Assignment- STRATIFICATION AND MIXING IN LAKES
Introduction:
Thermal stratification of freshwater lakes, as the name implies, is controlled by temperature- driven density gradients with depth. Thermal stratification and mixing are key physical processes that have important impacts on biogeochemical processes such as primary productivity, nutrient cycling and dissolved oxygen levels and therefore lake biology (e.g., fish habitat, phytoplankton ecology).
You will use a simple lake model (an aquarium) to look at stratification and mixing in lakes. The benefits of using simple model systems include reduced size and complexity, which make it easier to understand the mechanisms and processes at play. Model systems also facilitate experimentation that would otherwise be difficult or prohibitively expensive to carry out with a natural system. However, it is important to understand that models also have limitations due to their oversimplified nature; therefore unjustifiable extrapolations can be made from the model to nature if the observer is not careful to distinguish between the two. For example, the action of heat and wind will be illustrated separately, while in natural systems the two act in concert.
Furthermore, the aquarium used in today's exercise is not insulated and the sides and bottom are in contact with the air, which facilitates more rapid heat exchange between the model lake and its surroundings than is the case in natural systems.
References (This exercise is based on material from the following):
Vallentyne, J.R. 1967. A simplified model of a lake for instructional use. J. Fish. Res. Bd. Canada 24: 2473-2479.
Wetzel, R.G.; Likens, G.E. 2000. Limnological Analyses (3rd edition). Springer-Verlag, New York (429 pp.).
Apparatus:
1. An aquarium, corresponding to a lake basin, equipped with temperature probes and a digitial readout.
2. A stopwatch (or cell phone with timer, etc.)
3. A heat source (heat lamp) and stand. Be careful not to splash water on the lamp (explosion!)
4. Wind source (fan or blower)
5. Dye (methylene blue crystals)
6. Ice cubes
7. Slotted spoon or strainer (to remove ice)
Procedure:
1. Prepare a data table, either on paper or using excel, with columns for time (minutes elapsed and temperatures at depths of 1, 3, 5, 9, 13, 17 and 21 cm (corresponding to T0 through T6 on the display).
2. Fill the aquarium with cold tap water (as cold as you can get) and mix well. Record temperature at the different depths (hint: they should all be more or less the same); these are your initial or springtime conditions.
3. Turn on the heat lamp, record this time as the beginning of the experiment. Record temperature at the various depths in the model lake at intervals of 3 minutes, for a total of 18 minutes.
4. Effect of heat: With the lamp still on, sprinkle a small quantity of methylene blue lightly on one half of the surface of the model lake (exercise care when handling this chemical, avoid contact with eyes). Allow sufficient time (~3 minutes) for the dye to dissolve and sink so that most of the descending trails are in the upper third of the aquarium with a few extending into the middle third and fewer still to the bottom third. With some luck, you may be able to observe the position of the trails and the complexities of the water motion, particularly near the surface. Record temperature at the various depth (total elapsed heating time since start of experiment = 21 minutes)
5. Effect of wind: After the 21 minute total heating time has elapsed, introduce a light-to- moderate wind blowing first from one side and then the other so as to mix the surface waters into one homothermal mass. Avoid too strong a wind, which could destroy the thermocline that you are now creating. The epilimnion (with homogeneous distribution of colour) should now be 3-5 cm thick. Keep recording temperatures as a function of depth every 3 minutes for an additional 6 minutes. At the end of the 6 minutes (i.e., 27 minutes into the experiment), turn off the lamp and allow the currents to settle down. Record the depth of the epilimnion, as well as (summer) temperatures as a function of depth (in the "end of step 5" column).
6. Effect of cooling: To simulate the cool nights of fall when the warm lake loses heat to the cooler air, carefully add a layer of ice cubes to the surface with minimal disturbance of the water. It will be necessary to cover ~¾ of the surface, depending on the size of the ice cubes. Observe the descending convection cells sinking through the thermocline region, which in turn cause the thermocline to descend. When the thermocline has descended to about mid- depth in the tank, use the slotted spoon to carefully remove the remaining ice with as little turbulence as possible. Record temperature at the different depths (in the "end of step 6" column)
7. Effect of fall storms: Use a strong wind to turn the lake over (do not splash water onto the heat lamp!), simulating fall storms. When mixing is complete, record the temperatures.
8. Hand-in datasheet: Make sure your datasheet is filled out completely, including the names of your group members, and submit it to your TA before leaving. Ensure each group member also has a copy of the data (you will need it for the assignment).
Data:
1. Use the data you collected in conjunction with the data set provided on to the portal (under
Course Materials à Assignment #3 Dataset) to generate the graphs described below.
