Isolated from its parent stream, the lake lay wrapped in a mantle of rising vapor, its surface unmarred in the morning stillness. The air had grown cool during the night, but the lake held warmth from sunny days. In the hours before dawn, water molecules from the surface had been escaping their bonds of hydrogen linking them with their neighbors. They rose steadily from the surface, condensing in the chilly air, gathering in wraithlike billows swaying together or pulled apart by the gentlest of air currents. With the sun climbing beyond the blue hills, evaporative activity would soon increase until the last wisps of vapor faded. No matter how hot the day, the level of the pond would remain constant, for the dykes of sediment sealing both ends allowed a seepage of stream water to enter the shallow basin.
Fog steaming from the pond blanketed much of the valley and settled in swelling droplets on every stem and blade. It was one means by which water molecules could rejoin their river, for they condensed so heavily along the banks that tiny rivulets coursed into the running water. Bright dewy webs stretched against the gray sky; rocks bared in the meadow grass gleamed darkly. While salamander still hunted along the wet banks, the banjo plunks of green frogs and clicks of crickets grew silent one by one as the sky lightened. There was a last deep and sonorous roar from a bullfrog as the day began. The air still held too much moisture for most daytime insects to fly, but redwing blackbirds stirred in the cattails, with shrill whirring calls and a fluttering of bright wings. A lone blue heron stood silent on the shadowed shore, yellow eyes gleaming, with bill poised above a pickerel frog lying quietly at the surface.
In the lake the intimacy of water and life was clearly evident. Water swelled the tissues of larger plants and animals, suffused cells and giant molecules, and played a role in the invisible world of atoms and atomic fragments. At least two thirds of every living thing in the pond or on its shores was comprised of water.
Fluid water existed in cells, the structural units of all life, in tiny vacuoles or in protoplasm itself, but it also dwelt more importantly as a part of the many intricate membranes of a cell. There were membranes which sealed every cell as a unit; others wove through the substance of a cell and its nucleus, and some were even found inside of the smallest components used in the transformation of food energy or the growth of the cell. Nearly all major life activities took place on the surface of these membranes.
Each membrane part protein and part fatty material, held thin layers of water molecules arranged in a pattern resembling the structure of ice. The resulting layered membrane, no matter where it was in the cell, had distinct and remarkable properties of allowing certain substances to pass through, while refusing passage to others. Each cell then had an ability to regulate its existence by satisfying needs and eliminating products and wastes. Such an orderly molecular lattice influenced the basic activities of every cell, helping it adjust its life pattern to changing conditions of the environment. If they grew too hot, proteins would coagulate, and freezing in winter could destroy the delicate membranes and their vital functions.
Once a cell was dead, its organization fell apart. Although the clusters of water molecules continued to vibrate, their influence was no longer ordered and all the cell parts bobbed about at random. To this degree, water in the living cells, derived from lake, stream and rain, was life itself.
The demise and disintegration of a cell, in the presence of decay bacteria living in water, was no catastrophe but a promise of future life in some other form. The dead cell's compounds of nitrogen, phosphorous, and other elements set free were soon caught up in further chemical and biological events which made it possible for organisms to build, grow, work and reproduce. The same ingredients had been used and reused for over three billion years.
During its years of isolation the pond had changed. No longer were its plants and animals the same as those of the stream that fed it. Cut off from meandering flow, imprisoned stream animals disappeared as the stilled water failed to bring the food or the immense quantities of dissolved oxygen they required. Soon the lake attracted other forms of life to an existence not available elsewhere on the valley floor. Aquatic insects flying overhead plummeted toward the bright reflective surface until the water was heavily populated with whirligig beetles, back swimmers, water boatmen, and diving beetles. On the legs of some of these insects came the larva of red water mites and crustaceans, but examples of transfers of this sort were commonplace. Each heron wading into the water, each duck that came skittering in, brought eggs, spores, seeds, whole organisms and viable fragments of living things on their feet and feathers. An occasional killdeer stepping lightly into the shallows might leave behind a snail which, because it was both male and female, could give rise to unlimited progeny. After even a short time of this kind of immigration, life in the pond was intense. Seeds and spores had grown into plants which rose from the mud; small animals crawled over submerged vegetation and upon the bottom; clouds of plankton drifted through the water; predatory fishes entered by eggs, or through rivulets between stream and pond and had survived.
