John Cooke

Alive & Kicking

The Restless Lake

By John Cooke

There is personal satisfaction to be found in understanding the "secrets" hidden in nature's common things. Minnesota's lakes have a yearly seasonal cycle that profoundly infl uences their ecology and, therefore, their biology.

I know of only two materials that expand as they cool. When mercury or water change from liquid state to solid state (freezing), each expands and, therefore, becomes less dense. This expansion causes ice to float.

Imagine: without this expansion, ice would sink to the lake bottom and likely build year after year, causing the lake to be solid ice all winter and in spring and summer thaw only to the extent determined by the depth of the lake and the heat available.

You can forget the fish and most other lake life under these conditions. The lake food chain would be vastly altered if it survived at all.

But, thankfully, water does expand when it freezes, and we subsequently have ice houses and snowmobile races on the frozen surface!

As water cools, it is most dense at 4 degrees C (39 degrees F). At temperatures colder than 4 degrees C, the water begins to expand and become less dense and seeks the surface.

Let's begin the yearly lake cycle in the early spring. Bald Eagle Lake is covered with ice. The temperature is 4 degrees C under the ice to the lake bottom. The fish houses have been removed; but the few items left scattered on the ice are about ready to float.

As the lake warms and the melt continues, the whole lake enters a temperature phase of equal density at 4 degrees C. Conditions are prime for a phenomenon known as spring turnover.

The downwind surface water stacks, causing the deep water to roll to the surface. This wind-dependent movement brings mineral-rich deep water to the surface and oxygen-rich surface water to the depths. Some lakes appear muddy after turnover, but the stage is set for the spring algae bloom, supplying energy for the food web, which feeds the lake life.

Solar warmth continues to warm the upper water and a thermocline develops. What is thermocline? Lake swimmers are fully aware of an abrupt point in depth where the water is suddenly colder: this is thermocline.

The wind and powerboats mix the warming surface water.

With stronger winds, the thermocline goes deeper. Warmer water continues to heat as summer progresses, to the delight of water sports fans.

Fall sets in and winter arrives.

Surface water begins to cool as cold wind blows across the land and lakes, causing surface water to become more dense than deeper water. Fall turnover occurs. Again, the deep lake water seeks the surface and the less dense water from below is taken to the top.

As the lake continues to lose heat, water at the surface reaches 4 degrees C and heads for the bottom. When the whole lake is at 4 degrees C, the surface continues to lose energy and ice rapidly forms, expanding and floating as surface water above cools to 0 degrees C.

Life below the ice survives in the cold, non-freezing environment. Metabolism is slowed to "hibernation" levels. If the winter is long, oxygen levels in the depths may become depleted; fish trapped below the ice may die (winter kill.) The lake next door is like you, waiting for spring! And the cycle continues year after year in our climate.

How different our world would be if ice did not expand upon freezing, but took its place at the lake and river bottom like other types of matter that become more dense as they cool.

Guest columnist John Cooke taught high school biology for 30 years and is pleased to share his insights wit h our readers. More information on this subject can be found at Water on the Web at http://waterontheweb.org/index.html