In the spring,
after ice-out in
climates, the water column is cold and
with depth. The intense sunlight of spring is absorbed in the water
column, which also heats up as the average daily temperature of the
air increases. In the absence of wind, a temperature profile
with depth might be expected to resemble Figure 2 (see the Light
section), decreasing exponentially with depth. However, density,
another physical characteristic of water, plays an important role in
modifying this pattern.
from most other compounds because it is less dense as a solid than as
a liquid. Consequently ice floats, while water at temperatures just
above freezing sinks. As most compounds change from a liquid to a solid,
the molecules become more tightly packed and consequently the compound
is denser as a solid than as a liquid. Water, in contrast, is most dense
at 4°C and becomes less dense at both higher and lower temperatures.
The density/temperature relationship of fresh water is shown in Figure
3. Because of this density-temperature relationship, many lakes in temperate
climates tend to stratify, that is, they separate into distinct layers.
of the upper Midwest and at higher elevations, the water near a lakes
bottom will usually be at 4°C just before the lake's ice
cover melts in the spring. Water above that layer will be cooler, approaching
0°C just under the ice. As the weather warms, the ice melts.
The surface water heats up and therefore it decreases in density. When
the temperature (density) of the surface water equals the bottom water,
very little wind energy is needed to mix the lake completely. This is
After this spring turnover, the surface water continues
to absorb heat and warms. As the temperature rises, the water becomes
lighter than the water below. For a while winds may still mix the lake
from bottom to top, but eventually the upper water becomes too warm
and too buoyant to mix completely with the denser deeper water. As Figure
3 suggests, the relatively large differences in density at higher temperatures
are very effective at preventing mixing. It simply takes too much energy
to mix the water any deeper.
is useful to visualize a more extreme example of density stratification.
Imagine a bottle of salad dressing containing vegetable oil and
vinegar. The oil is lighter (more buoyant) than the vinegar which
is mostly water. When you shake it up you are supplying the energy
to overcome the buoyant force, so the two fluids can be uniformly
mixed together. However, if allowed to stand undisturbed, the
more buoyant (less dense) oil will float to the top and a two-layer
system will develop.
some cases, such as happened at Ice Lake in April, 1998 and 1999,
the surface water may warm up rapidly immediately after ice-out,
causing the lake to stratify thermally without completely mixing.
This prevents atmospheric oxygen from reaching the bottom waters.
As a consequence, the entire water column never reaches 100% oxygen
saturation. This can be observed for Ice Lake by comparing temperature
and oxygen profiles from March 5, 1998 (still frozen), April 18,
1998 (the lake was completely ice-free on April 11, 1998), and
April 30, 1998.
progresses, the temperature (and density) differences between upper
and lower water layers become more distinct. Deep lakes generally become
physically stratified into
three identifiable layers, known as the epilimnion,
(Figure 4). The epilimnion is the upper,
warm layer, and is typically well mixed. Below the epilimnion is the
metalimnion or thermocline region, a
layer of water in which the temperature declines rapidly with depth.
The hypolimnion is the bottom layer of colder water, isolated from the
epilimnion by the metalimnion. The density change at the metalimnion
acts as a physical barrier that prevents mixing of the upper and lower
layers for several months during the summer.
of mixing depends in part on the exposure of the lake to wind
but is most closely related to the lakes size. Smaller to moderately-sized
lakes (50 to 1000 acres) reasonably may be expected to stratify and
be well mixed to a depth of 37 meters in north temperate climates.
Larger lakes may be well mixed to a depth of 1015 meters in summer
(e.g., Western Lake Superior near Duluth, MN).
that although "thermocline" is a term often used synonymously
with metalimnion, it is actually the plane or surface of maximum
rate of decrease of temperature with respect to depth. Thus, the
thermocline is the point of maximum temperature change within
As the weather
cools during autumn, the epilimnion cools
too, reducing the density difference between it and
(Figure 5). As time passes, winds mix the lake to greater depths, and
the thermocline gradually deepens. When
surface and bottom waters approach the same temperature and density,
autumn winds can mix the entire lake; the lake is said to "turn over."
As the atmosphere cools, the surface water continues to cool until it
freezes. A less distinct density stratification than that seen in summer
develops under the ice during winter. Most of the water column is isothermal
at a temperature of 4°C, which is denser than the colder, lighter
water just below the ice. In this case the stratification is much less
stable, because the density difference between 0°C and 4°C
water is quite small. However, the water column is isolated from wind-induced
turbulence by its cap of ice. Therefore, the layering persists throughout
the winter. You will need to download the Quicktime
Plug-in from Apple to view these movies.
1 - Here's what happens when warmer water (green) enters the surface
of a lake in winter. The second addition shows that the warm water
is buoyant (less dense) than the cold water and therefore rises.
2 - Here's what happens when colder water enters a summer-stratified
3 - Same movie 2 without the dyed green epilimnion.
4 - See what happens to the epilimnion (mixed layer) and thermocline
during a storm. Did the lake mix?
5 - Same as movie 4, but with increased turbulence. See what starts
to happen when the class 5 tornado hits.
6 - Shows how stream sediment entering a lake or reservoir deposits
its load. Why does some material stay in the upper layer and some
crash to the bottom?
