A DEPRESSION in the ground that fills with water to form a shallow pond spontaneously attracts a population of plants and animals. The initial conditions in the pond favor certain species. These species multiply, altering the environment in ways that encourage the growth of other organisms, which also thrive for a time, thereby preparing the pond for still other forms of life that similarly become transient links in a fascinating chain of biological succession.

An inspection of the surface of the water barely suggests the continuing drama of successional change, even if the pond is clear. Ideally observations should be made below the surface of the water without disturbing the environment or the population. One can do so in comfort with a submerged observatory forming an integral part of a pond that can be filled and drained as desired. An observatory of this kind has been developed by Ernest C. Bay, professor of entomology at the University of Maryland. He explains the details of the facility and its use as follows:

"In 1967, while studying the ecology of chironomid midge larvae and their predators in temporary ponds, I became dissatisfied with the limitations of conventional sampling methods that are used to investigate changes in populations. A tadpole or a crayfish placed in a jar reveals little more about its place in the world than can be learned about the behavior and natural dominion of a shark in an aquarium. I longed for a way to actually enter the ecosystem as an observer.

"Diving apparatus is obviously impractical. There is hardly enough water in the shallows of a pond to cover a man lying prone in scuba gear. Moreover, the presence of an observer would alter the environment. Ultimately I hit on the idea of a waterproof bunker of concrete, constructed as a feature of the pond, from which I could observe unobtrusively the interactions of prey and predator at any moment of the day or night and where nature rather than I would determine the quantity, selection and availability of food. I named the proposed structure a pond benthobservatory. Benthos is the Greek word for organisms that live on or in the bottom of bodies of water.

"Before undertaking the construction I consulted Theodore W. Fisher, a colleague who was also engaged in aquatic research at the University of California at Riverside, where I was then stationed. Together we worked out the details of a practical observatory. The pond was made by excavating an area of about 400 square feet to a depth of 18 inches. A hole about five feet wide and seven feet long was dug in the center of the pond to a depth of about five feet. The bottom of the hole was covered with a slab of concrete from which a set of anchor bolts protrudes. The bolts engage the watertight observatory that was subsequently installed in the hole [see illustration at right].

"The structure consists of a plywood room covered on the outside with fiberglass cloth cemented in place. The room is entered through a hatch in the roof. The ceiling is insulated with fiber-glass wool protected by vinyl upholstery. Fresh air is drawn into the observatory through louvered slots in the walls of the removable roof section. The air is circulated and exhausted by a system of squirrel-cage blowers. Electricity enters the structure through a buried cable that connects to continuous strip outlets on three walls above the windows. Two 18-by-24-inch windows of plate glass are built in each long wall and one is built into each end wall.

"No artificial light is provided inside the structure, but an eight-foot fluorescent fixture is installed below the portion of the roof that overhangs the front of the observatory. This fixture is covered with a red sheet of transparent plastic to minimize the influence of white light on organisms that are observed at night. During the day the interior, which is painted battleship gray, is illuminated by sunlight that filters through the water. During late afternoon the rays of the sun enter the west windows through a prism of water that casts a giant solar spectrum on the facing work surface.

"The work surface consists of desk-high shelving 12 inches wide and two inches thick that girdles the room just under the windows. The corners of the planks are mitered to form a structural framework that helps to resist the inward thrust of the surrounding mud. Each of the shelves is supported by triangular brackets cut from the same planking. The observatory is entered through the hatch by first stepping on the shelving, then on a stool and finally to the floor.

"The pond is filled with irrigation water from a nearby reservoir. The water, which can be diverted through a filter of sand and gravel, enters the pond through an adjustable float valve and leaves by percolation through the soil. The pond can be drained quickly through a four inch plastic pipe. Rapid draining is occasionally needed for maintenance and for altering experiments. The drainage system is also designed to accommodate overflow if the float valve should stick or be accidentally damaged. The pond is normally filled to within an inch of the top of the windows. The lower edges of the windows are recessed below the pond's muddy bottom.

"After a population has become established in the pond casual visitors who enter the observatory are invariably awed. The hatch is closed to exclude direct sunlight and reduce reflections. Within minutes the eyes become adjusted to the relative darkness. An eerie glow from the windows fills the interior. Depending on the stage of biological succession, clouds of daphnia and copepods may hang suspended in mid-water, while individuals jiggle in front of the windows. Dragonfly nymphs and beetle larvae can be seen stalking their prey through fields of bubbles anchored to the sediment.

"Frequently we fence off sections of the pond in front of one or more of the windows with plastic screening that can be extended from the observatory to the edge of the pond. In these experimental enclosures the visitor can see fishes skimming below the surface in search of the scant remaining organisms that continue to multiply in the sections beyond the screen. Masses of snail eggs affixed to the windows glisten in the sunlight like clusters of yellow-centered beads in a gelatinous case.

