The Natural History of an Unnatural Lake
Lake Springfield, this is your life
May 27, 1977
In most ways, Lake Springfield—an unremarkable man-made water supply like dozens of others in Illinois—is the most interesting thing in Springfield, not excluding the General Assembly. I found it so, anyway, and wrote about it in its several aspects.
This piece is (I see now) a surprisingly in-depth explainer about the biology of the capital city's water supply lake. My editor at IT in those days, Alan Anderson, Jr., is a fine science writer himself; it is not clear that an editor less comfortable with science would have bought it.
For clues to the nature of Lake Springfield it is necessary to reread the story of its origin. The lake is man-made, as much an artifact as an office building or an automobile. Before 1930 it did not exist, except in the minds of the politicians and planners who argued for its construction as the answer to the city's nagging water problems. And eventually (by the year 2277 if current estimates are correct) the lake will again cease to exist, as the basin fills in with sediment washed into it from upstream and the weeds and trees reclaim the land once their own.
* * *
Lakes are among the most transitory of landmarks. This is especially true of manmade lakes, whose lives are measured not in eons or even in centuries but in decades—a mere blink of the eye of time.
Physically, too, Lake Springfield is a modest thing, barely a blemish on the face of the earth. It varies in depth from only a few inches in the Sugar and Lick Creek arms to thirty feet in a hole near the dam. Were the lake's 4,300 acres reduced to a pond sixty feet square, it would be proportionally only as deep as a piece of paper is thick. Yet within this small space there is a diversity of environments and creatures that is astounding. Lake Springfield is home to millions of plants and animals, some familiar, some so strange that they rival the exotic fauna of Australia or the Amazon in eccentricity.
* * *
Before it was flooded, much of the Sugar Creek valley was under cultivation. Fences and farm roads crisscrossed it, and a covered bridge spanned the creek. Over the centuries the curling, curving path of the stream had created a wide, shallow dish in the earth, a gentle valley admirably suited for an artificial lake. The valley rose only two feet per mile for six miles upstream from the dam site. At its widest, it measured some 6,800 feet east to west, and at the planned pool elevation of 560 feet would form a reservoir seven square miles in area that would hold more than 21 billion gallons. At that level only 17 percent of the total water area would be shallow (five feet or less) water.
The valley floor was—still is—made of twenty feet of stream-deposited sediment that lies atop a bed of clayey sandstone, which in turn sits atop alternating layers of shale, sandstone and coal. It's a good natural lake bed, though workers did run into trouble building the dam and spillway. The ground there was mostly a sandy shale which was sturdy enough when dry but which turned soft when saturated with water. Engineers had to use concrete to seal the subsoil strata against seepage.
The trees that originally stood in the valley were cut down, of course, and their stumps ripped out. The stumps, like the roadbeds and fences and ravines that once scored the valley sides, like Sugar Creek itself— now invisible and mostly forgotten—still shape the underwater environment of Lake Springfield. Fish like the largemouth bass migrate vertically up and down the sides of submerged hills and roads according to the time of day and the temperature of the water. Stump fields left behind by sawmen have become prime feeding grounds, and the old stream bed and the gullies that emptied into it offer pockets of cool water in the summer and refuge from the ice in winter.
Underwater topography is only one ingredient in the complex recipe of life in the lake. Water, the sunlight that warms it, the chemicals dissolved in it, the plants and animals that live and die in it, the wind that moves it—all these things, interacting in ways so complex that scientists have only lately begun to understand them, shape the Lake Springfield ecosystem.
* * *
It is nine o'clock in the morning in the central lake basin just north of Lindsay Bridge. As the sun climbs over the trees and shines full on the water, millions of phytoplankton—one-celled plants—are borrowing the energy pouring into the lake and, by the process of photosynthesis, converting carbon dioxide, water and inorganic nutrients into simple sugars. As a byproduct of this alchemy, the phytoplankton pass back into the water the oxygen needed by fish and other animals. At nine o'clock there are nearly 8 parts per million (ppm) of oxygen in the water near the surface here; farther down, where the decomposition of bottom waste by anaerobic bacteria sucks oxygen out of the water and where light for photosynthesis cannot reach, oxygen levels are lower. At a depth of 18 feet, the water contains barely 3 ppm of oxygen. In general, fish cannot live in water that contains less than 5 ppm of oxygen.
As the sun rises higher in the sky each day, the level of oxygen in the lake water rises with it. Then, when the sun sets, photosynthesis stops, and with it the production of oxygen. Lake-dwelling animals continue to use it, however, so oxygen levels sink to their lowest at about six in the morning, after the long, sunless night.
