Dedicated To Tribal Aquaculture Programs | ||||||
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 | September 2006 ~ Volume 57 | ||
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Topics Of Interest:
Pond Mixing
By: John A. Hargreaves, Southern Regional Aquaculture Center
The upper layers of pond water absorb light, so most aquaculture ponds will develop stratification during the summer. This condition is characterized by extreme differences in water quality— especially temperature and dissolved oxygen concentration— between surface and bottom waters. These differences in water quality can affect fish culture. Many fish species will move to areas of preferred water quality and may be “squeezed” into relatively narrow bands of the water column.
Mechanically mixing pond water creates a more uniform environment and increases the pond volume that can be occupied by fish.
Most levee-type aquaculture ponds are shallow, so strong winds blowing over the pond surface mix the water somewhat. However, wind is unpredictable and unreliable for pond mixing. Winds are often light in the summer when the respiration rates of pond organisms are high, the demand for oxygen is greatest, and mixing would be most beneficial. Winds also are usually calmest at night, when the dissolved oxygen concentration is low and wind blowing across the pond surface would add oxygen to the water.
Mechanical mixing of pond water is more reliable than wind. Although aeration devices are widely used in commercial aquaculture ponds, pond mixing devices are not commonly used. Pond mixing is also known as water blending, water circulation, artificial circulation, or destratification. There are several mechanical techniques for reducing stratification.
Stratification—the Layering of Pond Water
Stratification is caused by the absorption of sunlight energy by pond water. The intensity of sunlight decreases exponentially with water depth. Surface waters are brightly lit and deeper waters are dimly lit. Water absorbs about 30 percent of the energy in sunlight as heat. As surface water absorbs this energy it becomes warmer and less dense than the water that lies beneath. The result is warmer, less dense layers of water that float over the top of cooler, more dense layers of water. Stratification becomes much more pronounced as turbidity increases because turbidity restricts the penetration of sunlight into the water column. The primary cause of turbidity is suspended matter (organic solids such as algae or inorganic solids such as clay particles) in the water column. During the summer, it is not unusual for the surface waters of aquaculture ponds to be 10 to 20 degrees warmer than bottom waters.
The maximum depths of aquaculture ponds range from 3 to 5 feet for levee ponds to 10 to 15 feet for watershed ponds. During the summer, both types of ponds will stratify during the day and destratify at night. Maximum stratification usually occurs between 2:00 and 4:00 p.m. on calm days. Ponds deeper than 6 to 10 feet may not mix all the way to the bottom at night, which causes a persistent layer of poor quality water that is temporarily disrupted only during storms strong enough to “turn” the pond water. Without natural disturbance or mechanical mixing, deeper watershed ponds can have a cold, deep layer that remains throughout the summer, even though the surface layer stratifies and destratifies daily.
Although stratification usually refers to water layers with different temperatures, the term also can refer to layers with different chemical concentrations—specifically, dissolved oxygen concentration.
Because there is more light near the surface, photosynthesis by algae is also greater near the surface. Photosynthesis produces oxygen. What little oxygen is produced at the pond bottom is often consumed by the decomposition of organic matter. Therefore, surface water contains much more dissolved oxygen than bottom water. On calm, sunny, summer mid-afternoons the dissolved oxygen concentration can range from more than 15 mg/L at the surface to less than 4 mg/L at the bottom. The temperature-related density stratification prevents the surface dissolved oxygen from mixing into the deeper waters. Chemical stratification affects the accurate measurement of dissolved oxygen concentration.
Severe dissolved oxygen stratification can force fish into shallow water where they are susceptible to cannibalism, predation by wading birds, or the effects of warmer water.
Potential Benefits of Mixing
Mixing is thought to benefit both the pond ecosystem and fish growth and survival. Although the benefits described below have not all been conclusively demonstrated by research, the results of lake remediation projects suggest that mixing can improve fisheries and perhaps fish production in aquaculture ponds.