"The dramas that unfold in this Lilliputian world can be brought close to the eye with a specially mounted binocular microscope. The instrument is supported by a movable fixture that can be maneuvered both vertically and horizontally for inspecting any area of a window. The microscope includes an accessory objective lens for focusing on organisms several inches from the glass.

"Among the more commonly viewed organisms are tubifex worms. Feathery gilled Branchiura are often seen undulating gracefully, with their heads buried in mud. Ostracods and beetle larvae browse among their fellow creatures.

"Bloodworm larvae can be studied in their tubes as their predaceous cousins, tanypodine larvae, wander among grains of sand in search of paramecia and other microscopic prey. One wonders what protects tanypodine larvae from the voracious hydras into which they repeatedly bump. The mere touch of a mosquito larva triggers the hydra's deadly sting! At certain times of the year hydras attach themselves in curtain-like colonies to the observatory's windows, where their feeding habits and seasonal population can be studied in intimate detail.

"A number of organisms other than the hydra attach themselves to the glass. For example, snails deposit their eggs here. Snail eggs serve as a substrate on and in which the larvae of midges construct tubes and to which other organisms are attracted as if to a miniature coral reef in a sandy waste. For the study of these creatures the glass windows serve the function of a microscope slide.

"Of particular interest among the many aquatic insects that use masses of snail eggs for a transient resting station is the beetle Laccophilus terminalis. This animal lays its eggs among snail eggs. Although the beetle also lays eggs elsewhere, the masses of snail eggs provide the most revealing site: the stage for a drama that is related to overpopulation. The ecological succession of the pond begins soon after the pond is filled. During each succession various organisms appear and for a time assume dominance. Frequently some organisms vanish as abruptly as they came, without evident cause. It is much like observing the evolution of life on earth in microcosm, complete with the development and extinction of species. Some species exist only briefly, like insects on land. Others seem to adapt and persist through various changes in the pond's evolution. The Laccophilus beetles are among these.

"Whenever the pond at Riverside was drained and refilled, except during the winter, Laccophilus beetles were attracted to it almost immediately. Mosquitoes and midges also appeared promptly. They laid many eggs during the first few evenings after the pond was filled, but then the rate declined. The newly filled pond was usually clear at first, but it became murky during the period when midges and mosquitoes laid eggs.

"From within the observatory the microscope revealed that the apparently muddy water was actually a broth of swirling rotifers. These organisms appear brightly iridescent in sunlight. Moldlike colonies of sessile rotifers also attach themselves to various surfaces at this time. The colonies diminished as populations of midge and mosquito larvae became dominant.

"Occasionally the cause of the murkiness changes as the rotifers are replaced by unicellular green algae. Within days microscopic animals (mostly daphnia) appear, reproduce and clear the water by feeding on the plants. By this time snails enter the pond, presumably through the water inlet, and begin laying eggs on various surfaces including the windows. The masses of snail eggs become favored sites for egg-laying by Laccophilus beetles. Adult females can be observed alternately surfacing for air and returning along a specific path to a particular mass of snail eggs. Presumably they follow a spoor. An average of two eggs are laid-during each visit. Every detail of the egg placement can be observed as easily as if the gelatinous mass of snail eggs were a crystal ball.

"The self-regulation of the beetle population becomes evident after the density of midge and mosquito larvae declines. Adult beetles continue to visit the snail-egg masses, but for a dramatically different purpose. At great effort they now extract their previously laid eggs and devour them! They completely ignore the snail eggs as well as the developing snail embryos. In time the snails deposit additional masses of eggs. The beetles rarely use these eggs for laying sites, although beetle larvae and adults continue to occupy the pond. Only after the pond has been dried and refilled, or during the normal succession the following year, do the beetles again consume their own eggs.

"As I have mentioned, the nymphs of dragonflies and beetle larvae prowl among the small bubbles anchored to the bottom of the pond The bubbles help to explain an almost unsuspected mechanism by which nutrients and food particles are distributed among the various levels in the community, particularly in pools that are protected from the action of the wind and waves. The bubbles are a by-product of the photosynthesis of microscopic algae that grow as a fine layer over sediment and other surfaces. Although the bubbles cannot be seen from above the water, they are a striking feature below it, particularly on a brilliantly sunny day. As the bubbles form and expand to critical size they detach from their moorings and rise to the surface, trailing bits of organic sediment. Shallow water that may appear to be perfectly clear and still when observed above the surface is seen from the observatory to be filled with effervescence as millions of tiny bubbles distribute food to the population above.

"During the course of 24 hours changes can occur within even the smallest pond that compare with those that span years m the terrestrial environment. There is no rain or wind within the pond, but these elements of weather as well as sunshine and cloud cover exert a strong, if indirect, influence on activity below the surface. Occasionally the combination of environmental factors strikes a balance that is ideal for a specific organism. Almost overnight the pond becomes the scene of the population explosion known as a bloom. The environment recovers within a matter of days.

