1. Project EDDIE: Lake Mixing
Carey, C.C., J.L. Klug, and R.L. Fuller. 1 August Project EDDIE: Lake Mixing Module. Project EDDIE Module 3, Version 1.
Module development was supported by NSF DEB

2. The heat in a lake comes primarily from solar heating.
Other heat sources:
Streams
Air
Ground
Sub-surface inputs (hot springs)

3. Light decreases exponentially with depth

4. So, wouldn’t we expect temperature to have the same pattern, since that light is getting converted to heat?
Yes, but that is not what we often find.

5. Why is measuring temperature at depth important?
If the temperature is isothermal throughout the profile, we can assume that the water column is able to mix
Why is mixing important?
Oxygen
Nutrients
Organisms and other particles
Conversely, if the temperature is not isothermal, we assume that the water column is stratified

6. For a north temperate lake in the northern hemisphere:
Mar
Isothermal,
Spring
Turnover
Jan
Inverse
Stratification
ice
Nov
Fall
Turnover
Jun
April
Isothermal
May
Jul
Summer
Stratification
Oct
Aug
Sep

7. Epilimnion
Hypolimnion
Thermocline
Metalimnion
Thermocline is the
depth where temperature
changes the most; depth controlled by solar radiation and wind-driven mixing (fetch)

8. Why is warmer water at the surface?

9. Mar Isothermal, Spring Turnover Jan Inverse Stratification ice May Nov
Fall
Turnover
Jun
April
Isothermal
Oct
Sep
Jul
Summer
Stratification
Aug
As the lake gets warmer, thermocline gets shallower
During the spring, summer, and fall, you continously see warmer water at the surface, and colder water at depth- due to the density gradient
However! in winter, when there is ice that is at 0oC, it is less dense than 4oC water; why ice floats. Inverse stratifcation!
Why a lake doesn’t freeze solid- allows fish to persist through the winter!

10. Why ice floats… It’s all due to Hydrogen bonding!
Why ice floats- colder water on top of warmer water because max density is at 4oC, not 0oC
Photo credit: Midge Eliassen

11. Stability -- the degree to which lake stratification resists mixing by the wind
Stability depends on the difference in density between layers
Schmidt stability- quantity of work required to mix the entire volume of water to a uniform temperature
How much wind energy is needed to mix the lake?

12. Slope decreases more rapidly- difference in water temperature between 25 and 26 is much greater than difference between 5 and 6oC

13. Which lake (A, B, C) has the greatest Schmidt stability?

14. Which lake (A, B, C) has the greatest Schmidt stability?
Stability of A > C > B

15. We can classify lakes based on how often they mix per year
Dimictic
Monomictic
Warm
Cold
Amictic
Oligomictic
Polymictic
Mixing regime

16. Dimictic = two periods of mixing per year:
summer stratification
fall turnover (mixing)
winter inverse stratification
spring turnover (mixing)
Typical of northern latitudes
Must have winter ice cover

17. Dimictic Temperate Lake - Summer and winter stratification
Spring
Summer
Fall
Winter
Temperature °C
Surface
Bottom
Lake Depth
Temperature °C
Temperature °C
Temperature °C
Slide courtesy of K. Webster

18. Dimictic = two periods of mixing per year: summer stratification
fall turnover (mixing)
winter inverse stratification
spring turnover (mixing)
Mountain Lake, VA; Horne and Goldman 1994

19. High-frequency (10 minute) temperature measurements from Lake Sunapee, New Hampshire
The new way of presenting these data; rather than lines that are connected, can use high-frequency data to see changes in time
Thermocline deepens throughout season due to increasing winds for lake
Jan
Apr
Jul
Oct
Jan
Photos by M. Eliassen; figure from C.C. Carey; data courtesy of LSPA

20. Lake Sunapee’s stability over the year

21. Comparison of Schmidt Stability to temperature profile heat map.
What factors might explain variation in Schmidt Stability during summer stratification?