- Improves Distribution of Dissolved Oxygen Through the Water Column
Effective mixing creates a homogeneous environment throughout the pond water column. In unmixed ponds with dense algae blooms, photosynthesis is confined to the surface layers because that is where light levels are most intense. In these ponds, photosynthesis at the surface will result in very high dissolved oxygen concentration near the pond surface and very low dissolved oxygen concentration at the pond bottom. Surface dissolved oxygen concentration often exceeds saturation (the equilibrium concentration of oxygen that water can “hold” at a certain temperature, salinity and barometric pressure). In this case, dissolved oxygen diffuses from the water to the atmosphere and is lost from the pond. Pond mixing conserves the dissolved oxygen produced by photosynthesis and distributes supersaturated surface waters throughout the pond. This increases the total supply of oxygen available for fish at night. Mixing at night, when the dissolved oxygen concentration is below saturation, helps diffuse oxygen from the air into water.- Minimizes Organic Matter Accumulation
Redistributing surface waters can have a positive effect on processes that occur on the pond bottom. The population of algae is constantly turning over so that a certain fraction dies and settles out every day. Once it settles to the pond bottom, this organic matter decomposes rapidly, creating a large demand for oxygen. So, the oxygen concentration of the water near the pond bottom tends to be low.Redistributing the oxygen in surface water layers accelerates the decomposition of organic matter so that excessive amounts don’t accumulate in pond sediment.
Without oxygen near the pond bottom, decomposition results in the production of various chemical compounds (such as volatile fatty acids and various fermentation products), some of which are toxic to fish and may reduce fish growth in some ponds. Therefore, mixing may prevent the accumulation of toxic chemical compounds.
- Reduces Density of Algae Blooms
Mixing may reduce the density of algae blooms by lowering the concentration of soluble phosphorus in pond water. Of all the nutrients added to freshwater ponds in feed or fertilizer, phosphorus is most important for algae growth. Theoretically, reducing phosphorus concentration can reduce the density of algae blooms, thereby reducing the risk of oxygen depletions and the potential for fish kills. Mixing maintains a high dissolved oxygen concentration near the pond bottom, which causes the development of a thin, oxygenated layer or “crust” at the sediment-water interface. This barrier prevents phosphorus from diffusing into the water and keeps it “locked up” in pond sediment. Although this process has been demonstrated in lakes and reservoirs, it has not been conclusively documented in fish ponds.- Shifts Composition of Algae Blooms
Mixing may affect the species composition of algae blooms. This is important with respect to the potential control of off-flavor caused by blue-green algae. When algae blooms become very dense, further expansion of the blooms is limited by the availability of light for photosynthesis. Algae blooms in most commercial aquaculture ponds are sufficiently dense to be limited by light. Algae blooms that are limited by light tend to favor the growth of bluegreen algae because they compete better for light than other kinds of algae. Blue-green algae are also favored by a stable water column, which occurs when a pond stratifies. Theoretically, then, mixing will limit the growth of blue-green algae. Although research on the species composition of algae blooms in mixed versus unmixed ponds is scant, there is some evidence to suggest that mixing can affect the species composition of algae blooms. If mixing can reduce the prevalence of blue-green algae in blooms, then mixing may be an effective way of managing off-flavors caused by blue-green algae.
Energy Considerations
The limited research on the subject showed that operating an axial-flow circulator for 6 to 8 hours during the middle of the day reduced the need for paddlewheel aeration by 58 percent. However, the combined power requirement of the mixer and the paddlewheel aerator was very similar to that of the paddlewheel aerator alone. Fish production in ponds with a mixer and an aerator was similar to that in ponds with only an aerator, so there was no economic justification for mixing in that study. However, the mixing efficiency of the device used was low. In fact, the mixing efficiency of most devices used to circulate pond water is very low, on the order of 0.1 percent. This can be explained by the fact that the energy is usually applied at a single point and turbulence emanates only from that point. The amount of energy required to mix a stratified water column to a uniform temperature (or, more properly, density) is actually quite low. The problem is that this density gradient extends throughout the pond. This presents a formidable engineering challenge, one that can’t be solved simply by using mixing equipment that creates turbulence at more than one point.
The effectiveness of mixing equipment can be increased by installing vertical baffles along the long axis of the pond to direct water flow and help create a uniform flow throughout the water column.
Mixing Devices
Many devices have been used to mix ponds. Most mechanical aerators provide some mixing, although their primary purpose is to add dissolved oxygen to the water. The types of mixing devices that are effective in commercial catfish ponds are constrained by the relatively large pond size (20 acres), shallow depth (3 to 5 feet), and intense stratification.
The most efficient mixing devices move very large volumes of water against a very low head (1/2 to 2 inches) compared to the amount of power applied.