The pond was not an entirely isolated microcosm, but was involved in a continuing exchange with the world around it. Every rain brought contributions from the land as minerals, silt, debris, and life. The shoreline was blanketed by old and decaying leaves that had blown down, become saturated, and slipped beneath the surface the previous autumn. Insects flew in to lay eggs that would hatch and grow as aquatic larva, to leave later, taking with them the product of a season of nourishment and growth. No sooner than did most of the insects emerge from the water than they were snatched from mid-flight by kingbirds and swallows swooping over the lake in daylight, or by low flying bats after dark. The pond was a major contributor to the valley world as well as a recipient, providing food for ring neck ducks as they dived to take mollusks and insects off the bottom, and for mergansers that caught fishes in deeper water. Surface feeding mallards and teal came to consume vegetation and raise their young in the shallows. Occasional ospreys snatched large fishes basking at the surface, and the many herons speared fish, frogs and even water snakes that had come to hunt crayfish. Raccoons walked the shore every night to scrabble for clams and crustaceans.
A lake or pond has an existence that simulates life itself. It is born abruptly, matures slowly, and eventually declines into senescence. Finally it dies, its grave only a richer shadow on the valley floor where it once gleamed in the sun.
The basin of each very young lake in the valley held open water with little sediment lying on the bottom. At times the entire valley floor had been inundated by surging floods, when fine particles of suspended clay settled into the crescent shaped depressions, carpeting their basins with thick ooze. The valleys themselves once gouged out by the action of glaciers once covering them. The melting of those glaciers released untold surging rivers of water which raised the levels of the oceans so long ago after the receding of the last ice age. Ages later several of the lakes possessed small tributary streams that carried additional loose material into the tranquil water. The oldest ponds lay quiet and thick with a suffocating blanket of clay and organic soil, a deposit often reaching within inches of the surface. It was here that dense stands of aquatic vegetation grew high above the shallow water. The luxuriant growth of these plants, filling old ponds, hurried them toward becoming swamps and bogs, and inevitable extinction. Up and down the valley the outlines of old meanders and ox-bow lakes were defined by parallel rows of vegetation following the contours of diminished shorelines. The lake of this pocket of the valley was neither new nor old, but mature and healthy, supporting a multitude of lives.
Where the water was only an inch deep, dense strands of spike rush bore tiny fruits, many now damaged and removed, taken by water fowl, marsh birds, and the ever hungry muskrat, who depended on aquatic vegetation more than any other mammal on the North American continent. In shallow water along the shore arrowhead and pickerel weed emerged to spread their broad, spear like leaves a foot or more above the surface. From the pickerel weed vertical clusters of blue flowers arose, but were dwarfed by the towering flame red spikes of loosestrife behind them.
The two plants of the shoreline covering more territory than all others were cattails and waterwillow. Long flattened swords of cattails fanned out from submerged, starchy rootstocks; some of the leaves extended over six feet into the air, providing shelter for a bittern which made its quiet and deliberate way through the dense vegetation in search of prey. The bittern paused briefly beside a vacated nest of a black duck crowning an old tussock; there it found a recently emerged leopard frog, still with stump of a tail from its old tadpole days. The long beak of the bird flicked out, removing the frog before it had a chance to react. With head low and outthrust, the bittern stalked on.
Waterwillow, with its tough, woody, looping stems was one of the major agents in the eventual demise of the pond. With each years growth its stems extended over the water, took root, until there was a thicket of almost impenetrable vegetation along the shore. Wherever stands of waterwillow were established, sediment accumulated to the surface and above, diminishing the shoreline year after year.
It was the plants of the shoreline that gave this and every pond a character of its own, and established suitable conditions for a multitude of inhabitants. Depending upon what vegetation grew and how abundantly, waves were reduced, light filtered, and refuge from predators was made available to lessor creatures of the watery world. Some animals used plant materials for nests and cases into which they could retreat, while others attached to plant stems could be transported along the shore if the plants broke free and drifted away. Rooted plants consumed nutrients from water and soil, freed oxygen into the pond and the air above, consumed quantities of carbon dioxide in daylight hours, and left their remains to enrich the shoreline. These larger plants also helped keep the water clear, and not only by minimizing wave disturbance of the muddy bottom; they gave off substances that reduced the surface tension of the film, allowing particles to drift to the bottom more quickly.
Because of the exceptionally high food content of many plants growing in or near the water, the pond drew vegetarians from all over the valley floor, far more than could be supported by the relatively sparse plant growth along the shallows of the rushing stream. Some of the softer plants, well out in the water, were eaten regularly by dozens of different animals, especially insects and turtles. A few fishes were vegetarians, as were several kind of water fowl that were frequent visitors. Other than the ubiquitous muskrat, however, few mammals came to the pond to feed--only deer and an occasional venturesome rabbit. Where beavers were established they also consumed vegetation, but they had not traveled widely and were absent from this pond.
