Winter World_ The Ingenuity of Animal Survival - Bernd Heinrich [6]
Water influences life as profoundly at the ecological level as it does at the cellular level. Every fall in the North Temperate Zone we can observe the ecological effects of the various physical properties of water. Most of the creatures of the earth experience water as a transparent liquid that runs downhill and that can only be contained by barriers. For part of the year some of us also see water as a white powdery matter that sticks to the trees and the sides of the hills and that makes the woods look like a fairyland. This substance can be stacked into piles, tunneled into, and made into dwellings for man and beast. It can accumulate and become so dense and deep that we can’t walk through it. It can shut out the light to plants and may crush them. In northern areas, when the tilt of the earth is appropriate, it may collect over long periods of time to create glaciers that transform the landscape, grinding down mountains and valleys. With a difference of just one degree Celsius, or less, water also can become a clear, glasslike substance that seals over the surface of lakes and allows us to walk across them with impunity.
The fate of almost everything in the winter world is ultimately determined by the crystallization of water. In a matter of a few hours that crystallization can change the physical surface of the earth, and in the course of millions of years it has profoundly changed the physiological, morphological, and behavioral characterizations of all organisms that have to contend with that magic transformation of liquid to crystal.
Every fall the winter world creeps up gradually and inexorably onto those inhabiting the Northern Hemisphere. As it does, the nights get progressively longer and colder. Less energy from the sun reaches the ground. First the water in topsoil freezes to form a solid cover (unless it’s already snow-covered). The swiftest-flowing streams and brooks are the last to freeze over because the cold air–water interface constantly mixes. The cold that causes the water to freeze comes from the air just above the water. The water is at least slightly warmer. When the water is stirred (as in swift-flowing streams), the surface doesn’t cool down to 0°C so quickly.
One night the inevitable happens: the bodies of water freeze solid. The temperature drops enough for water molecules on marsh grass stems, twigs, and leaves floating at pond’s edge to slow their molecular momentum enough that they drop into stable crystalline positions. The stems, twigs, and leaves then serve as nucleation sites for ice-crystal formation. Like billiard balls rolling into pockets, the water molecules lock into position, first indiscriminately on any object they encounter, then on other molecules that have come to rest, forming an ice lattice. What little energy these molecules had left is now released as heat, the heat of fusion, 76.7 calories for every gram of liquid water turned to ice. (This heat is not enough to cause any appreciable temperature rise in the pond or lake because it is so quickly absorbed by the much larger mass of water. However, the sudden freezing of a small droplet isolated from others often causes an appreciable “exotherm” of several degrees Celsius.)
The ice crystals being formed reach out like sharp fingers over the surface of the water. They meet, interlock, and by morning the whole pond may be covered with a transparent pane of ice that physically separates the water denizens from