Most people know of, or have heard of, “turnover”.
That’s when the lake water mixes from the surface to the bottom.
So what happens during the time when the lake isn’t “turning
The lake is stratified, that’s what.
A volume of water is heaviest at 4 degrees Celsius
(39.2 degrees F). That is just above freezing. The same volume
of water becomes lighter as it gets warmer. So in a lake, warm
lake water is at the top and the colder water is at the bottom
(except in winter—see below).
As the sun continues to heat the water at the top, the difference
in temperature between the top and bottom water becomes greater.
Eventually there are 2 distinct layers, the epilimnion
at the top and the hypolimnion at the bottom.
Between these 2 layers is a third, less distinct, transition layer
called the metalimnion.
Because of the temperature difference (and thus density difference)
between the epilimnion and hypolimnion, they don’t typically
mix together during the summer. It takes a major climactic event
to accomplish this, though the lake will mix in the autumn as
the surface water cools.
Often in the summer, the hypolimnion will become depleted of oxygen.
The bacteria responsible for decomposition consume the oxygen
and access to the atmosphere’s oxygen is cut off by the
why does water freeze from the top down?
If water becomes more dense as it gets colder, then it should
freeze from the bottom up, right? Well, water is most dense at
4 degrees Celsius (or 39.2 degrees F), which is warmer than freezing.
So as water continues to cool from 4 degrees C (39.2 degrees F),
it becomes less dense and rises back to the top, leaving the slightly
warmer water below.
At the surface, the cooler water is exposed to freezing air temperatures
and may eventually freeze. Once ice forms, the water beneath cannot
be mixed by the wind.
When the ice melts in the spring, the entire water column will
be at approximately 4 degrees C for a brief time. The lake will
mix thoroughly (“turn over”) with just a bit of wind.
A calm, warm day can heat the surface water and initiate the stratification
Stratification may also occur due to changes in salt content
as well as temperature. Oceans, particularly in places where freshwater
enters, may be stratified by salinity. The problems with Gulf
Hypoxia may be attributed partially to the inability of the
dense and salty bottom water to mix with the oxygen-rich, less
salty water above.
information, click here
Back to the Spring
2002 Water Line
Brought to you by the Lakes of
Missouri Volunteer Program