When Water Rolls OverWhen Water Rolls Over

An introduction to thermal stratification and turnover in lakes, and interpretation

of temperature profile data.

An introduction to thermal stratification and turnover in lakes, and interpretation

of temperature profile data.

Introduction – the curious properties of water

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.

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?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

When different temperature layers form, it is called stratification

A common stratification pattern in a temperate lake in the summerA common stratification pattern in a temperate lake in the summer

Upper, warm layer; well-

mixed

Upper, warm layer; well-

mixed

Commonly called the

thermocline, because the temperature

declines rapidly with

depth

Commonly called the

thermocline, because the temperature

declines rapidly with

depth

Cold, deep water,

isolated from mixing

by the thermocline

Cold, deep water,

isolated from mixing

by the thermocline

Mixing is caused by wind. Why doesn’t the wind mix the whole lake?

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.

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 densityLow density

Thermocline – density barrierThermocline – density barrier

High densityHigh density

Seasonal changes: winter and summerSeasonal changes: winter and summer

Winter and summer data from Ice Lake, MinnesotaWinter and summer data from Ice Lake, Minnesota

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!

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!

Temperature °C

Seasonal Changes: spring and fallSeasonal Changes: spring and fallWhat 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)

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 density.

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)

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 density.

A typical temperate lake during turnoverA typical temperate lake during turnover

Turnover pattern in a deep temperate lake

Turnover pattern in a deep temperate lake

What are the effects of lake turnover?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.

• 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?What affects lake turnover?

The timing, duration, frequency, and extent of turnover are affected by several factors:

The timing, duration, frequency, and extent of turnover are affected by several factors:

Wind Solar radiation input Depth

Lake size Lake bottom topography

Air temperature

Presence of winter ice

Streams

entering lake

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 lakesTypes 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.

• 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 lakesTypes 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.

• 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?

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?

13th Lake, North River, NY: a dependable dimictic lake.What part of the annual cycle do you think this is? 13th Lake, North River, NY: a dependable dimictic lake.What part of the annual cycle do you think this is?

My name is Moose

My name is Moose

Let’s practice looking at some real data

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 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.

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!

• 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?

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?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?

SvalbardSvalbard

Lake LinneLake 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

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?

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?

Provided by Dr. Al Werner, REU project, Svalbard

Lake Linne 2006-07 Water Temperatures, Mooring G

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?

Provided by Dr. Al Werner, REU project, Svalbard

In conclusion…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.

• 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…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%202006/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/)