Limnology Lab
ESS 328: Limnology
2 Nov2016
Thermal stratification in model lakes
Readings: Chapter 2 and 7 in Dodds and Whiles
INTRODUCTION
This exercise uses ordinary fish aquaria to illustrate the hydrodynamics of water in lakes
as it is subject to heating, cooling, and the action of wind. Specifically, we will examine how
light, ice, and wind interact to control thermal distributions in lakes during a complete annual
cycle (i.e., from winter ice cover to spring turnover to summer stratification to autumn turnover).
We also will examine the effects of wind energy on water currents. Of course, it is important to
remember that the aquaria are very simplified models of lake systems. Although these models
enable us to control thermal distributions and water currents with ease, we must keep in mind
that sloping sides and basin irregularities modify water movements in real lakes. Furthermore,
thermal stratification in real lakes is a much slower and more variable process.
MATERIALS
1. Aquarium with depth marked in 1 cm increments from surface to bottom
2. Thermometers
3. Lamps with 200-Watt bulbs suspended above the water
4. Styrofoam to insulate the tank
5. Dyes to trace water movement
6. A fan or hair dryer to create wind
7. Stopwatch
8. Strainer to remove ice
LABORATORY PROCEDURES
1. Add ice to your aquarium and mix until the lake is uniformly 40 C. Allow the system to
come to rest with a thin layer of ice floating on the surface.
2. Apply a moderate wind (1st wind) with a fan to the surface of the water by blowing from
one side and then the other. After a minute or two, record the temperature at depth
intervals 0.5, 1, 1.5, 2, 2.5, 3, 3.5, 4, 5, 7, 8, 9, 10, 13, 17, and 21 cm. Is the temperature
uniform? What is this temperature profile called?
3. Remove the ice with a strainer. Apply moderate to strong wind (2nd wind) for a few
minutes. Record temperatures. Is the temperature uniform?
4. Turn on the lamp. Record temperatures at the same depth increments after 0, 5, 10, 15,
20, 30, 40, 50, and 60 minutes, OR until the surface water temperature reaches 250C.
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5. When the surface water temperature reaches at least 250C and with the lamp still on,
scatter 6-9 drops of food coloring evenly over the surface of the water.
6. While most of the dye is still in the upper third of the water column, apply more wind
gently (3rd wind), blowing from one side and then the other. After a few minutes, the
upper portion of the water column should be well mixed. At this point, turn off the light
and stop the fan to allow the currents to slacken. Then record temperatures at each depth
interval. Also record the depth of the homogeneous colored layer.
7. Turn the fan on again (4th wind). Blow gently from one side only so that the surface
water builds up on one side resulting in a tilt of the thermocline of about 10 – 15 degrees.
You are creating an internal seiche, an oscillation of the thermocline. Turn off the fan
and time the period of oscillation using a stopwatch. A period is the amount of time
required to complete the up and down motion. Take the average of three measurements.
8. Allow the system to come to rest. Next, carefully add a layer of ice to the surface with
minimal disturbance of water. Observe the descending convection cells sinking through
the thermocline region. When the thermocline is at mid-depth, carefully remove the ice
and record temperatures at each depth.
9. Finally, apply a strong 5th wind to turn the lake over. When mixing is complete, record
the temperatures.
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STRATIFICATION LAB DATA SHEET
Depth
(cm)
Temp
After
Wind1
Temp
After
Wind2
Temp Before Wind 3 - During Heating
D1
3
g/cm
0
5
10
15
20
30
40
50
60
Temp
After
Wind3
D2
3
g/cm
Temp
After
Ice
0
0.5
1
1.5
2
2.5
3
3.5
4
5
6
7
8
9
10
13
17
21
3
Temp
After
Wind5
4
CALCULATIONS, DATA ANALYSIS, AND DISCUSSION QUESTIONS (25 POINTS)
1.
