ABOUT 10,000 ponds and small lake are created each year in the U.S Most of them are on farms an are maintained primarily for watering livestock. Many ponds that are careful managed, however, also become facilities for fishing, boating, swimming, ice skating, aquatic gardening and so on.

Careful management is essential if pond is to serve these purposes, because every open body of water becomes a active universe of plants and animals The character of a pond's population changes from day to day and year to year, not always in directions that pleases the human user. When the direction of the change is disappointing, the use may say that the pond has become "polluted" or "sick." In fact, every pond is a healthy environment for the organisms living in it, although they may not appeal to the owner.

To encourage the development of a desired population, the owner must make a limnological study of the pond, which entails determining the physical, chemical and biological conditions of the water and modifying them to meet the preferences of the desired population. An example is a small pond that was created 15 years ago on a farm near Keystone, Ind., and figured in a prizewinning science-fair project undertaken by Betty Sue Settle, the daughter of a teacher and farmer who made the pond. Miss Settle, now a student at Ball State University, writes:

"Our pond was established in 1955 on the slope of a field about half a mile from our house as a water supply for cattle pastured at the back of our farm. The water was pumped to the animals so that they would not contaminate the supply. In its second year the pond was stocked in the proper proportion with prey and predator fish: fingerlings of bluegill and largemouth bass. The fingerlings developed rapidly and provided fine fishing until the extremely cold winter of 1963, when the water froze to a thickness of 13 inches. None of the fish survived, even though more than five feet of water remained under the ice. In later years all efforts to restock the pond failed. Fingerlings did not survive in the water for more than a few days. Late in 1966 I decided to find out what had gone wrong. To that end I bought a few books on basic ecology.

"Our pond is roughly oval in shape, about 250 feet long, 75 feet across at the widest point and roughly eight feet deep in the middle. Like all ponds, it consists of four more or less distinct environments or zones, each uniquely hospitable to certain groups of plants and animals. For example, large plants take root and do best in shallow water near the edge. The upper layer of water beyond these plants, which receives the most light, encourages the growth o plankton, including algae, protozoans diatoms, crustaceans and rotifers. The middle layer below receives less sun- light and is cooler; it supported our fish. The bottom layer, which exists only at the deepest part of the pond and tend to merge into the middle layer where the water becomes warm in midsummer, is about a foot thick, dimly lighted, cold and low in dissolved oxygen. This zone is inhabited largely by microorganisms that feed on and thus help to decompose particles of organic debris that settle from the upper layers.

"To investigate the physical, chemical and biological conditions of the pond throughout the year I established four collection stations, equally spaced along the major axis of the ellipse. The positions were determined by reference points on the bank, such as particular trees and nooks. Specimens were collected from these stations at the end of each month. In summer a collecting vessel could be moved to the center of the pond by a pulley that rode on a suspension rope anchored to the banks. Station No. 1 was at the east end of the pond at what was thought to be the deepest point. At Station No. 1 and Station No. 2 specimens were taken from the surface and the bottom layer, whereas at Station No. 3 and Station No. 4 they were taken from the bottom only, because of the shallowness of the water.

"Observation of the physical conditions was limited to temperature, transparency of the water and thickness of ice (when ice was present). Transparency was measured by lowering an aluminum plate on a rope until the plate could not be seen and then raising it until the plate became visible. The distance was determined by calibrations on the rope. In winter blocks of ice were cut with a large handsaw and carefully removed for measurement of the thickness with a steel tape.

"Chemical observations included the determination of dissolved oxygen, the relative acid-alkaline balance in units of pH and the measurement of free carbon dioxide. The Winkler method of oxygen determination has become the most generally used means of measuring dissolved oxygen in ponds because of its convenience and the fact that it yields sufficiently accurate results. To make this measurement I put 250 milliliters of pond water in a bottle and added by pipette two milliliters of manganese (II) sulfate and two milliliters of alkaline iodide solution. The brown precipitate that formed was then dissolved by two milliliters of concentrated sulfuric acid. The resulting manganese (IV) sulfate reacted with potassium iodide in the alkaline iodide solution to form soluble iodine. The iodine was titrated with a standard solution of sodium thiosulfate, using starch as the indicator.

