Limnology Lab ESS 328: 2 Nov 2020

Thermal stratification in model

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 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. Salt to change water density 9. 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 gently with as little surface disturbance as possible. Apply moderate to strong wind (2nd wind) for a few minutes. Record temperatures. Is the temperature uniform and what is this pattern called? ______

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4. Turn on the lamps. Record temperatures at the same depth increments after 5, 10, 15, 20, 30, 40, 50, and 60 minutes, OR until the surface water temperature reaches 250C.

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 of about 10 – 15 degrees. You are creating an internal , 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

Temp Before Wind 3 - During Heating *Temp *Temp *Temp *Temp *Temp Depth After After D1 After D2 After After (cm) g/cm3 g/cm3 Wind1 Wind2 0 5 10 15 20 30 40 50 *60 Wind3 Ice Wind5

0 0.5 1 1.5 2 2.5 3 3.5 4 5 6 7 8 9 10 13 17 21

3 Water density as a function of temperature. Whole degrees are listed down the left hand side of the table, while tenths of a degree are listed across the top.

4 CALCULATIONS, DATA ANALYSIS, AND DISCUSSION QUESTIONS (20 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 , thermocline, and 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 estimate TRM, prepare a table as follows:

Depth Density Temp at Density at Temp at Density at Density Interval Difference bottom bottom Top Top Difference (cm) / .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 TRM maximal? What insight does the TRM plot give regarding the development of stratification and where in the water column a thermocline develops?

5 4. Period of an Internal Seiche-

A. 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:

2L T  g(Dh  De )

Dh / Zh  De / Ze

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 analyses 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: Note the figure caption is a concise phrase describing the figure, followed by complete sentences if needed for further clarification. The figure should be able to stand on its own without reference back to the text of the manuscript.

Figure X. Temperature as a function of depth during summer and winter simulations.

Temp (C)

0 5 10 15 20 25 0

3

6

9

12 Depth (cm) Depth

15

18 Summer Winter 21

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