Plots of Temperature vs. Depth – Use a graphics software (e.g., EXCEL) to make plots of
temperature vs. depth. Make a separate graph for data from: Wind 1 (ice cover), Wind 2
(after removing ice), Prior to Wind 3 (just after heating), Wind 3, Prior to Wind 5 (after
adding and removing ice), and Wind 5. These graphs describe the thermal cycle of a
temperate, dimictic lake. Include these graphs along with the answers to the following
questions:
A. Identify the epilimnion, thermocline, and hypolimnion on the plot after Wind 3.
B. What processes determine the thermal patterns with depth during summer?,
winter?, fall?
C. Why is the temperature profile prior to Wind 3 different from the profile after
Wind 3?
D. What is the ecological significance of thermal stratification in summer?
E. What is the ecological significance of fall turnover?
2.
Plots of Density vs. Depth – Using the table of water density as a function of temperature,
plot water density as a function of depth for after Wind 2 (after removing ice) and before
Wind 3 (after heating) on the same graph. Include these graphs along with answers to the
following questions:
A. Using your understanding of the relationship between water temperature and
density with depth, why do some rivers flowing into reservoirs flow down into the
hypolimnion whereas others flow across the surface?
3.
Thermal Resistance to Mixing – This concept, developed by Birge in 1910, is defined as
the amount of work required to completely mix a column of water consisting of two or more
layers of different density. In practice, thermal resistance is estimated by comparing the
difference in density between the top and bottom of a defined thickness of water to the
density between water at 40 and 50C (i.e., where the density difference is least).
To do this, prepare a table as follows:
Depth
Interval
(cm)
Temp at
bottom
Density at
bottom
Temp at
Top
Density at
Top
Density
Difference
Density
Difference
/ .000008
0-1
1-2
2-3
Etc.
Plot the standardized density difference (last column) vs. depth. Do this for data from just
prior to Wind 3 and after Wind 3. Where is Thermal Resistance to Mixing maximal? What
insight doe the TRM plot give regarding the development of stratification and where in the
water column a thermocline develops?
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4.
Period of an Internal SeicheA. What was the period of the internal seiche you generated in the tank?
B. In your own words, define seiche. What is its limnological significance?
5.
Calculate the theoretical period of an internal seiche. The period of a seiche is the time
required for an oscillation to complete one cycle of an up-and-down motion. The period (T)
of a uninodal, internal seiche in a rectangular basin of uniform depth, when the two layers
have thicknesses of Ze and Zh (e=epilimnion, h=hypolimnion) and mean densities of the
epilimnion (De) and hypolimnion (Dh) is given by:
T
2L
g ( Dh  De )
Dh / Z h  De / Z e
Where L = length of the basin in the direction of the wind and g = acceleration of gravity
(980 cm/sec2).
Use density information obtained from your aquarium to estimate the theoretical period of an
internal seiche. The length of the aquarium (L) = 50 cm. The depth of the aquarium is 29cm.
What was the average period? Compare your theoretical calculation of the period of an
internal seiche to your direct estimate in lab. Why might they differ?
6.
What are the benefits to understanding the hydrodynamics of a real dimictic lake by
modeling them in lab? What are the disadvantages?
7.
Bonus: Try to create a time-depth isopleth diagram of temperature. These plots are a
much more efficient use of space than making numerous individual plots of temperature vs.
depth separately for each time, which may obscure any seasonal patterns. Graphical analysis
should clarify and elucidate patterns. Use your judgement to determine which isopleths make the
most sense given the data, and then connect uniform values with smooth lines. Remember,
isopleths can never cross, but they can exit and enter at the boundaries of the system (i.e., water
surface or sediment, if the system is a lake).
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EXAMPLE OF HOW A FIGURE SHOULD LOOK:
Figure X. Temperature as a function of depth during summer and winter simulations.
Temp (C)
0
5
10
15
20
25
0
3
Depth (cm)
6
9
12
15
18
Summer
Winter
21
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