Transcript Physical Properties of Water

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/lakemixing.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
Depth (m)
10
20
30
40
0
20
40
60
80
100
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
For a north temperate lake in the northern hemisphere:
0
0 ice
Mar
Isothermal,
Spring
Turnover 5
Temperature (oC)
10
15
Oct
20
25
Sep
2
Depth (m)
4
Jun
Nov
Fall
Turnover
6
May
8
10
Jan
Inverse
12
Stratification
14
April
Isothermal
Aug
Jul
Summer
Stratification
Temperature (oC)
0
2
0
5
10
15
20
25
Epilimnion
Depth (m)
4
Metalimnion
6
Thermocline
8
10
12
14
Hypolimnion
Thermocline is the
depth where temperature
changes the most; depth
controlled by solar radiation and
wind-driven mixing (fetch)
Why is warmer water at the surface?
Density (g/cm3)
1.000
0.998
0.996
0.994
0.992
0.990
0.920
0.900
0
10
20
30
Temperature (oC)
40
0
0 ice
Mar
Isothermal,
Spring
Turnover 5
Temperature (oC)
10
15
Oct
20
25
Sep
2
Jun
Nov
Fall
Turnover
Aug
6
Jul
Summer
Stratification
May
8
1.000
10
Jan
12 Inverse
Stratification
April
Isothermal
14
Density (g/cm3)
Depth (m)
4
0.998
0.996
0.994
0.992
0.990
0.920
0.900
0
10
20
30
Temperature (oC)
40
Why ice floats…
It’s all due to
Hydrogen bonding!
Photo credit: Midge Eliassen
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?
1.000
Density (g/mL)
0.998
0.996
0.994
0.0003
0.992
Density difference (per oC)
0.0002
0.990
0.920
0.0001
0.900
0
5
10
15
20
25
Temperature (oC)
30
35
Which lake (A, B, C) has the greatest Schmidt
stability?
0
B
Depth (m)
5
A
10
C
15
Lake A
Lake B
Lake C
20
25
2
4
6
8
10
12
14
Temperature (oC)
16
18
20
Which lake (A, B, C) has the greatest Schmidt
stability?
0
B
A
10
C
15
Lake A
Lake B
Lake C
1.000
Density (g/cm3)
Depth (m)
5
20
Stability of A > C > B
0.998
0.996
0.994
0.992
0.990
0.920
0.900
0
10
20
30
40
Temperature (oC)
25
2
4
6
8
10
12
14
Temperature (oC)
16
18
20
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
Temperature °C
Summer
Fall
Winter
Temperature °C
Temperature °C
Temperature °C
Lake Depth
Surface
Bottom
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
Apr
Jul
Oct
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.
Modified From Hutchinson and Löffler (1956)
6000
Altitude (m)
5000
polymictic
4000
3000
amictic
Usually
warm
monomictic
2000
1000
transitional
0
90
80
70
60
oligomictic
transitional
50
40
30
Degrees latitude
20
10
0
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.
The buoys of GLEON: sensor platforms from around the world
Yang Yuan Lake, Taiwan
Lake Sunapee, New Hampshire (USA)
Lake Rotorua, NZ
Lake Paajarvi, Finland
Trout Lake, Wisconsin (USA)
Lake Taihu, China
Lake Erken, Sweden
Slide courtesy of K. Weathers
Lake Mendota, (WI, USA)
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?