When Water Rolls Over
An introduction to thermal stratification
and turnover in lakes, and interpretation of
temperature profile data.
Introduction – the curious properties
of water
Did you ever wonder why ice floats? Most substances
get more dense as they cool. Not water! It is most
dense at 4°C (39°F). As it continues cooling
beyond that, density decreases.
So what, you ask? What’s
the big deal? Well, if water
didn’t have that one single
property, life on Earth
would not exist as we
know it. Let’s explore this.
How does this apply to lakes?
Tetlin Lake, AK
In a lake, warm water will stay near the surface,
and cold water will stay below…
except
when it doesn’t! If the temperature of the whole
lake is 4°C or below, the warmer (4°),
denser water will be at the bottom, and the
colder water, which might be frozen, will be
found at the surface.
Kluane Lake, Yukon, Canada
When different temperature layers form, it
is called stratification
Upper, warm
layer; wellmixed
Cold, deep
water,
isolated
from mixing
by the
thermocline
A common stratification pattern in a temperate lake in the summer
Commonly
called the
thermocline,
because the
temperature
declines
rapidly with
depth
Mixing is caused by wind. Why doesn’t
the wind mix the whole lake?
As the surface warms, the density
gradient increases. The different
densities create a physical barrier
that prevents the surface water
from mixing with the water below.
The thermocline (metalimnion) is
that barrier.
Low density
Thermocline – density barrier
High density
Seasonal changes: winter and summer
Temperature °C
Notice the reverse
stratification in the
winter, when the
lake is frozen.
Below the ice, there
is no stratification,
because there is no
solar input and no
effect of air
temperature.
In summer, Ice Lake
shows a very steep
thermocline!
Winter and summer data from Ice Lake, Minnesota
Seasonal Changes: spring and fall
What is happening here?
As the epilimnion cools because of cooling air temperature
and less solar input, the thermal stratification breaks down
and the thermocline starts to disappear. When the upper
temperatures cool to the same temperature of the
hypolimnion, the entire lake is the same density and now
the surface wind can mix the whole water body. This
process is called turnover.
This temperature profile is called an isotherm (iso = same,
therm = temperature)
A typical temperate lake
during turnover
Notice that the temperature does not have to be 4°C for
fall turnover to happen. It is dependent on the
temperature of the hypolimnion.
This same pattern is often seen in spring, when the surface
water warms to 4°, and the whole lake is the same
Turnover pattern in a deep
temperate lake
What are the effects of lake turnover?
• Nutrients that were trapped in the hypolimnion are distributed
throughout the lake.
• Oxygen that has formed near the surface from phytoplankton and
aeration is distributed to the deep water. This allows fish to move to
deeper water, and is essential for winter fish survival.
• At the same time, anoxic water is brought to the surface. In some
small eutrophic ponds with little oxygen, this can cause fish kills.
• Turnover often causes sulphurous gases and products of decay that
were in the hypolimnion to be brought to the surface and released,
which also enables fish to return to the depths. This can cause a
noticeable smell.
• The water may become temporarily cloudy (increased turbidity)
because of the nutrients and material brought up from the depths.
What affects lake turnover?
The timing, duration, frequency, and extent of
turnover are affected by several factors:
Solar radiation input
Lake size
Presence of
winter ice
That’s a lot of information! Let it sink in while looking at this beautiful lake.
Portage Lake, Whittier, AK, May 2012 (Portage Glacier in center)
Can you tell what part of the annual cycle this is?
Types of circulation in lakes
• Amixis – no circulation. These lakes are permanently frozen.
Found in the Arctic (few) and Antarctica, and some very high
elevations. Climate change is causing a decrease in this type.
• Meromixis – incomplete circulation. Usually a very deep lake
that has a stagnant bottom layer called the monolimnion.
Often this is caused by increased dissolved substances, which
increase the density of the bottom water. A density gradient
(independent of temperature) is formed below the
hypolimnion.
• Holomixis – The entire (holo = whole) lake is mixed during
turnover. These are the lakes we are exploring in this
presentation. There are several types of holomictic lakes,
caused by the factors mentioned earlier.
Types of holomictic lakes
• Oligomictic – Have poor (oligo) mixing. Usually found in the
tropics, with warm water at all depths and little or no
seasonal change.
• Polymictic – Mix often (poly) or continuously, affected mostly
by daily temperature fluctuation. Usually small, shallow lakes
found in warm climates, the desert, or high altitudes.
• Monomictic – one period of circulation per year. Two types:
Cold monomixis – found in polar regions and frozen much of
the year. Not enough summer warming for much stratification
to occur. Warm monomixis – lacking ice cover in winter, they
circulate in winter and stratify in summer.
• Dimictic – two periods of circulation per year – the classic
spring and fall turnover. Freezes in winter, stratifies in
summer. Most temperate lakes are dimictic.
Very often, a lake doesn’t fall neatly into
any one of these categories. Some years
there might be different circulation and
stratification patterns than others. Once
again, this is all dependent on the
conditions mentioned above. Can you
name them?
My
name is
Moose
13th Lake, North River, NY: a dependable dimictic lake.
What part of the annual cycle do you think this is?
Let’s practice looking at some real data
We’ll start with
Ice Lake, in
Grand Rapids,
Minnesota
This is Ice Lake on a single day after the spring turnover. Notice the fairly even
dissolve oxygen content (from mixing). Thermal stratification is just beginning.
