Lake Fairlee, just in from the Connecticut River on the Vermont side, looks like a giant tadpole, its tail swung north to meet Blood Brook, its head southwest, feeding the unnamed fork of the East Branch of the Ompompanoosuc River.
There is a certain day each autumn, usually in November on 427-acre Lake Fairlee and other large lakes hereabouts, when “fall overturn” takes place – an event not usually marked on human calendars yet absolutely crucial for aquatic life. Without fall overturn, it is quite possible that life on earth would be vastly different from what it is today.
But first, 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. These three zones remain as distinct as oil and vinegar; the winds of summer which agitate the surface do not mix them together. Diving through the epilimnion into the 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 – always in contact with the air – is richest in oxygen and lowest in nutrients, which tend to settle to the bottom of the lake. The hypolimnion gains all the nutrients that rain down during the summer, where they become food for the catfish, suckers, and all the decomposers that 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 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; fish would perish in lake-size blocks of ice, and human colonization of the Northern Hemisphere would undoubtedly have been greatly slowed.
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, which triggers fall overturn. The slightest wind stirs the lake from top to bottom, distributing nutrients and oxygen through the depths. Life in the lake responds unseen below the surface. Fish normally restricted to the top visit the bottom. Bottom dwellers rise throughout the lake.
As cold autumn weather continues to cool the lake below 39.2 degrees Fahrenheit, the overturn comes to an end. The lake stops circulating. The ever-colder surface water is now less dense than the rest of the lake, and this cooler water floats. Ice finally forms, generally by late December, and the lake is sealed off from the wind.
If fall overturn hadn’t already taken place, this skin of ice would be fatal to many animals that depend on the surge of oxygen and nutrients from fall overturn to make it through the long winter ahead.
In the spring, when the ice melts and the lake water warms to a uniform 39.2 degrees Fahrenheit, spring overturn begins. Once more, nutrients and oxygen are distributed throughout the body of water. Then, when April grades into May, the surface of Lake Fairlee warms, mixing of the waters ceases, and lake is again stratified into its three distinct layers.
We cannot see overturn happen, of course, but I imagine that the flock of bufflehead I saw last November, which had just arrived from central Canada en route to some mid-Atlantic estuary for a winter respite, found plenty of aquatic insects and small fish dispersed at nearly all depths. Ducks idled in the middle of the lake that day, and when I paddled my kayak toward them, they dove, powered by their large hind feet. Not far off, they surfaced – three males, mostly white, and four dull-colored, either females or young of the year.
With twilight closing in I turned back north toward Lake Fairlee’s public landing. A skittish blue jay bathed, while noisy crows soared above. Four herring gulls and an immature black-legged kittywake – a very rare inland visitor from the Arctic – eddied against a cranberry-colored sky.