22. Let’s focus on winter in Lake Sunapee…
Modified from Bruesewitz, Carey, Richardson, Weathers (2015)

23. monomictic -- one period of mixing
warm monomictic stratifies in summer and mixes all winter (no ice)
cold monomictic stratifies in winter (under ice) and mixes in “summer”
polymictic -- mix frequently throughout the year
Where would you expect to find lakes with these different mixing regimes?

24. Amictic -- Never mixes. Always stratified. Always covered with ice
Amictic -- Never mixes. Always stratified. Always covered with ice. Antarctica.
Lake Bonney, Antarctica (Photo courtesy of G. Simmons)

25. Are there lakes that are always “summer” stratified
Are there lakes that are always “summer” stratified? Maybe, but they usually mix occasionally, so they are called oligomictic.
Oligomictic = Thermally-stratified much of the year but cool sufficiently for rare short mixing periods.
They occur in the tropics and since there is no cold season, they do not have a cold hypolimnion.

26. Usually warm monomictic Modified From Hutchinson and Löffler (1956)
polymictic
amictic
cold monomictic
warm monomictic
dimictic
oligomictic
Usually warm monomictic
transitional
Modified From Hutchinson and Löffler (1956)

27. Lake mixing module goals
Interpret variability in lake thermal depth profiles over a year.
Identify lake mixing regimes based on figures of water temperature.
Compare and contrast lake mixing regimes across lakes of different depths, size, and latitude.
Understand the drivers of lake mixing and thermal stratification.
Predict how climate change will affect lake thermal stratification and mixing.

28. The buoys of GLEON: sensor platforms from around the world
Lake Sunapee, New Hampshire (USA)
Yang Yuan Lake, Taiwan
Lake Rotorua, NZ
Lake Paajarvi, Finland
Trout Lake, Wisconsin (USA)
Lake Taihu, China
Lake Mendota, (WI, USA)
Lake Erken, Sweden
Slide courtesy of K. Weathers

29. Activity A
Divide into groups, with at least one laptop per group and each group assigned to one lake
Access the color temperature figure for your lake, Excel data files (separate tabs for each lake), and student instructions handout.
Follow directions on handout for Activity A.

30. GLEON lake characteristics

31. Lillinonah
Acton
Lacawac
Annie
Feeagh
Mügglesee

32. Activity B
Divide into groups, with at least one laptop per group and each group assigned to one lake
Access the color temperature figure for your lake, Excel data files (separate tabs for each lake), and student instructions handout.
Follow directions on handout for Activity B.

33. Discussion What were the mixing regimes for each lake?
Which lakes had the highest Schmidt stability? What factors might relate to stability?
How would climate change affect stratification?
What are the implications of altered stability for the six study lakes?

34. Activity C How will lake thermal structure respond to altered climate?
To answer this question, we will use a lake model called GLM (General Lake Model) in which we can manipulate air temperatures and explore the effects on lake mixing and stratification

35. Figure from Hipsey et al. 2014

36. Air temperature simulations
We will add +3oC and +5oC to all air temperature observations for Lake Mendota, Wisconsin, USA during the ice-free period of 2011
We can then compare the resulting output from the baseline (simulated 2011) and +3oC and +5oC scenarios to see the effects on the Mendota thermal profiles over time
Create a figure that shows the time series of Schmidt stability from the three simulations on the same plot

37. Lake Mendota 2011: No change
Lake Mendota 2011: +3oC Air temperature
Lake Mendota 2011: +5oC Air temperature

38. Discussion
Compare the thermal heat maps for 2011, oC and oC. How are they similar, and how are they different?
What are the effects of the 3 and 5oC increases in air temperatures on water temperature over time at 0m? 20m? What limnological mechanisms might explain these patterns?
What are some of the assumptions that went into this model output? Are they realistic?

39. Discussion, continued
What is the effect of increased air temperatures on Schmidt stability? Why?
As air temperature continues to increase, are the effects on water temperature and stability likely to be linear? Why or why not?
What are the implications of higher temperature on lake oxygen concentrations? Phytoplankton? Zooplankton? Fish?