This is Ice Lake on the same day the previous spring, in 1999. Notice that there
is much more thermal stratification already, warmer surface temperature, and a
radical decline in dissolved oxygen (DO) with depth, to the point where aquatic
life, including fish, cannot survive below about 8 meters.
What can cause the 1999 situation?
There are a few possibilities!
• A rapid spring warm-up causing early thermal
stratification, before mixing was complete
• Lack of spring wind
• A sudden influx of nutrients, causing eutrophic
conditions as bacterial decomposition uses up
the oxygen
This is where you come in! Brainstorm some of
your own ideas and further questions!
These graphs are from
1 ½ months later, in
the same two years.
What do you see here?
Do you see the
difference in thermal
stratification?
What happened to the
DO level in 1999? Why
do you think there is a
spike at 4m depth?
Trout need 80% DO
saturation, and levels
below 5 mg/L (or ppm)
cannot sustain life.
Can you tell that
mixing of lake water is
extremely important?
Just for fun, let’s look at another type
of thermal stratification graphic
Can you tell
when the fall
turnover took
place?
What can you
tell about spring
turnover?
Has the lake
frozen by the
end of
December?
Lake Mendota is in Madison, WI. It is a hotbed for limnological
research at the Center for Limnology, UW-Madison
(limnology – the study of fresh water. Limne = lake, ology = study of)
Here’s some data from Lake Mendota
Compare the fall and spring turnover periods. Which showed more consistent mixing?
What can you tell about ice formation during the winter of 2010/11 vs. 2011/12?
Let’s go north now. Lake Linne (or Linnevatnet) is in part of Norway
called Svalbard that is in the High Arctic, at about 78°N. It is a
gold mine for geological and glacial studies.
Svalbard
Lake Linne
Lake Linne, Svalbard, Norway
Photo by Missy Holzer
Lake Linne is an Arctic, glacier-fed lake with no outlet. It is a holomictic
lake, but what kind? Let’s explore its thermal profile…
(a map of the depth of Lake Linne)
Temperature profiles were taken at the
lettered locations. The following two slides
show data from Mooring C, relatively
shallow water greatly affected by the
stream inflow; and Mooring G, deeper
water in the center.
Mooring G
Mooring C
image by Benjamin Schupack
Lake Linne 2006-07 Water Temperature, Mooring C
Provided by Dr. Al Werner,
REU project, Svalbard
What do you see in this graph (besides cold water)? The lake remained frozen until June.
After that (right side), you can see a lot of fluctuation. This is caused by wind, inflow from
the glacier stream, which is laden with sediment, and by the surface ice moving around as it
melted. In fall (left), the temperature is very uniform at all depths, with good mixing. Do you
see any summer stratification? Barely! How would you classify this lake?
Lake Linne 2006-07 Water Temperatures, Mooring G
Provided by Dr. Al Werner,
REU project, Svalbard
Be careful comparing this to the last one, because the temperature scale is smaller. How does
the bottom temperature in winter compare to the shallower water at Mooring C? Do you see
less spring fluctuation? Why? What can you conclude about summer temperature in 2006 vs.
2007 from these graphs? What kind of “mixis” does Lake Linne demonstrate?
In conclusion…
• You can learn a lot by studying the temperature profile of
a lake!
• Most lakes undergo periods of mixing, or turnover,
though there is a large amount of variation; every lake is
unique.
• Learning how to interpret graphics is very important in
science. Practice practice!
• Comparing the temperature profile with other data, such
as dissolved oxygen, dissolved solids, suspended
sediment, biological activity, land use, and more will
reveal the lake’s secrets.
• Comparing data over the course of several years can yield
valuable information about weather patterns and climate
change.
Questions for further exploration…
• What do you think might happen in a dimictic temperate lake
that used to freeze in winter, and now no longer does?
• What about a polar lake (such as Lake Linne), if the climate
became warmer? Remember, Lake Linne is glacier fed, and
increased temperature means increased meltwater inflow.
• What causes a steep thermocline? A shallow one?
• What might be the source of sulphurous gases in the
hypolimnion?
• Some of the lakes in this presentation (Ice Lake, Lake
Mendota) are in cities. What might be the effects of increased
road salt that enters a lake, especially a deep lake?
• Finally, can you imagine what the world would be like if water
got increasingly more dense as it cools below 4°C and
freezes? Brainstorm and describe your ideas.
References
•
•
•
•
•
Cole (1979) Textbook of Limnology, C.V Mosby Co., St. Louis, MO
http://infosyahara.org/temp_mendota
http://www.waterontheweb.org
http://www.ourlake.org/html/thermocline.html
Schupack (2007), thesis
http://helios.hampshire.edu/~srNS/Svalbard/Student%20Theses/Ben%20Schupak%20
2006/Schupack%20final%20thesis%20compressed.pdf
• http://limnology.wisc.edu/lake_information/mendota_&_other_
Y.html
• Missy Holzer, PolarTREC teacher, Svalbard, Norway
• Dr. Al Werner, Mt. Holyoke College and REU program, Svalbard,
Norway
• Dan Frost, PolarTREC teacher, Svalbard
• The good folks at PolarTREC (http://www.polartrec.com/)