In one way or another, all the creatures in the lake depend on this daily replenishment of oxygen by the lake's green plants. Though there is some mixing of atmospheric oxygen with the water by wave action, most of the oxygen in the lake is produced in the lake itself. Just as crucial, the plants themselves are food for the animals farther up the food chain. Photosynthesis is the vital first step in the process by which green plants convert sunlight and inorganic nutrients into foods usable, ultimately, by all the lake's creatures. In short, in order for the lake to live, the plants must live.
* * *
The most abundant life forms in the lake are the least visible. They include one-celled plants like the blue-green and green algaes, the diatoms (yellow-green, pillbox-shaped algae), and a host of bizarre microscopic animals like the half-plant, half-animal Euglena. Together they constitute the free-floating plankton (from the Greek planktos, meaning "wandering") that forms the first and vital link in the lake's food chain.
The diversity of form and function among the plankton is incredible, their numbers inestimable. In the lake's inlets, its shallow, sunlit water and the sluggish currents have combined to make ideal plankton nurseries. It was from one such inlet behind CWLP's lakeside power plant that researchers in 1975 seined nearly 80,000 separate organisms (most of them diatoms) from each liter (about a quart) of water sampled. In open water, plankton populations are more modest, but even the lowest concentration measured that year was roughly 3000 organisms per liter.
The most common denizens of this microscopic menagerie are green algaes like Microspora and Ulothrix, single-celled plants colored grass-green by the chlorophyll contained in them. As many as 24,750 individual plants have been found to congregate in each liter of water in the upper levels of Lake Springfield (this near Lindsay Bridge), though in most parts of the lake the concentrations are closer to ten or fifteen thousand organisms per liter.
Also present in the lake is the only blue-green algae sampled, Oscillatoria, a filamentous variety that grows in dark-colored clumps that sway and twist with the currents. Much more substantial are the populations of diatoms. These algae have a delicately sculpted shell of silica. Lake Springfield water is home to the star-shaped Astrionella, the rod-shaped Fragilaria and the Tabellaria, which link together in chains that look like rows of dominoes.
In most parts of the lake, the plankton populations vary according to natural laws, being more common in shallow inlets and creek arms where sunlight and nutrients are plentiful, and less concentrated in the less generous environment of the deeper, open waters. There is one exception, however—one place where the natural rules have been suspended. It is in the lake's northern end, where warm water from the city power plant spews into the lake. There the plankton populations are thinner, less robust—"stressed," in the parlance of biologists. Populations along the dam and offshore next to the coal storage area and across the lake from the coal piles range from 10,000 to 15,000 organisms per liter; at the mouths of the discharge pipes, the populations ranged from five to six thousand organisms per liter.
* * *
More complex plants—starting with the relatively simple mosses and climbing up the evolutionary ladder to the vascular plants (seed-bearing species ranging from the ferns to trees) though not as common, are more visible. Unlike their more primitive cousins, these higher plants have abandoned the free-floating life and taken root along the shore, some in and some under the water. Together, these rooted plants define the littoral zone, the area in which rooted plants grow. In small ponds the littoral zone (the word comes from the Latin for "shoreline") may stretch from one shore to another; because of Lake Springfield's more complex topography, its littoral zone is haphazardly placed, tucked into an inlet here and a creek arm there.
The littoral zone can itself be divided into three zones, each defined by its depth and the kinds of plant life that flourishes in it.
The emergent zone lies closest to shore, in shallow water, and is so called because its floral complexion is made up of plants whose stalks and leaves, though rooted below the surface, emerge above it. Sedges, saw grass and the ubiquitous cattail call the emergent zone home.
The submerged stems of these plants form an underwater jungle which, like its terrestrial counterparts, is a haven for wildlife of all sorts. Certain protozoa and algae attach themselves to their stalks. Insect larvae, tiny crustaceans and the fry of many fish species take refuge there against the predations of those that would eat them. Some species have leaves underwater, and their photosynthesizing helps oxygenate the water. When the plants die, they decompose in the water, thus adding to the stock of nutrients from which the lower life forms take sustenance.
A little farther out (how far depends on the slope of the shore) is the deeper, floating leaf plant zone. Plants here, instead of rising above the water, float on top of it. In Lake Springfield, the most conspicuous resident of this zone is the lotus (Nelumbo lutea), commonly called the water lily. This plant has a tough stem that connects the single leaf to a tuber-like anchor in the mud. The leaf itself is broadly circular in shape and often grows to a diameter of three feet. The flowers are yellow and the conical seed pods—ingenious pieces of packaging whose flattened ends are pockmarked with chambers, each holding as many as two dozen black seeds—are favorites of handicrafters who use them in the construction of seed wreaths.