In some lakes, reservoirs and deeper watershed ponds, destratification equipment is operated to expand the pond volume that can be occupied by fish and to improve water quality. Destratifiers typically operate by lifting water from deeper parts of the lake to the surface, or by pushing water from the surface toward the bottom. Some destratifiers have large fan blades that move water vertically; others release air near the pond bottom to lift water. This equipment works well in lakes and reservoirs that are much deeper than aquaculture ponds, but destratifiers rarely destratify large lakes completely. Rather, they create a zone of well-mixed water near the destratifier.
In shallow aquaculture ponds, vertical water movement is not as effective as horizontal water movement. Therefore, this section will emphasize mixing devices that move water horizontally.
Paddlewheel Aerator
Adding dissolved oxygen to pond water is the primary purpose of paddlewheel aerators. They splash water into the air and generate turbulence adjacent to the aerator. The shaft usually rotates at a fairly high speed (75 to 80 rpm). In commercial catfish ponds, a paddlewheel aerator creates a zone of well oxygenated water that is a refuge for fish when dissolved oxygen concentration in most of the pond is very low. If paddlewheel aerators are operated continuously, particularly if several aerators are placed in strategic locations, the pond can become well mixed. However, paddlewheel aerators are not efficient mixing devices, which is true of aerators designed primarily to provide oxygen to the pond. Operating paddlewheel aerators during the day mixes water well, but causes a net loss of oxygen from the pond. This occurs because mechanical disturbance drives out dissolved oxygen if water is supersaturated with oxygen. More gentle mixing may permit more dissolved oxygen to accumulate and to be mixed into deeper water.Propeller-Aspirator Pump
Propeller-aspirator pumps consist of an electric motor connected to a hollow shaft with a propeller at the end. The motor rests on a float above the water surface and the hollow shaft extends into the water at an angle. There are holes cut in the part of the shaft that is above the water surface. As the shaft rotates and water is pushed out ahead of the propeller, air is drawn into the holes and ejected through the end of the shaft. These devices circulate water better than other mechanical aerators. Unlike paddlewheel aerators, which create a surface current and a return current along the bottom, propeller-aspirator pumps create a current along the bottom and a return flow along the surface.Axial-Flow Water Circulator
Various designs of axial-flow water circulators have been evaluated. They move water with wide (6-inch), large-diameter (24- to 30- inch) fan blades attached to a shaft that is connected to a gear motor mounted above the water surface. The unit has the fan blades mounted on a shaft within a casing 4 feet long and 3 feet in diameter. The circulator has three or four fan blade units, each with six blades. The shaft rotates at 120 to 140 rpm and is powered by a 2.5- or 3-hp gear motor. This combination produces a water discharge of about 12,000 gallons/minute. Axial-flow circulators should be mounted to a solid surface, such as a wooden platform, on the pond bottom. The circulator should be placed in a corner of the pond and should direct water flow along a long side of a rectangular pond.Low-Speed Paddlewheel
Low-speed paddlewheels are a central component of the "Partitioned Aquaculture System" (PAS) developed and evaluated by engineers at Clemson University. The PAS consists of a high-density fish culture raceway coupled with a shallow (1.5-foot), baffled, open pond area where luxuriant algae growth occurs. A high rate of algae growth is necessary to treat the large quantities of waste products produced in the fish culture raceway. The low-speed paddlewheel moves a large water volume at a constant and low velocity (3 to 4 inches/second) throughout the pond. A 3-hp electric motor connected to a hydraulic system powering four 16-foot paddles can move more than 22,000 gallons/minute.In many respects, a low-speed paddlewheel is an ideal mixing device because it can move a large volume of water against a very low head with very little energy. In conjunction with baffles placed along the pond length, it can create a flow field that is uniform throughout the water column.
Tractor-Powered “Side-Winder”
Tractor-powered paddlewheel aerators have been widely used for emergency aeration of commercial catfish ponds. The paddlewheel hub of the unit is attached to a drive shaft that connects to the power take-off (PTO) of a tractor. Most tractor-powered paddlewheel aerators move water away from the pond bank. However, some tractor-powered paddlewheel aerators, called “side-winders” or “bank-washers,” have long paddles mounted on a hub and create a current parallel to the pond bank. To maximize oxygen transfer, the devices are operated with paddlewheels submerged at an intermediate depth and at an intermediate engine speed. To obtain good pond mixing, paddles should be fully submerged and the tractor engine set at idle speed. Two tractors on opposite sides of the long levees of a pond can mix a pond completely in 2 to 3 hours.