2. Figure 1a: Use the data you collected to plot temperature vs. depth at the start (t = 0 min), middle (t = 9 min) and end (t = 18 min) of the heating period, and after the application of wind, such that you end up with four different lines on the same graph. Plot the graph with "Depth" on the y-axis and temperature on the x-axis. Depth values can either be plotted as negative values, or in Excel you can select the option to "plot values in reverse order" under the "edit axis" options. This way the surface of the lake will be at the top of the graph and the deeper portions of the lake near the bottom of the graph, which is a more intuitive way of looking at this type of data. Now repeat this with the data provided to create Figure 1b.
3. Figure 2: As above, plot temperature vs. depth before the addition of ice (i.e, collected at the end of step 5, equivalent to mid-summer), just before the removal of ice (step 6), and after the application of a strong wind (step 7). In this case you should then end up with 3 lines (or temperature profiles) on the same graph. Do this with both the data you collected and the data provided.
Questions:
1. Refer to Figure 1 to answer this question. During the initial heating phase of the lake, you likely observed an exponential decrease in temperature with depth. Why is that? (Hint: think about how solar radiation is absorbed as it penetrates into the lake water column.)
2. Was the application of heat alone (step 4) sufficient to establish the tripartite thermal structure of epiliminion, thermocline and hypolimnion normally observed in temperate lakes during summer? What was the role of wind (step 5) in establishing this three-part thermal structure? Use Figure 1 to support your answer.
3. Do the graphs generated using your data (Figs. 1a and 2a) look like the ones generated from the well-behaved data provided (Figs. 1b and 2b)? If not, what are some potential reasons for any differences observed?
4. The density of water is a function of both temperature and salinity. For pure water (i.e., salinity = 0), density (in units of g/mL) can be calculated using the following equation:
Density = 1 - T + 288.9414/(508929.2 *(T + 68.12963))*(T - 3.9863)2
What is the density of water at temperatures of 5, 15, 20 and 25 °C? Report your answer to five decimal places. A simple data table is sufficient for this question. You may wish to use a spreadsheet to complete these calculations, but please show your steps in completing the density calculation for at least one of the four temperatures.
5. Consider two lakes of identical morphology (i.e., identical depth, area, volume and basin shape). The first is a temperate lake with an epilimnetic temperature of 15 °C and a hypolimnetic tempature of 5 °C ; and the second is a tropical lake with an epilimnetic temperature of 25 °C and a hypolimnetic temperature of 20 °C. You may answer parts "A" to "C" with a simple 1-word answer.
a. Which lake has the greater difference in temperature between surface and bottom?
b. Given your answer to question 3 above, which lake has the greater density difference between epilimnion and hypolimnion?
c. Which lake would you expect to exhibit more stable stratification and therefore be more resistant to mixing?
d. Is it the temperature difference or the density difference between epilimnion and hypolimnion that is more important in determining stability and resistance to mixing?
6. It has been predicted that climate change will result in longer duration of summer stratification (i.e., more time between spring and fall turnover), as well as increased rates of primary productivity and respiration in temperate lakes. Explain why this could be cause for concern, and what measures you might recommend to minimize any detrimental impacts.
You can calculate the heat budget of a lake from the difference in water temperature between the winter minimum and summer maximum. Each layer of water in the lake has to be considered separately (because temperature varies with depth) and the heat absorbed by each layer summed up to obtain the total heat absorbed by the lake. The table provided already includes the various layers or depth intervals that you will need for your calculation, simply follow the steps (A to C) outlined below.
A. For each row (layer), calculate the volume of water in the layer. The aquarium used for the experiment has a length of 40 cm and a width of 25 cm (and a total depth of 23 cm); Recall also that the volume of water in each layer can be obtained using the formula:
Volume (cm3) = length (cm) x width (cm) x thickness of layer (cm)
B. Next fill in the relevant data on the minimum temperature recorded at each depth (i.e., at T =
0) and repeat for the maximum temperature recorded at each depth (i.e., at end of step 5). Calculate the temperature difference between the maximum and minimum values recorded at each depth.
C. Now you are ready to calculate the amount of heat absorbed by each layer of water. Use the following formula, along with the water volumes calculated in step (A) and the temperature change calculated in step (B):
Heat absorbed (J) = Temperature change (K) x Specific Heat of water (J g-1 K-1) x Water mass (g)
You may assume in this case that for water, 1 cm3 = 1 mL = 1 g, such that the mass of water in each layer is the same as the volume in cm3 you calculated in step (A). The specific heat of water is equal to 4.179 J g-1 K-1.
The heat budget for the model lake can now be obtained simply by adding up the heat absorbed by each layer.
How much heat, in joules, was absorbed by the model lake between (late) winter and summer?
The lamp used to heat the model lake had a power output of 300 W (equivalent to 300 J per second).
Calculate how much heat, in joules, was applied to the lake by the lamp over the time it took to reach maximum temperature? Hint: you will need to convert the time to seconds and remember that the lamp output was 300 W (or 300 J s-1).
Finally, calculate the percent of the applied heat that was absorbed by the lake, and if this value is not equal to 100%, very briefly explain why this might be.
Attachment:- Dataset.xlsx