BIOGRAPHY OF A RIVER

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9-12 HELP! LAKE OVERTURNING!OBJECTIVESThe student will do the following:1. Define the terms associated with this lesson.2. Explain the process of overturning and its importance toaquatic life.BACKGROUND INFORMATIONOverturning refers to the phenomenon of a lake exchanging waterfrom the bottom to the top and from the top to the bottom. Thisprocess is very important to the health of the lake’s aquatic life.Overturning is also an important factor in the recovery of a lakeafter it has been contaminated with a pollutant. Lakes and pondsare lentic systems, which means that the surface water is standingas opposed to lotic or flowing systems, such as rivers. It is muchharder for a lake to recover from pollution than it is for a river, evena slow flowing one, because the time period for a total overturningof a lake may be as frequently as annually or as long as 10 to 100years. During this time, pollutants may build up and reach lethallevels, especially if the influx of contaminants is constant or in largevolumes.SUBJECTS:Science (Biology, Earth Science,Environmental Science)TIME:2 class periodsMATERIALS:student handouts, one per student4 large sheets of drawing paper and 4boxes of crayonshot plate2 beakersfood coloringtable salt2 large glass containers with openmouthrefrigerator or ice cubesfanladlefunneltubingTo understand the process of overturning, it is important to know two different categories of zones intowhich lakes may be divided. One category is based on the depth of the water; the other is based on thetemperature of the water.The first set of zones is determined by the depth of the water that, in turn, determines the amount of sunlightthat is able to penetrate the water. The amount of sunlight determines the amount and type of plant life andthus the amount and type of aquatic animal life. The three zones in this classification are:y y
1. the littoral zone: shallow waters along a lakeshore where rooted vegetation grows; largestamount of sunlight penetration.2. the limnetic zone: open water zone of a lake through which sunlight penetrates; located parallelthe lake bottom and extending between the littoral zones; contains algae and other microscopicorganisms.3. the profundal zone: deepest area of a lake; located from the limnetic zone to the bottomof the lake; no sunlight penetrates; contains decomposing vegetation.There is no set size for the zones because they change as other factors change. For example, if the lakefills with silt from construction runoff, the bottom will rise and the shoreline will expand causing a largerlittoral zone. Also, the limnetic zone will thin because light penetration will decrease.The second set of zones involved in the rate of lake overturn is based on temperature (thermal zones). Thethree zones within this classification are:1. the epilimnion: the upper waters nearest the surface of a lake.2. the thermocline or metalimnion: the transition zone between the top (epilimnion) and bottom(hypolimnion) zone.3. the hypolimnion: the deepest waters in a lake.The temperature zones of a lake change due to wind movement, solar intensity, and air temperature; thus,they are seasonal. During each season, the lake takes on different characteristics. The differences in watertemperature among the zones lead to the movement of water and results in the process of overturning. Acloser look at the seasonal changes shows how the process of overturning operates and brings changes tothe lake. Note that this kind of overturning only occurs in temperate lakes that partially freeze in winter andthaw in spring. Some lakes in other climatic settings can also overturn, but the mechanisms are different.Spring: As the ice from winter melts, the cold water sinks to the bottom. This is because ice is less densethan water, but water that is only slightly above the freezing temperature is denser than warmer water.Eventually the lake reaches a state in which all water is at the same temperature. There are no longer threedistinct zones, only one zone. This unlayered water is easily mixed by brisk spring winds, and the oxygenand nutrients from the bottom are evenly distributed throughout the lake. This period is called the springoverturn.Summer: The sun warms the surface of the lake, but the mild summer winds cannot mix this warmer waterwith the other zones. The lake again forms temperature zones, but this time the low-density surface layeris warm water. The thermocline has warm water above and cool water below. This reduces the amounty y
Page 4: B. Read descriptions of rivers in l
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Diseases Organisms Associated WithM
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lactose: a type of simple sugar tha
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A. Students could research the diff
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3-73Student /Teacher Sheet
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Turbidity affects fish and aquatic
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E. The disc should be lowered until
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CLEAN CLOTHES - CLEANENVIRONMENT?9-
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eutrophication: the process in whic
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Chiras, Daniel D., Environmental Sc
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B. Present terms and define.PROCEDU
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observe effects of iron on plants.
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THERMAL POLLUTIONOBJECTIVESThe stud
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D. The teacher should do the agitat
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BIOGRAPHY OF A RIVER

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