Operational Considerations
The type of mixing device selected determines the number of devices needed, their placement, and the timing of operation. It is important to emphasize that to be beneficial, most of the water volume in a pond should be circulated. Mixing only a corner of a pond or a small area near a paddlewheel aerator is of limited benefit. Most commercial catfish ponds are large (10 to 20 acres) and rectangular. These ponds are difficult to mix.
In general, mixing devices should be placed so that water is directed along the long axis of the pond. The effectiveness of many mixing devices can be improved by constructing one or more baffle levees within the pond, although baffle levees will increase suitable habitat for wading birds and other nuisance wildlife and interfere with normal harvest operations. Rather than retrofitting existing ponds with mixing equipment, the best method is to configure ponds during construction so that mixing is incorporated into pond design. The PAS is one example of this approach. The best time for operating equipment varies somewhat with the goal of mixing. If the goal is increasing the dissolved oxygen concentration at the pond bottom or conserving dissolved oxygen produced by photosynthesis during the day for use at night, then it is most effective to operate equipment for several hours during mid-day before maximum stratification occurs.
Intermittently operated mixing devices should be on for 3 to 8 hours during the day and turned off at night. In some production settings, such as the PAS, continuous mixing may be warranted. In this case, mixing at night will accelerate the diffusion of oxygen from the air into the water. Dissolved oxygen will be lost from ponds where some types of mixing equipment (such as paddlwheel aerators) are operated during the day. However, there is often more oxygen produced in continuously mixed ponds than in static ponds. Depending on the pond type, the species cultured, and the climatic conditions in a given location, timely mixing of ponds may be beneficial. Yet both technical and economic questions remain unresolved. At this point, it is not known whether the costs outweigh the actual benefits.
Solar Aeration ~ Pond Mixing Alternative
By Keeton Industries, PO Box 249 Wellington, CO. 80549, 970-493-4831
Note from the MTAN: Other alternatives are also available for pond mixing and aeration needs. Depending on the size of the pond, one such option is to install a solar aeration system such as the one described below.
Solaer systems are the cost effective environmental solution forLake Bed Aeration™.
Keeton Industries is the leader in innovative, technologically advanced systems. Our Solaer systems are one of a kind. Their applications ranges in size from ornamental ponds to lakes up to 5 acres. For lakes larger than 5 acres call us for a custom system quotation.
With unbelievable cost savings over electric powered aeration systems, they pay for themselves within the first year! Do not spend your hard earned money on utility costs for running your aeration system when you can have a system that will run for years with no additional costs.
SB-4: SB4 Solaer Lake Bed Aeration SystemThe SB4 is the ultimate patented solar aeration system from Keeton Industries. Designed for larger pond and lake applications up to 5 surface acres in size, the SB4 package system includes 3-120 watt solar panels, 2-828 high volume 24 VDC brushless commercial compressors, wiring harness, 30 amp charge control center, 4-valve adjustable manifold, 4-9 in. duraplates™ non-clogging maintenance free diffusers, large recycled palstic cabinet with sound reduction package, 2-225 amp hour deep cycle solar batteries, fittings package, 12/24 volt smart box converter, 1600 ft. of 1/2 in. polypipe and 4 in. solar panel mounting structure.
Liming Ponds for Aquaculture
By: William A. Wurts and Michael P. Masser, Southern Regional Aquaculture Center
The pH and mineral content of water are the result of interactions between the soil beneath a pond and the water used to fill it. Clay soils are often acidic. Because ponds are commonly constructed on these soils, the effect on water quality can be significant. Ponds with acidic bottom soils that are filled with poorly mineralized water characteristically have low alkalinity and hardness. When total alkalinity and hardness are below 20 mg/L pH productivity is usually reduced. Alkalinity concentrations below 20 mg/L often lead to large swings in daily pH values, which stress aquatic animals. Acidic soils contain high concentrations of hydrogen ions and/or aluminum relative to the concentrations of calcium and magnesium, which are important minerals for good water quality.
The acidity of pond soils can be neutralized and the productivity of the pond improved by liming. “Liming” refers to the application of various acid-neutralizing compounds of calcium or calcium and magnesium.
Liming ponds has three important benefits:
Liming may enhance the effectiveness of fertilization.