A lotus patch is a complex ecosystem in itself. In late summer the sluggish inlets of the lake (Hazel Dell is one, the Sugar Creek pond near Glenwood High School is another) fill with lotus until their leaves carpet the water from shore to shore. They are pretty to look at, though risky to drive a boat through, since their stalks are a match for the stoutest propeller. Close examination reveals a world as complex as that crowded around the stems of the cattails in the emergent zone. Snails, bugs of various species, and flying insects like the mayfly lay their eggs on the undersides of lotus leaves. Algae clings to the stalks, bass and bluegill feed on the creatures that seek shelter there, dragonflies use them as landing fields, snakes sun on them, and sunfish nest in their shadow.
Beyond the lotus and the duckweed is the submersed plant zone, the deepest and least visible of the three zones of the littoral. Plants at this depth live almost entirely underwater (only their flowers are pollinated above water). The submersed zone is inhabited by hornworts and other pond weeds which have adapted to their underwater environment. Their leaves and stems lack the thick, tough supporting fibers common to plants that must resist the pull of gravity to stand upright. Buoyed by water, the underwater plants have slender stems and thin leaves that offer as much surface area as possible to the scant sunlight that filters through the cloudy water above.
Beyond the submersed plant zone no rooted plant can live; there is not enough light. Obviously, the extent of the littoral zone (as measured from the water's edge) will vary from lake to lake according to the clarity of the water in it and the depth of the bottom. If the water is clear, submersed plants can get enough sunlight to survive at a depth of several feet. In waters like those of Lake Springfield, on the other hand, the relatively high concentrations of organic and solid matter carried by the water blocks off the sun, and plants can live only in shallow water.
Depth is not the only factor that determines where and how lake plants can take root. Because the prevailing winds in central Illinois blow from the south and southwest, the lake's windward shores (the two Forest Parks, Beach Park, and other spots on the northern and northeastern shores) are pounded by wind-driven waves. The waves make it impossible for plants to take root in the water, no matter how inviting those shores may be from a topographical point of view. The only plant life that can survive this barrage of water (other than the free-floating algae) is the filamentous algae that attaches itself in slippery, feathery tufts to the rocks that fringe the windward shores.
* * *
Each species of animals, like each plant, occupies a different corner of the Lake Springfield ecosystem. At the surface, mosquito larvae hang suspended from breathing tubes poked through the surface film. Pond skaters cruise atop the water in search of a meal, while diving beetles dart just below it. Whirligig beetles, common scavengers, oar themselves about with specially adapted middle and hind legs, simultaneously peering above and below the surface with two sets of eyes—one set above, the other below the visual barrier that separates the world of air from the world of water.
Beneath the surface, the mid-water region is as bare of animals as the surface is full of them. This zone is home to both the largest and the smallest of lake dwellers. The large carnivores like bass and turtles cruise the open waters in search of prey while smaller fish hug the shore, seeking the protection of plant cover. These large animals share their space with the plankton, which float through it suspended on the currents.
It is the lake bottom that is, to people at least, the most alien of the lake habitats. In quiet water in the shallows, the mud teems with life: crayfish, dragonfly and damselfly nymphs, mussels, earthworms and many other creatures. The living conditions in deeper water, however, are less hospitable. Too deep for light-hungry plants to roost, the mid-lake bottoms are dark, barren places, oxygen-poor and carbon dioxide-rich. Only worms and a few larval forms and a clam or two can live here—they and the millions of bacteria that feast on corpses of small creatures and large alike that rain steadily from above.
The process by which organic materials are produced from inorganic nutrients and sunlight and their subsequent conversion to use by organisms along the food chain is reversible—indeed, must be reversible. It's been estimated that if decomposition were to stop recycling nutrients locked up in dead organisms, all plant activity would halt in three months, starved by lack of fuel. Just as green plants are the agents of what could be called "composition," so bacteria and certain fungi are the agents of decomposition. As limnologist Robert Coker notes in his book Streams, Lake, Ponds: "Although we may disparage it as 'rotting,' [decomposition] is just as necessary . . . as is any other link in the whole chain of transformations that forms the great organic cycle . . . . The decomposition of organic matter should really have in our minds all the beauty of the sprouting seed and the growing plant. But it does not!"
The rotifers (so called because of the rotating cilia with which they propel themselves through the water), small crustaceans and other members of the zooplankton community are food for the lake's population of invertebrates, the next link in the food chain. The flatworm Dugesia tigrina, roughly three-quarters of an inch long, with a light-sensitive eyespot on its triangular head, has been found in Lake Springfield, as has the leech, Erpobdella punctata. The damselfly Enallagma, whose nymph breathes underwater by means of gills and which stalks its prey, is plentiful, as is the pond skater (or water strider) and ferocious, predacious diving beetles which feed on small water animals. The elmids, which crawl about on the mud bottom in search of prey, breathing from an oxygen reservoir they carry on their bodies, are also common, as are midges (fifteen species of which are spread throughout the lake) whose larvae are an important food source for fish. Worms, snails, mayflies . . . the list of macro-invertebrate species is fifty animals long.
