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In winter, the surface of a pond turns cold and hard, sometimes blanketed with snow, other times swept clean by raw, northwest winds. Ice makes solid the otherwise fluid boundary between air and water that allows us to walk, slide or skim along the surface on steel blades.
As impenetrable as ice seems, it forms a barrier that sustains life for the inhabitants sealed below. Ice is a relatively good insulator that slows the transfer of heat from water to frigid air. Flora and fauna trapped below that can withstand the minimum 37° F temperature will live to see next spring.
The ice that thickens as winter deepens started forming weeks earlier as cool air passed over warmer water, sapping heat from the pond and lowering the surface water temperature. The cooled surface water became dense and heavy, sinking to the bottom while warmer subsurface water rose to the top creating a temporary vertical circulation that hastened the heat loss. Once the water reached a uniform 37 degrees the vertical circulation stopped. The surface continued to cool, but convection no longer transferred heat from below. Conduction is a slower process and as the water temperature dropped to freezing, the water molecules aligned in a crystalline matrix and ice formed. Its thin solid tentacles reached out from shore toward the center. Given still days and cold nights, smooth ice soon covers and seals the pond until spring's warmth reverses the cooling trend and melts the frozen shield away.
By summer, the same surface forms a fine line between the freedom of flight and the confines of a watery world. It's a barrier that can be easily penetrated, but it's also a microhabitat used by animals, plants and insects adapted to life right on the boundary where water meets the sky.
Whirligig beetles – black, oval and the size of a pumpkin seed – spin on the water surface in large congregations. Each of their compound eyes is divided in half, top and bottom, so they can see equally well above and below the surface as they search for prey and avoid being preyed upon.
Spindly water striders skate as easily on water as we did on winter's ice. Water lilies rooted in the pond form circular leaves that float on the surface providing an excellent landing pad for dragonflies. Whirligig beetles lay their eggs on the underside of the lily's waxy leaves.
An often misidentified soupy lime-green layer that covers the pond in late summer is not algae, but Wisconsin's smallest flowering plant called duckweed. The minute flowers are seldom seen as the plant reproduces by budding. In fall, tiny mats of green scales break loose and drop to the pond bottom to overwinter. The new growth will rise to the surface again next year and eventually recover the pond.
Though many insects live most of their lives in water, they still depend on air to breathe. Backswimmers, water boatmen, giant water bugs and predaceous diving beetles periodically rise to the surface. Each in its own way grabs an air bubble before descending to continue underwater activities.
Nostrils from turtles that do not wish to be seen barely break the surface to inhale life-sustaining air. Green frogs, leopard frogs and American toads rest on top of the water to plunk, snore and trill in their efforts to solicit females. Swallows during the day and bats at night skim the water line in search of insects. Trout rise to and through the surface chasing mayflies while belted kingfishers dive down through it to catch fish. Dragonflies and damselflies hover above the water surface but dip their abdomens below to deposit eggs on plants and rocks. Puddle ducks paddle, dabble and tip up at the surface to feed. Speaking of refueling, two "jumbo transports" of the avian world, Canada geese and tundra swans, find rest and refuge at the surface during their long migratory flights.
Reflections play and cast off the tranquil surface extending light and space. Looking down, we can see cumulus clouds traveling overhead or double our pleasure in the mirrored collage of tree silhouettes surrounding a pond. At nighttime stars and moonbeams sparkle and shimmer as breezes ripple across the water. The small waves carry pine pollen, twigs, lost bobbers and colorful leaves across the surface to be washed ashore or temporarily suspended if they are trapped as the water freezes again in its annual cycle.
Marking time from day into night, week into month, season into season, the pond surface holds many stories of so many species that survive and thrive on the thin line between the vast spaces of air and water.
Anita Carpenter writes from Oshkosh and enjoys sharing different ways to look at nature.