Insects were abundant within the zone of emerging aquatic plants. Now that the sun had risen and the fog had lifted, the shoreline was busy with darting, brilliant hued damselflies and the flicking, powerful flight of dragonflies. A pair of damselflies hung over a cluster of sedges, the male grasping the females head by a pair of pincers at the tip of his abdomen. They alighted together on a single stem, the female immediately backing down into the water until she was completely submerged. There she deftly deposited eggs on the plant, but without the male would have been trapped beneath the tough and elastic surface film. He crawled and fluttered upward, still clinging to her, until she broke free and they flew off to repeat the process on another plant.
Most of the dragonflies flew in great, looping patterns in pursuit of airborne prey, often coming to rest on a bare shoreline stem; there they crouched, outstretched wings glinting in the sunlight, while their head with huge bulbous eyes swiveled rapidly, following the flight of a possible insect victim. Other dragonflies were occupied with a more regular flight, as they had been in the nearby stream, flying back and forth along the shore, facing the water and defending their established mating territories against invaders of their own kind.
Beneath the surface, predatory diving beetles and back swimmers darted about in search of smaller prey, although one very large beetle had just seized a tadpole twice its own size and was tearing it to pieces with its powerful jaws. Water scorpions, seemingly lethargic hung close to the surface with raptorial claws cocked back in readiness. Water boatmen, insects bearing a superficial resemblance to back swimmers with their long sculling legs, swam to the bottom where they swept algae and microscopic animals from the fine sediment. At times a water boatmen fed directly upon algal filaments through a piercing, protrusion tube that permitted it to suck plant juices.
Many submerged insects carried air with them as silvery bubbles or as shiny coats surrounding their bodies. This air was not consumed directly, but served as a unique non-living gill. The surface film of each bubble was a boundary membrane between the aquatic world and the dry atmosphere of the insect's breathing apparatus. Oxygen, dissolved in the water, diffused readily through the film into the bubble where it was drawn upon for respiration. Carbon dioxide in turn, was diffused outward as it accumulated as waste. Some insects carrying such physical gills could remain beneath the surface for hours; indeed, a few small kinds never replenished the silvery coatings their entire adult lives.
The success of other swimming insects in obtaining air was due to long tubular snorkels fringed with water repellent hairs around the aperture. When the snorkel protruded through the surface film, not only was water prevented from flooding in, but the hairs functioned as an anchor, keeping the animal in place while replenishing its air.
Fisher spiders shared with their distant insect cousins the ability to carry air beneath, although they did so only rarely and then when pursued by larger predators or when they in turn had an opportunity to capture a small fish. Otherwise they rested upon the surface, all eight legs radiating flat across the water, depressing the film slightly with their weight.
Through the corridors of quiet water formed by the erect blades of cattails and arching waterwillow stems, whirligig beetles coursed busily and water striders skated, resting for long intervals between each sudden thrust of their legs. Beneath them, gray bullhead catfish grubbed among plant fragments littering the bottom, oblivious to the insects shadows on the mirrored ceiling overhead. Satinfin shiners swam slowly by, gulping plankton that swarmed in the shallow water: they served as important links between the microscopic world and larger predators that sooner or later would catch them. A lone mud pickerel, only inches long, hung obliquely in the water, head up, waiting for an insect to come within range of its sudden inescapable rush.
The most common fishes in the warm, lighted water were yellow perch and sunfishes, especially crappies, bluegills, and a few brilliant blue and orange pumpkinseeds with scarlet tabs on their gill covers. Sunfishes, characteristic of lakes or ponds, lacked the streamlining of brook trout with its strong lateral muscles necessary for living in rapidly flowing water. Sunfish bodies were compressed and deep, allowing them to slip with ease among the tangled stems and masses of aquatic plants.
A pair of pumpkinseeds had just finished clearing a rounded depression in the muddy bottom close to shore, creating a light spot where bared sand and pebbles contrasted with the dark organic debris everywhere else. The female passively circled the nest, the male hovering on her right side, slightly to the rear. As he drew closer to her, he tilted away from the vertical, slanting under her belly, ready to release sperm containing milt whenever eggs were produced. This action occurred repeatedly, occasionally interrupted by an irritated rush from the female, driving him away a short distance but he returned persistently. Before long the female laid her first eggs cementing them to the clean pebbles growing cloudy from the milt of the male. As soon as she was finished, she took up a position in the center of the nest fanning the bottom with her fins to keep silt from settling and oxygenating them, in which each individual cell was beginning to cleave in two. The male would relieve her from time to time, in addition to being an aggressive protector of the nest, charging all intruders and driving away most. But when one of the parents left for awhile the other was not always successful in defending the brood. A minnow dashed in, scooped up an egg, and fled, pursued briefly by the remaining sunfish. Even with the nest left unattended so briefly there was sufficient time for other minnows, waiting nearby, to dash in and gulp as many eggs as they could.