"Standard manganese (II) sulfate solution is prepared by dissolving 368 grams of the chemical in distilled water to make one liter. Alkaline iodide solution is prepared by dissolving 500 grams of sodium hydroxide and 150 grams of potassium iodide in distilled water to make one liter. Sodium thiosulfate is prepared as a .025 normal solution because this concentration simplifies the calculation of weight equivalents for iodine and oxygen. I prepared the solution by dissolving 6.2 grams of sodium thiosulfate in distilled water to make one liter.

"During the reaction each equivalent weight of dissolved oxygen produces an equivalent weight of iodine. Therefore the titration of iodine can serve as a direct method of determining the number of parts of oxygen in a million parts of water solution. For example, if the addition of eight milliliters of sodium thiosulfate alters the color of the solution, the specimen contains 16 parts of dissolved oxygen in a million parts of solution. The starch solution, which functions as the indicator by turning blue in the presence of iodine, is made by dissolving .5 gram of starch in 100 milliliters of hot water and cooling the liquid to room temperature for use.

"The acid-alkaline balance of the water was determined within a quarter of a pH unit by means of a recently developed paper for testing solutions of minimal buffer capacity and low total solids. Known as pHydrion Lo Ion test paper, the material is available from Micro Essential Laboratory Inc., 4224 Avenue H, Brooklyn, N.Y. 11210. The test is made by immersing a strip of the sensitized paper in a special test tube that comes with the kit and, after one minute, comparing the hue of the strip with a calibrated color chart that is also provided.

"Free carbon dioxide was determined by adding 10 drops of phenolphthalein indicator solution to 100 milliliters of pond water and titrating with a .023 normal solution of sodium hydroxide until the water turned a faint but permanent shade of pink. A solution of this concentration is prepared by dissolving 910 milligrams of sodium hydroxide in distilled water to make one liter. Phenolphthalein indicator solution is prepared by dissolving .05 gram of phenolphthalein in 50 milliliters of ethyl alcohol and 50 milliliters of distilled water. The parts per million of free carbon dioxide in the specimen equal 10 times the number of milliliters of sodium hydroxide required to make the color appear.

"A monthly census of the microorganisms was taken at each of the four sampling stations. Collections were made with a four-liter aluminum pail. The four-liter specimen was filtered through a homemade plankton net of silk inside a 200-milliliter container, thus concentrating the organisms from four liters to 200 milliliters. Half of the concentrated specimen was then transferred to a clean bottle that contained; five milliliters of a 5 percent solution of Formalin. The density of organisms per cubic centimeter of pond water was then determined with a 400-power microscope and a homemade counting chamber.

"In winter the ice cover reached maximum thickness in December-and January, but during the 1967-1968 season it amounted to only seven inches. The temperature of the water at Station No. 1 and Station No. 2 varied from 27.2 degrees Celsius on August 15 to zero degrees C. on January 27. At Station No. 3 and Station No. 4 it ranged from 27.9 degrees to one degree. The slightly larger variation in temperature at the latter stations is explained by the shallower depth of the pond there. Four groups of plankton were identified and counted: rotifers, protozoans, diatoms and crustaceans, all of which are eaten by fish.

"All data were tabulated and subsequently plotted as graphs. The results immediately suggested that our fish had died from the lack of dissolved oxygen or a slightly acid condition of the water, or both.

"What had depleted the oxygen? The explanation became clear when I reviewed the history of the pond and the data I had collected. Dissolved oxygen in a pond comes primarily from two sources. Most of it is provided by green plants, notably those in the plankton. The gas is liberated during photosynthesis. A lesser amount diffuses into surface water from the air.

"Two demands are made on the pond's supply of oxygen. Animals consume a substantial part of it. Hence living green plants function as essential allies of the animals. A second demand is represented by the oxidation that takes place during the decomposition of organic debris. In ponds and lakes that are deeper than about 20 feet the debris settles to the bottom, exhausts the oxygen and thereafter becomes relatively inert. In shallower water, as in our pond, convection currents carry oxygen-rich water to the bottom, where it is consumed as long as debris is available. This process lowers the concentration of oxygen in the middle layer and the upper layer, which is the oxygen reserve that sustains fish. Much of the organic debris is of plant origin. In effect, when plants die, they become aggressive competitors of the animals, at least for the available oxygen.