Liming helps prevent wide swings in pH.
Liming also adds calcium and magnesium, which are important in animal physiology.
The Difference Between Alkalinity and Hardness
The difference between alkalinity and hardness is often confused. The misunderstanding relates to the term used to report them—ppm (mg/L). Total alkalinity indicates the entire quantity of titratable bases present in water, primarily bicarbonates, carbonates and hydroxides. The most important components of alkalinity are bicarbonates and carbonates. Hardness is the overall concentration of divalent salts (calcium, magnesium, iron) but does not identify which of these elements is the source of hardness. Calcium and magnesium are the most common sources of water hardness. Liming increases both alkalinity and hardness.
The Effect of Liming on Fertilization
Both recreational and commercial ponds are often fertilized to improve fish production. Fertilizers containing nitrogen, phosphorus and potassium (especially phosphorus) stimulate the growth of microscopic plants (phytoplankton) and animals (zooplankton), which, in turn, serve as food for animals in the aquatic food chain. In recreational ponds, an abundance of plankton supports larger populations of species such as largemouth bass and bluegill. In ponds used for commercial production of juvenile fish, plankton is the primary food source. Healthy phytoplankton blooms also absorb toxic nitrogen wastes and raise daytime dissolved oxygen concentrations, so they are important to water quality.
Perhaps the most common reason to lime ponds is to improve the response to fertilization. In ponds built on acidic soils and filled with fresh water of low mineral content, much of the phosphorus added in fertilizers becomes tightly bound in pond sediment where it is not available to support phytoplankton growth. Proper liming can improve phosphorus availability and greatly enhance pond productivity.
Liming and pH Swings
In ponds with acidic soils, filled with poorly mineralized water with low total alkalinity, liming will increase total alkalinity. This helps stabilize pH, which can swing widely from 6 to 10 during the day if total alkalinity is below 20 mg/L. Fluctuations in pH are the result of the interplay of photosynthesis and respiration. Nighttime respiration increases CO2 concentrations, creating carbonic acid and causing pH to fall. During the day phytoplankton absorbs CO2 for photosynthesis, causing pH to rise. Large, daily changes in pH can stress aquatic animals. Most aquaculture species can live in a broad range of alkalinity concentrations, but the desired alkalinity for many animals is 50 mg/L or higher. Liming to raise total alkalinity to the required or preferred ranges buffers the water and reduces swings in pH.
Liming and Hardness
Hardness concentrations are important to aquatic animals also. Calcium and magnesium are essential for bone and scale formation in fish. The most critical component of total hardness, however, is the calcium concentration or “calcium hardness.” Environmental calcium is crucial for osmoregulation, the biological process that maintains precise levels of internal salts for normal heart, nerve and muscle function. In low-calcium environments, animals can lose (leak) substantial quantities of these salts into the water. Most aquatic organisms can tolerate a broad range of calcium hardness concentrations, but a desirable range is 75 to 250 mg/L with a minimum concentration of 20 mg/L. Adding liming materials or gypsum increases hardness.
Deciding Whether to Lime a Pond
To determine whether a pond needs to be limed, first check total alkalinity. Collect a water sample from the first several inches below the surface, making sure the sample contains no bottom sediment (mud). Collect the sample in a clean quart container that has no chemical residues. The sample can be tested for total alkalinity with a swimming pool test kit. Or, the sample can be sent to a university laboratory or commercial testing company. Check with your county Extension agent for information about water testing.
If the total alkalinity of the water sample is less than 20 mg/L, the pond may benefit from liming. The amount of lime needed depends on the chemical characteristics of the bottom sediment. Take samples of the pond bottom and have them analyzed to determine the soil pH and the amount of liming material to apply. Collect the samples as you would for cropland. Take samples to a soil depth of 6 inches from several locations in the pond (an S-shaped pattern is usually used). In ponds less than 5 acres, collect at least ten samples per acre. In larger ponds, collect four to eight samples per acre. In a new pond, collect soil samples before filling. In ponds with water, push a length of PVC pipe into the bottom and remove the mud plugs from the pipe. Or, attach a can or small container to a long pole and scoop soil from the pond bottom. Combine the samples, mix them evenly, and spread the blended sample out to dry. After drying and crushing, mark the sample “pond mud” so the appropriate analysis can be made. Approximately 1 pint of dried, blended soil sample is needed for lab analysis. Contact your county Extension agent for information about soil testing services.