Small leeches clung to the tails of several sunfishes, streaming out like banners from the fan shaped fins. Other leeches looped along the bottom, searching for food in the mud. A large horse leech swam rapidly past in mid-water, its long body waving up and down like a flat ribbon, displaying a bright orange belly.
The underwater portions of all emergent plants were heavily populated. Life on submerged surfaces of stems, old sticks, rocks, and debris composed a distinct assemblage, quite unlike all other zones of life in the lake. Plants and animals there had to be fast growing, as their restricted world lasted for little more than the warm months of summer. Some were firmly attached, but others were not; all depended upon light, moderate temperature and the slight water movement of the shallow water. When a fish or turtle swam violently by, some of the encrusted plants and animals were swept off and became members of the drifting life of the plankton. Others retreated into tubes, sheaths or cups, or were firmly held by tough stalks, and soon returned to full feeding activity after being disturbed by strong local currents.
Dark smears of blue-green algae wrapped some of the stems, but on most a loose brownish coating was composed of microscopic diatoms, tiny plants housed in glasslike cases of geometric precision; like those of brook or stream, they were fresh water relatives of the diatoms that had drifted under the surface of the sea where the river had begun. A host of one celled animals, hydras, insect larva, and worms burrowed into the algal crusts seeking food and protection. Small crustaceans scrabbled across the coated stems, interrupted at times by the slow coursing of larger snails rasping away the film of diatoms with a flexible, toothed tongue. Clear elongated masses of snail eggs were everywhere; inside the transparent egg spheres tiny embryonic snails rotated smoothly and ceaselessly. Long green dragonfly nymphs, the largest predators on stalks, waited to shoot out hinged jaws until smaller, unwary victims came within range.
In some places the stems were heavily encrusted solely with animals, fixed permanently in place, at least for the duration of the season. Fresh water sponges created a rough and irregular mass, hugging the stem without rising in finger like projections as had the stream varieties. Other stems were enveloped by great gelatinous chunks decorated with hundreds of dark rosettes. Each rosette was composed of several soft, tube-dwelling moss animals that extended tiny tentacles arranged in a half circle; tiny beating hairs on the tentacles produced currents which flowed toward a centrally placed mouth, where quantities of minute plant and animal cells were removed from the water.
Short mud coated tubes lay along many stems. Inside each was the flickering, waving body of an elongated midge larva. Sinuous motion was its means of drawing a current into one end of the tube, bringing with it oxygen and food. Although the shoreline of the lake followed a smooth arc, the vegetation growing out into open water did not. There were places where waterwillow was indented toward the shore, forming coves in the tangle of plants. In such protected spots the surface was covered by a blanket of tiny floating plants, duckweed and watermeal, the smallest flowering plants in the world. The grain sized watermeal had no roots at all, but duckweed a trifle larger had fine, unbranched rootlets hanging a quarter inch below the buoyant leaves. The dense unbroken blanket enclosed a vast population of smaller plants and animals, of which some attached to the tiny roots while others swam through the maze of miniature vegetation. Insect eggs and larva, worms, tentacled hydra, snails, one celled animals, round bodied water mites, crustaceans, rotifers, algal filaments, and single celled diatoms and desmids lived together in protrusion and endless variety. But the pond bottom only a foot below was not so heavily populated, for sunlight failed to penetrate and little oxygen was present on the muddy floor.
The blanket supported another population on its upper surface, mostly insects and spiders that ran across the dry, cobbled surface of the tiny plants. Flies, mosquitoes, and damselflies alighted, depressing the matted surface slightly. Far more abundant were the springtails and jumping plant lice, extremely small insects that lived their entire lives in this environment. Disturbed by a turtle thrusting its head through the blanket, they bounded into the air in clouds of light specks describing arcs in the early morning sunlight. The turtle, head coated with duckweed, blinked a few times attempting to clear its eyes, but soon submerged, leaving no trace of its interruption as the floating plants spread once more across the surface film.
