky condition______ measurement location_____ day_____date_____ man-made or natural body of water_____ plant life along shore_____animal life along shore_____ plant life in the water_____condition of soil around water_____animal life in the water_____secchi disk disappears at_____ depth,_____ depth,_____ average depth_____

(The class can be divided into small groups to allow more than one group to take readings at the same place to see if they agree. Allow different groups to take readings in different places in the same body of water and compare readings. Can they find reasons for any differences they discover?)

Conclusions:
The average limit of visibility was __ m. What effect did the limit of visibility have on the plant and animal life in the result from the limit of visibility you measured? __ Did the conditions you observed result from the limit of visibility you measured?__

(When students begin to ask questions about the kinds of plant and animal life in certain bodies of water, the answers, in part, depend upon the clarity and purity of the water. Turbid water prevents light penetration, thus affecting all plant and animal life. It also determines, to some extent, the amount and kinds of life the body of water can support. Limited visibility in water generally indicates the presence of pollution in one form or another, either natural or man-made.)

(The use of the secchi disk in this lesson presents an opportunity for pupils to begin an investigation in aquatic life in your area. They will become involved in recording scientific data in the study of ecology. Whether investigating a pond, creek, river, or lake, a measurement of the limit of visibility in conjunction with other information about the body of water will help reveal some of the important ecological relationships in operation.)

Problem:


1. To determine the limits of visibility in bodies of water.

2. To compare the limits of visibility in different bodies of water and in different locations in the same body of water.

3. To calculate the average of two measurements in determining the limit of visibility.

4. To measure the distance of one meter from the eye to the surface of the water.

5. To hypothesize the effect of the limit of visibility upon plant and animal life in the water.

(Determine the parts of the problem you wish to solves based on the abilities and understanding of your students.)

Concepts:

1. A secchi disk is an instrument used to determine the limit of visibility in a body of water.

2. There are many factors which affect the accuracy of the reading. Among these are: the person observing, time of day, condition of the water surface, sky conditions, and the particular location where the measurement is taken.

3. The amount of visibility in a body of water has an effect upon the type and the quantity of plant and animal life found there.

4. Water that is cloudy or discolored is called turbid.

Materials:
Secchi disk, meter stick, notebook, pencil. (If you do not wish to compare your data to a purchased disk, just buy a disk. It is a good activity to make a disk, so you can compare data to purchased disk.)

The following activities may be performed:
1. In the classroom, set up a turbidity display showing water in jars with varying degrees of cloudiness. What substances can be used to make water turbid?

2. Try growing some algae, such as the kind used in fish aquariums, in water with varying amounts of turbidity. What conclusions can be made about plant growth in turbid water?

3. What are some possible causes of turbid water in rivers and lakes? What can be done to correct the problem?

4. You may purchase test kits made by Hach and other companies so you can test water in lakes and stream. Some tests you may run are for dissolved oxygen, C02, temperature, density, nutrients such as calcium, phosphorus, ammonia, pH, etc.

(Be sure to explain that light penetration affects the zones in a lake, the plants, animals that live in each zone, and whether respiration for photosynthesis is taking place.)

You have made a secchi disk and used it to study our lake. What did we find out about the secchi disk?

(It disappears at the boundary of light penetration.) We found out some other factors that affect plant and animal life in a lake. Can you list some of these?

(Conditions such as temp, time of year, zone of lake where disk was dropped, etc.) In our study you have made some comparisons of the lake with our school ecosystem. (Temperature, light, animals and plants are present.)

benthic zone - bottom ooze of a lake where much dead organic matter is converted into inorganic nutrients; little dissolved oxygen is left in the water near the bottom

compensation depth - the depth of a lake at which oxygen production by photosynthesis equals oxygen being used by all organisms

epilimnion - upper layer of warm water in a lake which may reach twenty-five degrees at the surface

eutropic zone - region of a lake in which more organic matter is formed than is consumed

eutrophic - lakes are relatively shallow and rich in organic matter and nutrients

humus - decomposed material in soil necessary for plant growth

hypolimnion - deeper layer near the bottom of a lake which never warms four degrees centigrade

limnetic zone - open surfaces of the lake dominated by floating algae, herbivorous rotifers, small arthropods, and carnivorous animals

littoral zone - shallow parts of the eutropic zone which contain various rooted aquatic plants such as water lilies, rushes, and sedges

nutrients - materials required by cells

oligotrophic - lakes characterized by greater depth, steeper sides, and a poorly developed littoral zone; these lakes are poor in nutrients and contain few organisms and little organic matter

optimal temperature - the best temperature at which a living thing can function

photosynthesis - the process by which plants convert carbon dioxide and water into simple sugar; chlorophyll and sunlight are essential to the series of complex chemical reactions involved

precipitation - form of moisture such as rain, snow, hail, and sleet

relative humidity - amount of water vapor in the air compared with the amount of water it can hold at a given temperature

thermocline - very narrow region of rapid temperature drop

This is the time this file has been accessed since 11/16/96.

The University of Tennessee at Martin is not responsible for the information or views expressed here.

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