"Our pond developed a massive population of plants, particularly those that grow well in shallow water. Various waterweeds, including cattails and small willows, sprang up after the pond filled, and by 1963 they had covered a substantial area of the water. The debris shed by this lush growth appeared to explain why we had lost our fish. To check this supposition we undertook a cleanup operation in August, 1967. It lasted for several weeks. I then resumed the testing routine.

"The cleanup caused an abrupt shift in the data. For example, the transparency of the water in the cleaned pond averaged 16 inches in winter and spring and 11 inches in fall, whereas previously it had averaged only six inches. Seasonal variations in transparency appeared to be caused by plankton in fall, snow cover in winter and mud in spring.

"The concentration of dissolved oxygen varies inversely with the temperature of the water, rising in winter and declining in summer. Before we destroyed the weeds the oxygen concentration ranged from a maximum of 9.4 parts per million at one station to 8.3 parts at another station on the same collection date, August 15. In January the concentration was well below saturation, the theoretical limit to which oxygen can dissolve in water at a given temperature, and below the limit fish can tolerate. After the cleanup it rose to 12.1 parts per million, substantially above saturation in summer and well within the limit required by fish. Ice accounted for some of the winter deficit, as indicated by the fact that the concentration rose when warm rams melted the surface.

"The pH was not appreciably affected by the cleanup. It ranged from 5 to 7 during the first few months and remained almost constant thereafter, somewhat on the low side for fish culture. Fish in this area cannot tolerate a pH below 5. It turned out that the pH of the water was close to that of the subsoil and was doubtless determined by the subsoil. We corrected the balance by adding pulverized limestone to the pond. The water is now almost neutral and so is suitable for fish.

"Final tests indicated that the temperature, transparency, dissolved oxygen, plankton density, pH and carbon dioxide concentration were all within the required limits for the culture of fingerlings. To check this conclusion I made a simple experiment. Three goldfish were placed in a small aquarium and given fresh pond water twice daily. In the beginning I added a small amount of prepared food to the water. This ration was gradually withdrawn as the fish became accustomed to eating plankton. They appeared to adjust easily to the change, and all of them thrived.

"We have not restocked the pond with fish, but we have extensive plans for it. I am away at school and will be for some time. My father, however, will return from teaching next year. He will then drain the water and bulldoze the area to increase its size and depth. The watershed will also be increased by running more drainage ditches into the enlarged basin. When this work is completed, we shall restock the pond. Thereafter we shall provide the maintenance that has been suggested by these experiments. Professor Eugene P. Odum of the University of Georgia has said that one of the best introductions to ecology is the study of a small pond. As an amateur who has tried it, I agree."

NOT ALL amateurs who might otherwise enjoy ecology have access to a farm pond. Even so, they can be of good cheer. According to Harold Abernathey, a city dweller of Waterloo, Iowa, it is possible to set up a serviceable pond in one's living room-if the experimenter is willing to settle for mosquitoes instead of largemouth bass. Abernathey writes:

"Our state has acquired a measure of renown for its tall corn and fat hogs, but somehow the sheer bulk and unparalleled ferocity of our mosquitoes have received scant acclaim. I hope to correct this oversight by explaining how to grow Iowa mosquitoes anywhere. All you need is a lump of Iowa soil that contains the eggs, and an apparatus that has been designed by Ernest C. Bay, a member of the Department of Biological Control at the University of California at Riverside.

"The main part of the apparatus consists of a pair of transparent plastic containers of about one-quart capacity with close-fitting lids. I bought a pair from a local hardware dealer. Cement the lids together, top to top, and cut a big hole in the middle, one that extends to within about a quarter of an inch of the flanged edge. You now have a ring with flanges on both sides.