Heavy masses of filamentous algae, rather than duckweed, lay in a few of the coves created by the indentation of shoreline plants. Glistening hemispheres of gas, greenish from the fine threads which kept them trapped, extended above the surface. Underwater the scene was one of delicate beauty, for the filaments rose gracefully from the bottom in long streamers, usually with an apex formed by a single luminescent bubble held captive in its attempted balloon ascent. The algal veils were almost still, but swayed slightly in the vague currents. Several shiners appeared and vanished behind the veiled curtains their silvery sides glinting momentarily as they caught the sunlight. The filaments were clean of attaching organisms, unlike the roots of the duckweed, although a few specialized creatures moved among them, feeding on the tubular cells by piercing their walls and sucking out the contents. One of the most highly adapted of these was the rotifer which wrapped two lobes loosely around an algal strand and swam briskly guided along by its living track.
Beyond the cattail and waterwillow the character of shoreline vegetation changed, Where the bottom sloped and grew deeper, plants capable of holding their foliage above water could no longer exist. Instead, plants rooted in the bottom sent up long flexible air-filled stems that terminated in floating leaves. Pond lilies and smart weeds were the most common, with a few pondweeds growing near the outer bend. Both produced two kinds of leaves: long slender ones underwater that offered little resistance to currents and passing animals, and broad flat leaves of great buoyancy on the surface. The cord like stalks and stems of these plants, as well as the thicker stalks of pond lilies, were tough enough to stand most any kind of wave action, and were coated with mucus that reduced damaging abrasion when they rubbed against one another. Because the buoyant stems of the larger pond lilies contained many air spaces, certain aquatic insects deposited their eggs deep within, insuring a protected oxygen rich environment for their vegetarian young.
Underneath each lilly pad was an assemblage of creatures and plants resembling that of the duckweed community, but because of the size of these leaves, much larger forms of life were included. Dozens of translucent hydras hung down, trailing long tentacles into the murky water beneath. On most of the hydras, small spool shaped one celled animals scurried about, finding protection within the range of their hosts stinging tentacles and an opportunity for capturing food. Sponges, moss animals, fresh water limpets, and a host of insects competed for space in this upside down world. Some of the larva mined the lily pads, leaving long sinuous cuts through which sunlight stabbed into the darkness below. Other holes had been cut by beetles alighting on the upper side. One such beetle had just completed a smooth round hole and now inserted her abdomen in it, depositing a semicircle of eggs on the under surface where they would later hatch into aquatic young.
The small minnows of the cattail and waterwillow zone were no longer common in deeper water, but sunfishes still threaded their way among the curving stems, keeping close to the surface, while predatory largemouth bass hunted near the bottom. Midway a large chain pickerel hung almost motionless, eyes directed forward along two grooves down its snout, focused on a young bluegill beginning to come into range. The sunfish veered away, but the pickerel did not give chase, remaining motionless except for a gentle fanning of its fins.
Floating leaves were only one indication of the support offered by the surface film, the result of strong attraction of water molecules, one for another. All molecules directly at the surface were linked to their lower neighbors through the hydrogen they carried, four bonds to each molecule. The effect was a strong and elastic film that supported distinct populations not only on top, but directly underneath, serving as a floor or ceiling for a multitude of living things.
The surface of the lake was of vital importance to its physical character as well as to its inhabitants. Its great expanse allowed a continuous passage of gas molecules back and forth between water and atmosphere. It was important to plants that water was a capacious absorber of carbon dioxide. Aquatic animals, on the other hand, could not tolerate too little dissolved oxygen, a gas available only from the activity of plants. some oxygen came from algae of the pond, but most filtered down from the atmosphere, where it had traveled from its origin in forests and field or in the microscopic plant life of a distant ocean. When wind ruffled the surface of the lake, gas exchange was enhanced, but never to the degree of the turbulent mountain brook, or even a riffle of the nearby stream.
There were seasonal effects at the surface: light fell at different angles in winter and summer, and with lowered temperatures, molecules of the water became less active and more densely packed. When the chill winds of winter cooled the surface even more, the increased weight of the upper layers would cause water to sink, creating an overturn that would bring warmer water up to be cooled in turn. In deep lakes which were distinctly layered all summer, the overturn was a phenomenon of major importance.
Once the water was thoroughly chilled and temperatures continued to fall in the air above, molecules of the surface would arrange themselves in loosely packed crystals with air spaces in between. Trapped air meant the ice would float, rather than sink as most solids, and would nearly seal the pond from further atmospheric influences. Gas exchange would diminish, seriously affecting life in the pond, perhaps even suffocating some of the fishes that required more oxygen than the water contained from an earlier season.
On top of the surface film, the smallest products of the plant world coated the water with a dusting of pollen grains and spores. They were important as food to a few of the creatures living beneath that were able to harvest them.