And yet the list is a short one compared to the animal populations of other lakes of this type—short enough, in fact, to cause concern among biologists. A 1975 investigation into the species and numbers of invertebrate animals described fifty species as tantamount to a "biological depression." Researchers suggested that "some common, persistent, agricultural chemical" might be a cause, since no source of industrial pollutants could be found.
This low number of species at the lower end of the food chain can have a bad effect on the animals—mainly fish—that either feed on them or feed on the animals that do. There is some evidence of such an effect. Two fish counts have been done at Lake Springfield in recent years, the first in 1963 and the second in 1972. The more recent census showed a dramatic decline (172 to 37) in the number of freshwater drum collected in the lake. One possible explanation: The drum feeds on mollusks, and mollusks are one of the invertebrates in short supply.
Generally, though, the lake's fish population is in good shape. The larvae of the midge, a mosquito-like fly, which is in ample supply, is a favorite food of many lake fish, and the biological depression diagnosed by researchers in 1975 has left most species' food source undiminished.
The 1972 census revealed that lake fish were larger, more numerous, and, judging from the samples brought up by the electric shockers used in such studies, healthier. There was a decline in the number of species found in the census compared with the 1963 total (from twenty-three to seventeen); the red shiner, bluntnose and flathead minnows, the hybrid bluegill/green sunfish, the black-stripe topminnow, the quillback carpsucker, and small-mouth buffalo had vanished. (Two species, the golden shiner and the madtom, a small catfish, showed up in 1972 for the first time.)
Two of the principal fish in the lake, the gizzard shad and the largemouth bass, showed big jumps in population between censuses. The shad and the bass live in a classic predator-prey relationship. The gizzard shad is a member of the herring family and a relative of the anchovy, a vegetarian that feeds by straining plankton-rich water through a sieve-like set of rakers in its gills. In season, schools of shad fingerlings are familiar sights on the lake. They swim and feed close to the surface where they find the plankton, their passage marked by a thousand tiny wakes. Once in a while the parade is enlivened by the finger-snapping sound of a shad slapping the surface, showing a glint of silver as it breaks into the world of sunlight for an instant before sinking back behind the water's dull brownish-green curtain.
The bass is a sunfish, the champion predator of the lake system, a fish much prized by fishermen for its strength and agility. Though the bass will dine happily on earthworms washed into the water after a rain, and even, if big enough, on young frogs and ducks, the bass lives on shad.
Occasionally a bass will attack a shad school, rushing up out of the dark water toward the surface, his momentum sometimes carrying him out of the water, scattering the panicked shad like a lion charging into a wildebeest herd, until, tired or sated, he retreats and allows the school to regroup and continue its grazing. Thus are the links in the food chain forged.
* * *
Lakes, like people, are born, grow old and die. That is what will happen to Lake Springfield. Indeed, it is happening now. As the lake ages, streams continue to feed the lake with plant nutrients. This reservoir of nutrient raw material is augmented by the recycled remains of existing plants and animal populations. The more life the lake supports, in other words, the more life it is able to support.
It is possible, however, for a lake to become too rich. Algae populations especially can run out of control and "bloom." or explode, living, dying, and accumulating on the bottom faster than bacteria can digest and recycle them. In old age, a lake's ecological balance is tipped a little too much, and the process begins to reverse. Dead and decaying algae feed anaerobic bacteria which, unlike the algae, consume oxygen rather than produce it. The water is able to support fewer and less desirable fish and may grow stagnant during the summer months. Sedimentation increases and land plants gradually advance onto once-submerged mud flats. The lake is said to be eutrophic.
In time the lake reaches a climax stage in which it again becomes dry land. For the main body of Lake Springfield this fate is many decades away: if the basin is dredged and the sediments piling up there removed, its useful life as a reservoir can be extended indefinitely. But in the shallow creek arms at the lake's southern and western extremities, we can already see a preview of a lake's climax stage. These areas, which have not been dredged, no longer hold open water. Grasses have filled what used to be coves and inlets, and dry-land trees are gingerly reaching for footholds farther and farther from shore.
A manmade lake has no natural outlet, and the silt washed into it by its feeder streams has nowhere to go but to the bottom of the lake basin. The lake will fill up. Men may choose to dredge it, but the lake, as an invention of men. can be maintained as such only by their intervention. Of all the life cycles played out in Lake Springfield, the grandest is that of the lake itself. ●
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