"A good site for a benthobservatory is one that has a constant supply of inexpensive water and is reasonably secure from vandalism. Ideally the site should be close enough to related facilities so that the observatory can be visited conveniently at various times of the day or week. If the site is near an existing pond or stream, care should be taken to protect it against flooding.

"The site should also be selected with the intention of constructing the pond around the observatory rather than placing the structure in an existing pond. Several advantages support this choice. For example, the observatory is much easier to assemble in a dry hole than in a wet one. Moreover, the designer has complete flexibility with respect to features such as flooding and drying the control of water level, the composition of the substrate, the sectioning of the pond for experimentation, the quality of the water and maintenance. With these factors in mind the designer should also choose a site that facilitates draining. Preferably the pond should be constructed somewhat above the level of the surrounding terrain, perhaps by erecting earth dikes.

"Where feasible the benthobservatory pond should be higher than an existing pond or lake and parallel to a tributary that can be dammed and partially diverted as a water supply. At a site of this kind some windows can face on the stream and others on a quiet backwater. The stream would doubtless provide the observer with a community of organisms that differs from the community found in a still pond. The arrangement has the disadvantage of inviting turbidity during rainy periods and possibly of flooding.

"Clear water, although it is aesthetically pleasing, is not essential. Most observations are made within a foot or two of the windows. Organisms within this range are easier to see against a background that is somewhat turbid than against a clear one. The effect is comparable to observing an insect against a plain sheet of paper rather than against a confusing background of twigs and leaves.

"Useful observations can be made at a distance of between five and seven feet in water that is maintained at a depth of 14 to 18 inches. The horizontal view rarely exceeds that distance. A pond of this size does not unduly restrict fishes, frogs, turtles and other large organisms the experimenter may wish to study.

"The observatory can be built by anyone who is reasonably handy. The construction details and the cost vary with size. The structure at the Riverside campus was made with six sheets of water- proof plywood five feet wide and nine feet long that were bought on special order. Standard sheets of plywood measure four by eight feet.

"The structure was prefabricated and preassembled in the carpentry shop complete with fans and electrical outlets. It was then dismantled for interior painting, the installation of windows and the application of fiber glass to the exterior surfaces. The glass windows were set in frames of 14-gauge galvanized sheet steel. The frames were sealed into the wall cutouts with silicone-rubber cement. The glass was then similarly cemented into the frames against a gasket of 3/8-inch Tygon tubing that had been previously cemented to the inner angle of the frame. A matching pressure plate was then fastened to each sheet of glass with screws.

"Oversized sheets of fiber glass were applied to the plywood. The excess is used for overlapping the corners of the structure during final assembly. The observatory is completed by cementing the overlaps in place, bolting the floor sills to the concrete slab and sealing the bolts with aquarium cement. Soil is filled around the structure to the base of the windows. Within a day, after the fiber glass has cured the pond is ready for filling.

"The observatory requires little maintenance other than occasional cleaning. Exterior maintenance involves only the occasional cleaning of windows and pond renovation when the experimenter recycles the environment or undertakes a special experiment. If the mud contains sharp sand, the danger that the glass will be scratched when the windows are cleaned can be minimized by covering the bottom of the pond with a layer of fine topsoil.

"The windows can usually be cleaned from the roof of the observatory with a short-handled sponge of the type used for waxing floors. Make downward strokes. Then raise the sponge through clear water to wash off the sand that may have been picked up from the bottom.

"If hard algae adhere to the glass, drain the pond and scrape off the organisms with a razor blade. The growth of algae on the glass can be discouraged when the observatory is not in use by installing waterproof shades of black polyethylene sheeting over the windows. Support the shades at the top of the windows by lengths of inexpensive pipe. Weight the bottom edges of the sheets. The growth of rooted vegetation can be restricted by burying sheets of galvanized steel just below the surface of the mud. Holes can be cut in the sheets to serve as planters.

"In some climates it may be necessary to drain the pond and abandon the observatory during the winter. With the coming of spring a completely new set of underwater phenomena can be observed in progression through the summer and the fall. Although pond inhabitants and events vary with location, no pond remains sterile. Where there is water there will be life. For this reason a benthobservatory is certain to yield rewarding adventures."

Bibliography

FRESH-WATER INVERTERRATES OF THE UNITED STATES. Robert W. Pennak. The Ronald Press Company, 1953.

A GUIDE THE STUDY OF FRESH-WATER BIOLOGY. James G. Needham and Paul R. Needham. Comstock Publishing Associates, 1955.

MANAGEMENT OF ARTIFICIAL LAKES AND PONDS. George W. Bennett. Reinhold Publishing Corporation, 1962.

THE LIFE OF THE POND. William H. Amos. McGraw-Hill Book Company, 1967.

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