Creatures of the surface lived not on wet water but upon a smooth, yielding film, prevented from plunging through by water repellent pads on their feet. The normal animal inhabitants were mostly fisher spiders and insects-whirligig beetles with their divided eyes, one pair up and another pair directed down into the water, water striders, springtails, and water measurers. But there were also accidental members of this population in the presence of crickets, bees and land beetles struggling as captives of the film; only a few were able to climb upon a leaf or floating twig and so escape the clinging grasp of the surface. An elongated rove beetle fell from an overhanging branch, landed on the water, and immediately shot across the surface, coming into contact with a stem of waterwillow. It had no apparent means of propulsion, but emitted a wetting agent that destroyed surface tension in front of its body; the pressure from the remaining tension against the rest of its body pushed it forward without any particular accuracy, but with considerable speed.
On the underside of the film, plants were mostly single celled algae and bacteria, but animals lived there in great variety, hanging down or crawling across the mirrored ceiling, only slightly dimpling it with their weight. Pond snails and flatworms flowed evenly across the wide expanse, the snails every now and then dropping straight down to the bottom several feet below. Other snails having generated a gas bubble beneath their shells, slowly rose in stately fashion to the surface. A few very bright green hydra hung from the surface their color deriving from algae living within their cells. Specially adapted water fleas held to the surface, upside down, with bristles caught in its tension; they rowed along rapidly with beating antennae, scooping pollen grains from the other side.
For other water fleas living as member of the plankton lower down, the film was a great hazard when they found themselves accidentally thrust above it. If they were trapped in its firm grip, there was only one means of escape: they might molt their outer shell-like skeletons and slip down into the fluid beneath, leaving the tiny transparent skeletons on the surface glistening in the sunlight. The cast skins along with pollen, fungi and other organic bits floating on top of the water, were collected indiscrimanently by mosquito larva cruising just beneath the film. The larva were caught in turn by minnows and predatory insects rising from below.
The last zone of vegetation extending into the pond was composed of deep plants, rooted in the bottom yet never reaching the surface. Most were slender and simple, but one, the bladderwort, was one of the most unusual plants in the lake. Underwater it appeared delicate and vine like, bearing many rounded bladders, each with a mouthlike opening guarded by sharp bristling hairs. As the plant grew, its bladders were first of all small and green, turning red with age and increasing in size, finally becoming almost black. Its presence was a sure indication that plankton was abundant, although in the dense tangle of bladderwort that filled some shallows of the pond, the traps were so efficient that the plankton population was noticeably diminished in the immediate vicinity. Whenever a rotifer or crustacean entered the slit of the bladder, its escape was blocked by the hairs and eventually it succumbed to digestive juices of the plant, although perhaps not for a day or two. In some bladderworts capture was made more certain by a sudden inflation of the bladder, sucking in water and nearby animals caught in the flow. That the traps were effective was attested to by the presence of a dozen or so creatures in most bladders, the largest which could capture this number in less than two hours. Why did such wholesale trapping take place? Animal food was not essential to bladderwort, but if one of the plants was denied sufficient quantities of plankton, its growth was stunted; it flourished where microscopic life was abundant.
Because of a general lack of currents in a lake, clouds of plankton drifted through the open water away from shore. These, the smallest of lives were present in every possible shape and degree of specialization. Close to the surface, green spindle shaped cells swam, each propelling itself with a single hair like organ, although another tinier flagellum extended partly from the cell body. The greenness was that of chlorophyll, showing a dependence upon the sun in the manufacture of food from water and carbon dioxide, but the motile whip suggested an animal trait. In the front of each of these cells was a red eyespot, slightly off center; when the creature rotated on its own axis, the eyespot was directed now at the lighted surface, now at the shadowed bottom, causing corrective and appropriate action to be taken by the flagellum to insure the cell remained at the surface.
There were many other such cells, but not all green and not all solitary. Some lived in little brown bottles of their own manufacture; others clustered in spherical colonies in which there was some degree of coordination to allow the entire sphere to behave as one.
Plant plankton, whether single cells or many, motile or completely passive, were the dominant forms of drifting life, but not only because they were the most abundant. They were essential links between radiant energy of the sun, and the multitude of lives this one body of water supported. They were grazed upon by planktonic vegetarians, which in turn by slightly larger predators, continuing through a series of events to the largest of the ponds carnivores-fishes, turtles, frogs and birds. But at every step there was a tenfold loss of available food energy, so the large predators were few, while the tiny plants blanketed the pond in drifting pastures of incalculable numbers.
