Many of the physical and chemical properties of water must be considered in its management. Some of these properties are temperature, pH, hardness, dissolved oxygen, source of the water in the pond, uses made of the water, and where it goes if it flows from the pond.
Water temperature as it affects the spawning of fish is discussed in a later section (see page 12) as is the effect of temperature on the survival of trout (see page 11). Water temperature also must be considered when using chemicals in the management of a pond. As is mentioned in other sections of this bulletin, some fish toxicants and herbicides are more effective when the water temperature is above 60 degrees F in the top two feet. Some herbicide labels discourage application when water temperatures exceed 75 degrees F because there is an increased risk of oxygen depletion due to decomposition of dead vegetation, and a fish kill could result.
Finally, copper compounds used as algaecides may be safe for fish at recommended rates but may kill fish eggs or fry (newly hatched fish). Thus, temperature provides a clue as to when fish are spawning and may influence the timing of algaecide applications. Water temperature should be measured at a depth of one foot.
The amount of dissolved oxygen (DO) in water is measured in parts per million (ppm) and is inversely related to water temperature. The chart presented here shows the decrease in dissolved oxygen as water temperature increases.
Pond water is not pure, nor are the Ohio ponds at sea level, so the amount of DO will be slightly different. Pond fish require about 4 ppm of DO, and other organisms in your pond, both plants and animals, also require dissolved oxygen.
When the dissolved oxygen level in the upper 4 feet of the pond drops to or below 4 ppm, fish and other organisms will start to show stress. Fish will come to the surface and appear to gulp air, and snails, crayfish, and other organisms may actually climb out on the bank or up on emergent objects.
Oxygen depletion can be caused by a number of factors, including the decomposition of aquatic plants that have died of natural causes, large decomposing masses of aquatic plants killed with a herbicide, run-off waters rich in nutrients and organic matter such as that from a livestock feedlot or a poorly maintained septic system, or from pond turnover.
| Amount of Dissolved Oxygen (ppm) in Pure Water at Different Water Temperatures (at sea level) |
|
|---|---|
| Temperature (F) | Approximate PPM |
| 35 | 13.4 |
| 40 | 12.7 |
| 45 | 11.8 |
| 50 | 10.9 |
| 55 | 10.2 |
| 60 | 9.7 |
| 65 | 9.2 |
| 70 | 8.7 |
| 75 | 8.3 |
| 80 | 7.9 |
Other pond water properties are also important, though less so than temperature and dissolved oxygen. Usually Ohio ponds have a pH (hydrogen ion concentration) from 7 (neutral) to 9.5 (alkaline). Most freshwater fish are able to survive and reproduce when the pH is between 5 and 10. Most Ohio ponds range from 75 to 200 parts per million in total alkalinity (hardness). The hardness of water may influence the effectiveness of some herbicides and is discussed on page 20 of this bulletin.
Finally, management of the watershed that contributes run-off water to a pond is important. Land uses that leave the soil surface unprotected may contribute large amounts of sediment. Some chemicals applied to crops in the watershed, especially insecticides, may be detrimental to the fish and other organisms in the pond as well as to water quality.
Microscopic plants and animals, called plankton, are basic fish foods. The production of plankton is directly related to the fertility of the water. Plankton are eaten by small fish and other animal organisms such as aquatic insects and their larvae, which in turn are eaten by larger fish. If any link in this food web is weak or missing, an unbalanced condition will result and fish production will suffer. The density of the plankton population also determines the depth to which light will penetrate the water. This affects the growth of undesirable plants (weeds), which is discussed later.
The principal source of water for most Ohio ponds is run-off from the pond's watershed. This run-off carries fertile matter from soils into the pond. As a result, the fertility of the soils in the watershed determines fertility of the pond water and the density of the plankton population. Most Ohio ponds do not need additional fertilizer. An exception may be ponds where most of the watershed is forested. Also, where springs or water pumped into the pond from a ditch, a tile drain, or a well provides the principal source of water, low fertility may be a problem.
The "dipstick" method can be used to measure the fertility of a pond. You can make a dipstick by nailing a shiny can lid to the end of a stick at least three-feet long and marking one inch graduations on the stick. Immerse the dipstick vertically in the water until the image of the can lid begins to blur. You are actually measuring the depth of light penetration. A plankton population that permits light to penetrate 15 to 18 inches deep is an indicator of good fertility.
Pond fertilization is a common practice in the South and among people who raise fish commercially. However, due to the good fertility of most Ohio soils, ponds used for recreational fishing usually do not need to be fertilized. Owners prefer to improve the fertility of the watershed and let nutrients be carried into the pond with run-off water.
If you determine that your pond needs additional fertility and are willing to make the commitment a fertilization program requires, here's how to proceed.
