With patches of snow on the ground and wood smoke drifting in the air, late-autumn changes are everywhere. Many species of birds have left for southern warmth; weasels and hares have put on winter white; other creatures have retreated to burrows and dens. Smaller lives die, their span complete by the frost, leaving egg, larva and pupa.
Another major autumnal event goes unseen in the large lakes of the north. Though hidden, it is every bit as pronounced. It is called the “autumnal overturn,” and in water it means major changes in temperature, oxygen and the vertical migration of living things below the surface. The overturn affects a host of aquatic organisms, including fish, algae, plankton, aquatic worms, insects and crustaceans. Yet with the exception of the most determined anglers, probably few people are aware of it.
Fishermen know of the overturn because it can affect their fishing. Some base their angling habits accordingly. In August, on lakes, they are apt to drop bait and to cast lures in comparatively shallow water kept well oxygenated at the surface thanks to wind, waves, rain or rushing tributaries. In February, however, the deeper portions of the lake hold promise.
How is this possible? In a nutshell, it is because of the movement of water molecules caused by dropping temperatures. In summer, when water molecules are warmed, they become excited and bounce around ricocheting off one another, thereby occupying a greater volume. When water molecules are chilled in autumn the water near the lake’s surface becomes dense and heavier. It sinks, displacing the less-cold water below, which in turn rises and chills. When cooled below 39 degrees F., however, water stops sinking. Water molecules at the surface soon to drop to 32 degrees, align themselves into crystals and form ice. Ice crystals themselves follow the rules of increased density, becoming heavy, but their interlocking geometric array encloses pockets of air, so the overall mass is lighter than water. “Solid” ice therefore floats on top of fluid water that remains unfrozen at 39 degrees. (It’s a good thing it does. If water were to sink when frozen—as other liquids—it would accumulate on the bottom, building upward an impenetrable mass and making life impossible for aquatic plants and animals. But before the water turns cold enough for ice to form on the lake’s surface, lower air temperatures keep cooling the surface water and oxygen continues to be absorbed; the cooler water sinks, sending newly chilled water down and warmer water up. The resulting convection currents eventually result in thermal equilibrium and a lake becomes uniformly oxygenated.
With water now thoroughly mixed, aquatic animals can be active everywhere, even at the bottom, which in deep lakes is a relatively lifeless region in summer. During the fall overturn, which takes several weeks or more, many species of fish migrate downward and have the entire lake to swim around in. Ice fishermen, in their heated shanties, can drop lines into the deep and be rewarded with fat and healthy fish.
Fish aren’t alone in heading to the depths. They typically are preceded by snails, crayfish, aquatic worms and over-wintering insect larvae, causing a rapid population increase in the depths. Fish follow the food supply and feed copiously in deep water. The slow autumnal overturn gives plankton (tiny plant or animal organisms that move mostly with currents) time to pass through several generations. Each generation shows a series of sequential anatomical changes to adjust to changes in their fluid environment. For example, many plankton organisms in the deeper and denser water of winter are smaller and more compact than their warm-water counterparts. In summer plankton have greatly increased body surface area, which keeps them from sinking in the less dense water.
Dramatic shape changes can be found in succeeding generations of many other lake creatures. Consider the Daphnia, an energetic little water flea, a type of crustacean. Summer and winter forms of Daphnia are so different from one another they once were thought to be separate species. In winter generations of Daphnia are compact, rounded and easily supported by the dense, cold water; but summer generations have greatly enlarged, widened bodies equipped with long spines and oil droplets that serve as flotation devices under less-dense conditions.
In late fall, temperatures drop, currents flow, fish move, plankton and other creatures take on new forms—all below the lake’s surface, escaping the human eye. We know winter’s coming when we smell wood smoke, but a peek into a lake’s depths would provide another signal.