In the spring,
immediately after ice-out
climates, the water
column is cold and nearly isothermal
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
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
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
progresses, the temperature (and density) differences between upper
and lower water layers become more distinct. Deep lakes generally become
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 (its fetch),
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 the hypolimnion
(Figure 5). As time passes, winds mix the lake to greater depths, and
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
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.
some videos that demonstrate density stratification.
- Movie 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.
- Movie 2
- Here's what happens when colder water enters a summer-stratified lake.
- Movie 3
- Same as movie 2 without the dyed green epilimnion.
- Movie 4
- See what happens to the epilimnion (mixed layer) and thermocline
during a storm. Did the lake mix?
- Movie 5
- Same as movie 4, but with increased turbulence. See what starts
to happen when the class 5 tornado hits.
- Movie 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?
- Movie 7
- An estuary is a 2-layer system with freshwater overlying salt
water. Here we see how freshwater behaves when added to each layer.
- Movie 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
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.
the data files under each lake page
or review the entire data set using the Profile
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 the holomictic
(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
and is separated from the mixolimnion
(the zone that mixes completely at least once a year) by the chemocline
(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.