7 - An estuary is a 2-layer system with freshwater overlying salt
water. Here we see how freshwater behaves when added to each layer.
8 - Same as movie 7, but here we introduce water that is saltier
than the upper freshwater layer. Example: Hurricanes can "throw" huge
amounts of saltwater into coastal lakes. What happens to this water
and what might its impact be?
(spring turnover summer stratification fall turnover
winter stratification) is typical for temperate
lakes. Lakes with this pattern of two mixing periods are referred to
as dimictic. Many shallow lakes, however,
do not stratify in the summer, or stratify for short periods only, throughout
the summer. Lakes that stratify and destratify numerous times within
a summer are known as polymictic lakes.
Both polymictic and dimictic lakes are common in Minnesota.
installing the RUSS unit in Ice Lake we have made an interesting
observation. Spring turnover is incomplete. There was not enough
mixing in spring, 1998 or 1999 to completely re-aerate the entire
water column to 100% saturation. On the other hand, Lake Independence,
a lake of comparable depth (15-18 meters) but much larger in size
(more fetch) and less sheltered from the wind, mixed completely.
We suspect that most aquatic scientists would not have expected
to see Ice Lakes bottom water, nearly saturated with oxygen
in fall, 1998, to be anoxic by mid-winter and then persist in
this state until the following fall. Once stratified thermally
in summer, even the barrage of severe thunderstorms that occurred
near Ice Lake in summer, 1999, lacked the energy to dramatically
decrease the thermocline or increase the oxygen content of the
hypolimnion. Heat and Oxygen
budget section of Ice Lake.
was cold and windy enough during fall, 1998 for Ice Lake to mix
thoroughly, bringing oxygen to the bottom waters (to about 100%
saturation). This is likely typical for Ice Lake during most autumns,
although it is possible for a cold, calm period to allow the lake
surface to freeze before the water column has been fully exposed
to the atmosphere and re-charged with oxygen.
Upper Lake Station of Lake Minnetonka, Lake Independence and thousands
of other Upper Midwestern lakes that are relatively deep (>10 meters)
and reasonably large (>100-200 acres or 40-80 ha) are probably dimictic,
leading to complete re-oxygenation of the water column for at least
some period of time. Ice Lake, though small (41 acres or 16.6ha), is
sheltered and deep (16 m) for its size. Lakes that have formed in former
open pit mines in Northeastern Minnesota are unusually deep for their
size. Lakes with these characteristics probably only mix completely
once a year in the fall for a brief period before freezing. Some of
the deeper mine pit lakes (>75 meters deep) probably never mix completely
to the bottom, although data are sparse.
common are lakes that circulate incompletely resulting in a layer of
bottom water that remains stagnant. To distinguish them from
(mixing from top to bottom) lakes, these partially mixing lakes are
referred to as meromictic. They mix partially,
in the sense that they may have extensive mixing periods which go quite
deeply into the hypolimnion, but they do not turn over completely, and
a layer of bottom water remains stagnant and anoxic for years at a time.
The non-mixing bottom layer is known as the monimolimnion
and is separated from
(the zone that mixes completely at least once a year) by
(Figure 6). The stagnant, and typically anaerobic, monimolimnion has
a high concentration of dissolved solids compared to the mixolimnion.
In general, meromictic lakes have large relative depths. These lakes are typically small and sheltered from the wind
by the morphology of their basin. In this case, the density differences
caused by temperature are smaller than density differences due to the
high dissolved solids (salts) concentration of the monimolimnion. Large
lakes that rarely freeze over are also typically monomictic, mixing
throughout the fall, winter and spring and stratifying in the summer.
visualize this effect, try dissolving several tablespoons of table
(NaCl) in hot water. Add a few drops of food coloring and then
a mayonnaise jar half-full. Now, very gently add cool tap water
a small measuring cup to fill the glass. Set up a second jar half
full with clear, cool water and then add the colored hot water
to fill the glass
- but don't add the salt. Compare the stability of the density
in the two systems by gently shaking or stirring the water
Minnetonka (Minneapolis, MN)
acres (5,670 ha)
Lake (Sandstone, MN)
acres (200 ha)
Independence (Minneapolis, MN)
acres (344 ha)
Lake (Duluth, MN)
acres (200 ha)
x 106 acres (2.6 x 106 ha)
x 106 acres (6.0 x 106 ha)
Lake* (Grand Rapids, MN)
acres (16.6 ha)
x 106 acres (5.8 x 106 ha)
x 106 acres (2.0 x 106 ha)
x 106 acres (8.2 x106 ha)
acres (49,900 ha)
Mead (NV largest US reservoir)
acres (66,096 ha)
lakes & ponds
Lacs Lake, MN
acres (53,648 ha)
Louis River and Duluth-Superior Harbor
acres (4,856 ha)
mixed because of
Pit Lake (Ely, MN)
acres (56 ha)
Pit Lake, (Crosby, MN)
acres (23 ha)
Lake (Minneapolis, MN)
acres (7.3 ha)
Lake (Itasca State Park, MN)
acres (5.0 ha)
Soda Lake (Fallon, NV)
acres (160 ha)
variable from year to year
about ARCHIMEDES's principle at EXPLORATORIUM
and a shockwave demonstration of density
and water displacement.