Seasonal Lake Stratification
Lakes within the Great Lakes region undergo seasonal
changes with regard to their temperature-density profiles. These
changes directly influence various characteristics of the lake. To
begin with, one must understand the temperature-density relationship for
water. For the most part, as water increases in temperature it becomes
less dense. Conversely, water becomes more dense as it decreases
in temperature. The exception to this rule is that water reaches
its maximum density at approximately 4° Celsius. Below 4°C,
as water cools the number of water molecules joined together by hydrogen
bonds to form loose clusters increases. Because of the formation
of these structured aggregates, below 4°C water actually becomes less
dense as it cools. The molecules in ice form a very structured, open
framework, so ice itself is less dense than water and, consequently, it
floats. With this concept in mind, seasonal or thermal stratification
within lakes can be explored
Changes in the temperature profile with depth within
a lake system is called thermal stratification. This profile changes
from one season to the next and creates a cyclical pattern that is repeated
from year to year. Let us begin with spring. After the ice
melts on a lake, the lake water is generally the same temperature from
the surface to the bottom. Wind allows circulation and mixing of
the lake water. Surface water can be pushed to the lake bottom and
bottom water can rise to the surface (Figure 1). This circulation
pattern is very important in that it allows relatively large amounts of
oxygen to reach the bottom of the lake. Otherwise, oxygen would have
to reach the bottom by the relatively slow process of diffusion.
The mixing of the lake water at this time of year is called spring overturn.
As air temperatures rise in late spring, heat from the
sun begins to warm the lake. As the amount of solar radiation absorbed
decreases with depth, the lake heats from the surface down. The warm
water is less dense than the colder water below resulting in a layer of
warm water that floats over the cold water. The layer of warm water
at the surface of the lake is called the epilimnion. The cold layer
below the epilimnion is called the hypolimnion. These two layers
are separated by a layer of water which rapidly changes temperature with
depth. This is called the thermocline (or metalimnion). The
three distinct layers of water, each with a different temperature or range
of temperatures, is an excellent example of thermal stratification within
a lake system. Figure 2 below shows how the depth of the epilimnion
increases through the spring and into the early summer.
During the summer the epilimnion will reach a maximum
depth and stratification will be maintained for the remainder of the summer.
The warm water, abundant sunlight, and nutrients brought up from the lake
bottom during spring overturn provide an ideal environment for algae growth
within the epilimnion. Algal blooms tend to give the epilimnion a
greenish hue. Stratification during the summer acts as a deterrent
to complete lake mixing. Wind circulates the surface water, but the
warm water of the epilimnion is unable to drive through the cold, dense
water of the hypolimnion. As a result, the water is only mixed in
the epilimnion (Figure 3).
Without mixing to provide dissolved oxygen, the lake
bottom, lacking enough light for photosynthesis to occur, tends to have
a very limited supply of oxygen during the summer. Respiration by
animals and bacteria can deplete the dissolved oxygen at the bottom of
the lake. A stratified lake of this nature is said to be in summer
stagnation. Dead algae sink to the lake bottom and are decomposed
by bacteria. This accelerates the depletion of dissolved oxygen in
the hypolimnion as aerobic bacteria use oxygen to decompose the wealth
of organic material raining down from the epilimnion. During summer
stagnation the lake bottom can become anoxic (i.e., without oxygen) and
anaerobic bacteria begin to decompose organic material without the aid
of dissolved oxygen. If dead algae accumulate at a faster rate than
bacteria decompose the organic matter, sediment deposited in the lake will
be rich in organics. This is likely because without thorough mixing
to provide the surface water with nutrients from the bottom, the algae
eventually begin to limit the available nutrients in the epilimnion. Lack
of available nutrients can cause large die-offs of algae, adding to the
organic matter on the lake bottom. Frequently, anaerobic bacteria
produce hydrogen sulfide gas (H2S), so the organic-rich
sediment may have the odor of “rotten eggs”. Some of the sulfur in
the H2S may combine with iron to form pyrite or “fool’s
gold” (FeS2). For example, when a core composed
of fine sediment is taken from the bottom of Lake Michigan and is cut open,
commonly dark laminations are observed, which disappear within an hour.
The dark material is likely pyrite that oxidizes (combines with oxygen)
to iron oxide when exposed to air.
As autumn approaches and temperatures decrease,
the epilimnion begins to decrease in depth (Figure 4). Eventually
the epilimnion gets so shallow that it can no longer be maintained as a
separate layer and the lake loses its stratification. Thus, as in
the spring, the lake water in the autumn has generally uniform temperatures
(about 4°C in late autumn), and wind can once again thoroughly mix
the lake water. In addition, surface water, which is in direct contact
with the cold air, gets cooled faster than the water below. This
cold, dense water sinks and further helps to mix the lake, and once more
oxygen and nutrients are replenished throughout the lake (Figure 5).
This process is called autumn overturn.
As winter approaches, the surface water is eventually
cooled below 4° C. At this point, the water no longer sinks.
The water molecules begin to align themselves (form more hydrogen bonds)
to solidify. As water temperatures at the surface reach 0°C,
ice begins to cover the surface of the lake. During the winter, ice
cover prevents wind from mixing the lake water. Again, stratification
can occur. A layer of low density water colder than 4°C, but
warmer than 0°C forms just under the ice. Below this water, the
remainder of the lake water is usually near 4° C. At this point,
a lake is said to be in winter stagnation (Figure 6). As spring approaches,
the seasonal cycle begins again.
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LIVING WITH THE GREAT LAKES
BROUGHT TO YOU BY:
GRAND VALLEY STATE UNIVERSITY
DEPARTMENT OF GEOLOGY
ALLENDALE, MICHIGAN 49401