Project EDDIE: Lake Mixing

Carey, C.C., J.L. Klug, and R.L. Fuller. 1 August 2015. Project EDDIE: Lake Mixing Module. Project EDDIE Module 3, Version 1.

http://cemast.illinoisstate.edu/data-for-students/modules/lake-mixing.shtml.

Module development was supported by NSF DEB 1245707.

The heat in a lake comes primarily from solar heating.

Other heat sources:•Streams•Air•Ground•Sub-surface inputs (hot springs)

Light decreases exponentially with depth

Percent of surface light

0 20 40 60 80 100

Dep

th (

m)

0

10

20

30

40

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.

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

Temperature (oC)0 5 10 15 20 25

Dep

th (

m)

0

2

4

6

8

10

12

14

AprilIsothermal

JulSummerStratification

Aug

Jun

SepOct

JanInverseStratification

ice

MarIsothermal,Spring Turnover

NovFallTurnover

May

For a north temperate lake in the northern hemisphere:

Temperature (oC)0 5 10 15 20 25

Dep

th (

m)

0

2

4

6

8

10

12

14

Epilimnion

Hypolimnion

Thermocline

Metalimnion

Thermocline is the depth where temperaturechanges the most; depth controlled by solar radiation and wind-driven mixing (fetch)

Temperature (oC)

0 10 20 30 40

De

ns

ity

(g/c

m3 )

0.900

0.920

0.990

0.992

0.994

0.996

0.998

1.000

Why is warmer water at the surface?

Temperature (oC)0 5 10 15 20 25

Dep

th (

m)

0

2

4

6

8

10

12

14

Temperature (oC)

0 10 20 30 40

De

ns

ity

(g/c

m3 )

0.900

0.9200.990

0.992

0.994

0.996

0.998

1.000

AprilIsothermal

JulSummerStratification

Aug

Jun

SepOct

JanInverseStratification

ice

MarIsothermal,Spring Turnover

NovFallTurnover

May

Photo credit: Midge Eliassen

Why ice floats…

It’s all due to Hydrogen bonding!

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?

Temperature (oC)

0 5 10 15 20 25 30 35

De

ns

ity (

g/ m

L)

0.900

0.920

0.990

0.992

0.994

0.996

0.998

1.000

Density difference (per oC)

0.0001

0.0002

0.0003

Temperature (oC)

2 4 6 8 10 12 14 16 18 20

De

pth

(m

)0

5

10

15

20

25

Lake ALake BLake C

A

B

C

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

Temperature (oC)

2 4 6 8 10 12 14 16 18 20

De

pth

(m

)0

5

10

15

20

25

Lake ALake BLake C

A

B

C

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

Temperature (oC)

0 10 20 30 40

De

ns

ity

(g/c

m3)

0.900

0.920

0.990

0.992

0.994

0.996

0.998

1.000

Stability of A > C > B

We can classify lakes based on how often they mix per year

• Dimictic• Monomictic

– Warm– Cold

• Amictic• Oligomictic• Polymictic

Mixing regime

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

Dimictic Temperate Lake - Summer and winter stratification

Spring

Summer

Fall

Temperature °CTemperature °C

Surface

Bottom

Lake D

ep

th

Temperature °C

Winter

Temperature °C

Slide courtesy of K. Webster

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

High-frequency (10 minute) temperature measurements from Lake Sunapee, New Hampshire

Jan Jul OctApr Jan

Photos by M. Eliassen; figure from C.C. Carey; data courtesy of LSPA

Lake Sunapee’s stability over the year

Comparison of Schmidt Stability to temperature profile heat map.

What factors might explain variation in Schmidt Stability during summer stratification?

Let’s focus on winter in Lake Sunapee…

Modified from Bruesewitz, Carey, Richardson, Weathers (2015)

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?

Amictic -- Never mixes. Always stratified. Always covered with ice. Antarctica.

Lake Bonney, Antarctica (Photo courtesy of G. Simmons)

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.

Degrees latitude

0102030405060708090

Alt

itu

de

(m

)

0

1000

2000

3000

4000

5000

6000

polymictic

amictic

cold monomictic

war

m m

on

om

icti

c

dim

ictic

oligomictic

Usually warm monomictic

transitionaltransitional

Modified From Hutchinson and Löffler (1956)

Lake mixing module goals

1. Interpret variability in lake thermal depth profiles over a year.

2. Identify lake mixing regimes based on figures of water temperature.

3. Compare and contrast lake mixing regimes across lakes of different depths, size, and latitude.

4. Understand the drivers of lake mixing and thermal stratification.

5. Predict how climate change will affect lake thermal stratification and mixing.

Lake Rotorua, NZ

The buoys of GLEON: sensor platforms from around the worldLake Sunapee, New Hampshire (USA) Y

ang

Yu

an L

ake, Taiw

an

Lake Taihu, China

Lake Erken, Sweden

Trout Lake, Wisconsin (USA)

Lake Mendota, (WI, USA)

Lake Paajarvi, Finland

Slide courtesy of K. WeathersSlide courtesy of K. Weathers

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.

GLEON lake characteristics

Lillinonah Acton

Lacawac Annie

Feeagh Mügglesee

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.

Discussion

1. What were the mixing regimes for each lake? 2. Which lakes had the highest Schmidt

stability? What factors might relate to stability?

3. How would climate change affect stratification?

4. What are the implications of altered stability for the six study lakes?

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

Figure from Hipsey et al. 2014

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

Lake Mendota 2011: No change

Lake Mendota 2011: +3oC Air temperature

Lake Mendota 2011: +5oC Air temperature

Discussion

1. Compare the thermal heat maps for 2011, 2011 +3oC and 2011 +5oC. How are they similar, and how are they different?

2. 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?

3. What are some of the assumptions that went into this model output? Are they realistic?

Discussion, continued

4. What is the effect of increased air temperatures on Schmidt stability? Why?

5. As air temperature continues to increase, are the effects on water temperature and stability likely to be linear? Why or why not?

6. What are the implications of higher temperature on lake oxygen concentrations? Phytoplankton? Zooplankton? Fish?