"You will also need a funnel of clear plastic with a top somewhat narrower than the lids. Cut off the spout and cement the base of the resulting cone over the hole in the ring. Assemble the containers to the ring to form a unit. The cone now extends into one of the containers. This container will function as a collection vessel. The other container, which forms the base of the apparatus will become the pond [see illustration at right].

"The pond must be given a mud bottom and a filter. The filter consists of two elements: an inverted Petri dish of clear plastic that fits inside the pond, and a disk of nylon scrim. Perforate the bottom of the Petri dish with a generous number of small holes and place it upside down inside the pond. Cover the perforations with the scrim. Later you will cover the scrim with a layer of gravel and soil and fill the pond with water. First, however, you must have a way of circulating water through the pond.

"Circulation can be maintained by a novel air pump that is connected to the pond by two short lengths of Tygon tubing. Drill a pair of holes for the tubing through the side of the container-pond, one close to the bottom and one close to the top. The diameter of the holes should be slightly less than the outside diameter of the tubing. The push fit ensures a leakproof joint. Make a notch in the edge of the Petri dish for accommodating the lower hose.

"The pump consists of a small plastic vial. The lid of the vial is drilled for a snug fit with the inlet tubing of the pond, and the bottom of the vial is similarly drilled to admit the outlet tubing of the pond. A third hole is drilled in the side of the vial near the bottom. This hole accepts tubing from an air compressor of the kind designed for small aquariums.

"When the system is filled with water, bubbles of air pass up through the vial and escape through a hole that is made in the top of the collection vessel. The bubbles supply dissolved oxygen to the water and simultaneously create the desired circulation without agitating the pond, provided that the water level of the pond is at the level of the inlet tube. The water filters continuously through the mud bottom. Organic debris collect harmlessly under the Petri dish. The opening in the top of the collection vessel should be covered with a screen of nylon scrim to prevent the escape of mosquitoes. An access hole can be made in the top and closed with a rubber stopper.

"The apparatus can be charged with mud or soil from any location where mosquitoes breed. In our part of the country they breed in unlikely places such as animal tracks and hoofprints of almost any size that catch and hold rain water for a few days, the inside of wooden kegs and hollow stumps, the sides of plow furrows, low places in city lawns and so on. The eggs evidently remain viable for a long time. I have hatched mosquitoes from lumps of hard clay that appear to have been dry for months and from scrapings taken inside an old rain barrel that I know had not been used for several years. (Scrapings are mixed with soil that has been sterilized in an oven.) I get good crops from lumps of froze soil in the dead of winter.

"To set up a culture I cover the scrim of the clean apparatus with a layer of soil about an inch thick and add distilled water to the level of the inlet hose. If the water is kept at room temperature wigglers appear within a week and the adults mature a few days later. Two or three days after the adults make the way through the cone and into the collection vessel they are ready to lay eggs I provide no special lighting, nor do I

otherwise tamper with their development. When a crop hatches, I anesthetize the adults by admitting carbon dioxide to the upper chamber. The insects fall inside the ring at the base of the cone, where they can be counted, removed for weighing or otherwise manipulated.

"Mature mosquitoes feed primarily on the juices of wild plants. It is interesting to observe their preferences by giving the adults access to screened enclosure where plants of various kinds are grown Incidentally, only the females suck blood. This behavior provides the experimenter with a reliable method of distinguishing the sexes. The females lay eggs in the absence of males, but (I am told) the eggs of some species are no fertile unless the female has partaken o animal blood at least once.

"The apparatus can be used for culturing various aquatic insects, of course as well as other organisms that inhabit freshwater ponds. An excellent article by Bay that describes some of these experiments appears in Turtox News, Volume 45, Number 6,1967."

Bibliography

FUNDAMENTAES OF ECOLOGY. Eugene P. Odum. W. B. Saunders Company, 1953.

MANAGING FARM FISHPONDS FOR BASS AND BLUEGILLS. Verne E. Davison. Farmers' Bulletin No. 2094, U.S. Department of Agriculture. U.S. Government Printing Office, l9S5.

A GUIDE TO THE STUDY OF FRESHWATER BIOLOGY. James G. Needham and Paul R. Needham. Holden-Day, Inc., 1966.

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