Starting in late March or early April, apply 80 to 100 pounds of fertilizer per surface acre of water. Inorganic fertilizers balanced for the three main components (N-P-K) such as 10-10-10 or 12-12-12 are suitable. Avoid fertilizers that contain lime, gypsum, or tobacco dusts as "inert" ingredients, and supplemental ingredients such as herbicides or insecticides. The first application should produce a plankton "bloom" that is green or brown. Check depth of light penetration. If no bloom results, repeat the application in 10 days. Once you have achieved the desired level of fertility, make additional fertilizer applications as determined to be necessary by the dipstick test. Your pond may require fertilizer applications as often as every two to four weeks. Continue your program until mid-August. Do not apply fertilizer after August 15.
Over-fertility is possible. It can result from too much fertilizer or from the natural introduction of high-fertility materials such as barnyard run-off, silo drainage, or septic system effluents. Excessive fertility may exist if light penetration is less than 12 inches and can contribute to fish kills. Fertilize only when needed, do not over-fertilize, and do not start a fertilization program unless you plan to continue it. Improperly done fertilization may actually increase undesirable weed growth and contribute to other problems.
Pond owners are frequently interested in providing artificial aeration in their ponds. The benefits of artificial aeration are:
The two most significant sources of dissolved oxygen are the atmosphere and photosynthesis. The diffusion of oxygen from the atmosphere is slow, except under conditions of strong turbulence. Transfer of oxygen from the atmosphere into natural waters will occur when the water is undersaturated with oxygen. If the surface film of water is saturated with oxygen there will be no further diffusion until oxygen diffuses from the surface film into the overall body of water. Therefore, oxygen transfer is dependent on the amount of water turbulence at the surface.
The most important source of oxygen is that produced by aquatic plants during the process of photosynthesis. Some factors that control the rate of photosynthesis include temperature, light, nutrient concentration, species of plants, abundance of plants, and water turbulence. Light penetration, to a large extent, is regulated by turbidity, and in many ponds the major source of turbidity is plankton. Therefore, the abundance of plankton is a primary factor affecting light penetration and the production of oxygen. Oxygen production by plankton is greatest near the surface and decreases with water depth because of self-shading. Ponds with high densities of plankton have higher rates of oxygen production near the surface, but ponds with lower densities of plankton have oxygen production occurring at greater depths. Therefore, ponds with low levels of plankton have more stable oxygen regimes, but are less productive because plankton is the base of the food chain in ponds.
Also known as diffusers, subsurface aerators consist of an onshore compressor that pumps air through a hose placed in the deepest part of the pond. This creates a column of air bubbles that are released at the bottom of the pond. As the column of bubbles rises to the surface, it moves water from the bottom of the pond to the surface where it is exposed to atmospheric oxygen which then dissolves into the water. The mixing of water moves oxygenated water to the bottom of the pond where it is used by aerobic bacteria to decompose dead organic material such as vegetation. The bubbles impart very little dissolved oxygen into the water. The primary benefit is the mixing effect.
Agitators such as paddlewheel devices and fountain sprayers can be used to create substantial turbulence which results in atmospheric oxygen dissolving into the pond. Fountains consist of a float, nozzle or sprayer head, and a pump that draws water from the pond and sprays it into the air. They are usually powered by an electric pump, or less commonly, by a windmill device. If the water is drawn from the bottom oxygen-poor layer of the pond, fountains can also reduce the layering that develops between oxygenated and unoxygenated water. Surface turbulence can also be created by paddlewheel devices powered by an electric motor or a power takeoff (PTO) from a tractor, or by churning the water surface with an outboard motor propeller. The process of mixing:
In order to get the maximum benefit from an aerator, it should be properly located and sized. Recommendations for these specifications are based on practical experience. Research-based findings vary due to the wide range of conditions under which aerators have been used. In one study, three sizes (1/3, 3, and 5 horsepower) of electrical, spray-type surface aerators were used, and each failed to appreciably raise the dissolved oxygen concentration in the 1.4-acre study ponds within four hours. But a 1/3-horsepower aerator in a 1/10-acre pond quickly raised the DO concentration and prevented a fish kill. A rule of thumb suggested by one manufacturer is 1.5-2 horsepower per surface acre. In situations where water quality is especially poor, a minimum of 2 horsepower per acre should be provided.
Considerations in sizing and locating aerators include:
Pond aeration has been promoted as a method of aquatic vegetation control. To date, the benefits supposedly derived by aeration for controlling aquatic weeds or algae have not been fully demonstrated. Artificial aeration may be helpful in reducing algal blooms in ponds and lakes plagued by blue-green algae problems. Also, there may be some indirect benefit of aeration if nutrient-rich water in the bottom of a pond is moved to the upper layer where it serves to increase the density of the phytoplankton which in turn increases the shading of the water which can then reduce the growth of aquatic vegetation. Research has shown that a well-oxygenated layer of water at the pond bottom can keep the mud oxidized. This can keep phosphorus in an insoluble form which is unavailable for plants. The practicality of doing this in small impoundments remains undetermined.