Rotifers, common animals of the plankton, were not much larger than the plant cells, but were composed of several dozen cells, the same number always the same for each different kind. A rotifer swam vigorously by extending two rounded crowns out in front of its head, each fringed with short hairs beating in synchronized waves. Inside the transparent body of the rotifer was a deeply seated pair of complex jaws, busily grinding fine all food particles swept into the mouth between the two crowns.
Tiny crustaceans were even more abundant and were more complex despite their small size. Most swam by means of Y-shaped legs or muscular antennae which rowed or beat the water, thrusting the animals along in a jerky fashion. The smallest crustaceans had finely feathered legs beneath a tear shaped body; legs with such closely spaced bristles that individual bacteria were filtered out of the water to serve as the prime constituent of their food. Water fleas, also small crustaceans were oval and compressed, each with a short projecting beak, above which a single multifaceted eye quivered and rotated in its internal socket. A few water fleas carried a saddle-like chamber giving them a humpbacked appearance; within the chamber were two or three eggs or developing larvae, already beginning to kick and move.
The variety of other animal plankton was almost endless. In addition to the more common crustaceans, one celled trumpet shaped creatures swam smoothly along, most of them a delicate blue; there was scarlet water mites whose spherical bodies tumbled through the water; larval parasites seeking a host; tiny immature insects; and juvenile fishes, their bellies still swollen with yolk. The numbers of each kind of plankton organism waxed and waned with seasonal events; what was common today might have been rare a week before, and almost certainly would be diminished a month hence.
The distribution of most of the plant plankton and many of the animals, was influenced by temperature. In the cold dense water of fall and winter, the tiny organisms were more buoyant than they were on this midsummer day when the water was warm. If they lacked special adaptations and were unable to swim, they tended to sink. Some of the crustaceans would swim obliquely upward, then drift slowly down to a certain level determined either by pressure, coolness, or a lessening of light before they became active again. Other plant and animal plankton were able to lessen their weight and approximate the density of water by manufacturing oil droplets that served as minute buoyant balloons, encased in their bodies. Most of the geometric plant cells, diatoms and desmids, not only had such droplets, but extremely thin shells to diminish their weight. Animals eating these plants often incorporated the oils directly into their own bodies, so they to were not as heavy as they seemed. A few planktonic animals manufactured gas bubbles which kept them afloat even more effectively. Other members of the plankton community relied upon increased friction to resist sinking: with long hairs, flat blades, elongated bodies and appendages, body surface area was increased until they sank far more slowly than organisms with a more compact shape.
The gentle eddies created by wind and temperature differences in the pond aide in keeping plankton suspended, but did not cause enough mixing to prevent a certain amount of layering among the population. Some microscopic plants were always close to the surface, but others only flourished in deeper water just off the bottom, where the water was cooler. Animal plankton with their capacity for independent movement, migrated vertically each day, rising at night and sinking during the daylight hours, each form leveling off at the layer most favorable to its particular needs
and its stage of development.
Throughout the lake the plankton populations were not evenly distributed either in depth or in area. In certain areas available nutrients had been depleted by microscopic plants. As plant numbers fell, a decline in the grazing animal plankton population followed. In other spots nutrients remained abundant, perhaps flowing off the surrounding land, and the drifting populations flourished. No matter what the conditions, their numbers and variety far exceeded anything the parent stream had so far produced, or would produce until meeting the sea in a distant bay.
One insect larva, that of the phantom midge, linked both the surface plankton and the murky bottom in which it burrowed out of sight during the day. At night it rose close to the surface where with swift shimmering movements it thrust its transparent body in pursuit of smaller, weaker animals which it captured with a great hooked beak. At either end of its body, the larva contained a pair of silvery air sacs. The pressure within the buoyant sacs was altered to keep the insect in one plane, so its energies were expended for rapid horizontal movement rather than overcoming a gentle gravitational pull.
The thick sediment where the midge larva took refuge during the day was alive with organisms, especially near the boundary between mud and water. Thin smooth roundworms squirmed among the fine particles; water bears crawled through the ooze on stumpy, clawed legs. Blood worm midge larva vibrated in loosely constructed mud burrows, using a hemoglobin like pigment to extract oxygen from the impoverished layer of water lying above the muddy silt where this vital gas was exhausted by bacterial decomposers. Decay was the single most important occurrence on the bottom, for it allowed the recirculation of essential compounds into the world of life. This detritus was nutrient rich in living animals and organic matter and omnivorous fish such as carp could grow large, gleaning the harvest of life between its gill rakers as it indiscriminately mined the bottom of the lake.
