Fall overturn in Vermont lakes

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(Host) According to commentator Ted Levin, this time of year the water in your local pond or lake undergoes an environmentally critical transformation.

(Levin) Four hundred twenty-seven-acre Lake Fairlee, just in from the Connecticut River, looks like a giant tadpole, its tail swung north, its head southwest. There is a certain day each autumn, usually in November on Lake Fairlee and other large lakes, when “fall overturn” takes place, an event not usually marked on human calenders yet absolutely crucial for aquatic wildlife. Without fall overturn, life on Earth would be vastly different from what it is today.

But first, let’s jump back three months. In summer, the waters of Lake Fairlee are stratified into three distinct zones: the top, or epilimnion; the middle, or metalimnion; and the bottom, or hypolimnion. Each of these three zones circulates separately, and the winds of summer which agitate the surface do not mix them together. They remain as distinct as oil and vinegar. Diving through the top epilimnion into the middle metalimnion is always a shock to any swimmer; the temperature suddenly drops ten to fifteen degrees Fahrenheit along an invisible boundary less than ten feet below the surface.

Of the three layers, the epilimnion the top layer that’s always in contact with air – is richest in oxygen, lowest in nutrients, which tend to settle to the bottom of the lake. On the bottom, the hypolimnion gains all the nutrients that rain down during the summer, where they become food for the enormous biomass of decomposers, which respire in the sediments.

With the cooler days of autumn, however, the boundaries between the three layers become less distinct. As surface waters cool, they become denser, which causes them to sink in the lake. The warm top layer or epilimnion shrinks as the lake gives up heat. If it weren’t for a unique property of water, this process would continue until the water at the bottom of the lake reached 32 degrees Fahrenheit and froze. Our ponds and lakes would freeze from the bottom up.

But water reaches its maximum density not at 32 degrees but at 39.2 degrees. Sometime in November, Lake Fairlee is a nearly uniform 39.2 degrees Fahrenheit. Now, fall overturn begins. The slightest wind stirs the lake from top to bottom, distributing nutrients and oxygen throughout the depths.

As cold autumn weather continues to cool the lake below 39.2 degrees Fahrenheit, however, overturn comes to an end. The ever-colder surface water is now less dense than the rest of the lake, and this cooler water floats. The lake stops circulating. Ice finally forms, generally by late December, and the lake is sealed off from the wind.

I can’t see overturn happen, of course, as I paddle toward the middle of Lake Fairlee. Ahead, a flock of bufflehead ducks have just arrived from central Canada en route to some mid-Atlantic estuary for a winter respite. They dive, powered by large feet. Then they surface, not far off–three males, mostly white, and four dull-colored birds, either females or the young of the year. And I imagine they’re finding plenty of insects and small fish dispersed at practically every depth of Lake Fairlee–thanks of course, to “fall overturn.”

This is Ted Levin from Gillette Swamp in Thetford Center, Vermont.

Ted Levin is a writer and photographer specializing in natural history.

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