Segmented worms burrowed head downward in softer sediments; , extending the upper half of their bodies out of chimney like tubes that rose above the mud surface. Here they performed an important function by turning over the bottom sediments; without such activity, the bottom would build up more rapidly, hastening the demise of the pond. When the oxygen levels were especially low, these tubifex worms undulated briskly, creating currents of water that helped circulate what little of dissolved gas was available, until the bottom appeared to flicker with the pale flame of their yellowish bodies. There were other curious shaped segmented worms: some with long, drawn out proboscis that probed the sediment; others with tufts of gills at the end of their transparent bodies; some predatory, and some that fed upon the organic matter of the bottom much like their larger relatives, the earthworms on shore.
Flattened dragonfly nymphs walked slowly, spraddle legged, to keep from sinking in the loose sediment. Each breathed by drawing water into a capricious chamber in the rear of its body. Usually the insect expelled water gently, but if danger threatened, it could be shot out with force to propel the nymph out of harms way. Crustaceans of several kinds occupied the same soft bottom layer as the nymphs, sifting through the organic ooze for nourishing particles. Some were small with two hinged shells like miniature clams; others were larger, and lay on their sides, kicking vigorously until clouds of sediment rose about them.
The nature of the bottom varied with depth; in the shallower approaches to shore, with oxygen more plentiful, animals increased in number and variety. Amoebas and their shelled relatives groped through the fine silt, rounded pill clams strained the tiniest of suspended plankton through their long pink siphons, and loose particles resting on the mud surface were worked over by a multitude of crustaceans large and small.
Over long years of settling, the finest sediment lay in the center and deeper parts of the pond, but gravel, sand and lime marl produced by plants grew more pronounced closer to shore. Plant fragments and fallen leaves, and all kinds of drift material, carpeted the shallow bottom along the shoreline. Life was abundant here: plants manufactured food from dissolved gases and minerals in the lighted zones, while animals depended mostly on the fine decaying organic litter for nourishment. Wandering furrows of snails crisscrossed the fine sediment, sometimes ending abruptly as a snail released its hold and slowly ascended to the surface buoyed by a bubble of trapped gas. At the surface it performed a somersault, exchanged the waste gas for a gulp of fresh air and sank gently to the bottom, where it commenced feeding again.
The bottom was a region where the dominant organisms were bacteria and one celled animals. In deep places so little oxygen was available because of lack of circulation in the water that decay was not able to proceed all the way, resulting in odorous gases that at times rose as bubbles to burst audibly at the surface; and was often seen as a distinct trail of bubbles caused by the rooting of carp consuming large quantities of nutrient rich detritus. Often the gas escaped from both gills and the fish's anus as it ingested large quantities of the disintegrated organic matter.
The shoreline with its plants, the surface film, and the bottom with its burrowers and bacteria were only the margins of the lake; its bulk lay in the water itself. The most numerous creatures of the open water were plankton, clouding the water a pale yellow. The larger, more visible animals were fewer in number, often drawing near to the plant choked shore for food and shelter. There were wanderers among them, especially the predator bass and pickerel that followed from one end to the other. Turtle swam freely, popping up for breath anywhere across the still surface. Now that the sun was high and gave warmth, every log and protruding rock or stump had its full quota of basking turtles, some lying on top of others in their attempt to draw heat. Small, dull mud turtles and musk turtles lay close to the water; but higher up, often balanced precariously, were painted turtles and great red bellied turtles, stretching their legs into the air, allowing the suns heat to penetrate their blood streams where it would be transmitted elsewhere to their heavy bodies. Without the ability to regulate heat, the turtles depended on the warmth of the sun to maintain a sufficiently high pitch of bodily activity.
A great blue heron cruised overhead, flying low over one crowded log provoking the turtles to action. All splashed into the water alerting the others to danger. Within seconds all had slid into the water, sculling deep for safety. But the bird pumped on overhead, and soon one turtle after another broke through the surface to inspect the air and shore. Slowly they clambered out on the log again, often upsetting the balance of another sending it careening into the water. Soon all were back drying and warming as before.
This was the young lake, unique in the world, but with patterns repeated a million times over in the northern continent, patterns that could be seen in all the ox-bows and ponds of the river valley. Like the stream it was a world of water, yet different in the opportunities it offered to life. It originated with the stream and might die without returning to it, but in more than one place the stream had changed course and abruptly entered an ox bow it has forsaken ages before. The proximity of pond to stream allowed some animals and seeds to be transported far downstream from the still water, perhaps lost forever, but also possible colonizers to another lake perhaps miles down stream. Like a great restless thing the stream snaked its way eventually meeting other streams until its arrival to the sea. And this stream like many others, would run to that great body of water, very different from both the lake and the stream in existence and the life it contains, the mighty river.