In some areas, specific tests for “pond mud” are not available. However, there is a simple and reasonably accurate way to estimate the amount of liming material needed in a pond. Submit the sample and request the recommendation for alfalfa production. The amount of liming material needed to grow alfalfa will be very close to the minimum required for producing most aquatic animals. Another method is to apply 1 to 2 times the amount of liming material used to farm row crops in the surrounding area.
Choosing Liming Materials
Materials such as agricultural limestone, basic slag, slaked lime, quick lime and liquid lime have been used to lime ponds. While all these compounds neutralize soil acidity, some are more practical or effective than others. It is not advisable to use quick lime or slaked lime. They are more expensive and can cause pH to rise rapidly to levels that can harm aquatic life.
Basic slag is a satisfactory liming material, but it is not commonly available and its effectiveness may vary significantly from load to load. A substance known as silicate slag is not an acceptable material and should not be used to lime recreational or commercial production ponds.
Liquid lime is popular among some farmers. This product is made by suspending finely powdered agricultural limestone in water. The small particles react more rapidly with the acid in soil and water and produce quick results. However, because this mixture is half water, it takes twice as much liquid lime as agricultural limestone to achieve the same results. Liquid lime can cost much more than agricultural limestone.
Finely crushed agricultural limestone is usually the best material to use. It is cost-effective and readily available.
Both pond alkalinity and hardness can be increased by adding either calcitic or dolomitic limestone. It is difficult to add too much agricultural limestone to a pond. At a pH of 8.3 or greater, calcium combines with carbonate to form limestone and drops out of solution. Limestone does not dissolve well in ponds where soil acidity has been neutralized and water pH has stabilized at or above 8.3.
Neutralizing Value and Efficiency
Commercial liming materials vary in their ability to neutralize soil acidity—their neutralizing value (NV). Pure calcium carbonate is the standard used for assigning relative neutralizing values to each of the liming compounds. Calcium carbonate is considered to have an acid neutralizing value of 100 percent. Agricultural limestone may have NV values between 85 and 109 percent depending on its specific chemical composition. Slaked lime has an NV of 136 percent. Neutralizing values for the liming materials previously discussed may fall between 55 and 179 percent.
Finely crushed agricultural limestone is composed of different sizes of particles. Small particles react faster and dissolve more rapidly and completely than large particles. Therefore, the neutralizing efficiency (NE) of agricultural limestone depends on the fineness of the mixture. The particle fineness and associated neutralizing efficiency are determined by passing limestone through a series of sieves. Particles that pass through a 20-mesh sieve but that are retained by a 60-mesh sieve have an NE of 52.2 percent. Those passing through a 60-mesh sieve have an NE of 100 percent. The various quantities of each particle size grouping and their associated NE values must be averaged to arrive at an overall NE rating.
If the liming requirement, neutralizing value (NV) and neutralizing efficiency (NE) are known, it is possible to calculate the precise amount of lime needed. Divide the amount of liming material recommended (tons per acre) by the product of the neutralizing value and the neutralizing efficiency (NV x NE).
Timing and Application of Liming Materials
To be effective, liming materials should be applied evenly over the bottom of the pond. The best, and easiest, time to lime a pond is before it is filled with water. A liming truck or tractor-pulled liming wagon can be driven around in the dry pond to spread the lime evenly over the entire bottom. It is not necessary to disc the lime into the soil, but this will accelerate its neutralizing activity.
If the pond contains water, lime should be applied evenly over the entire pond surface. Lime is loaded onto a boat or barge and then shoveled or washed uniformly into the pond. Often a sheet of plywood can be attached across the front of one or two small boats and the lime placed on the plywood. For small ponds of less than 1 acre, liming trucks can be backed up to the edge of the pond and the lime distributed with the spreader on the truck. This method works best if the truck can move around the entire pond and broadcast the lime evenly.
Agricultural lime does not dissolve quickly in water and will sink to the bottom. Liming a pond filled with water has an immediate effect on water quality. It increases pH, reduces soluble phosphorus, and reduces free carbon dioxide. Increasing the pH may cause the water to clear of suspended particles (mud), which can help pond productivity by increasing the light available to plants.
However, liming a pond shortly after fertilizing may remove phosphorus from the water, which could prevent a phytoplankton bloom from developing.
Recreational ponds are typically fertilized in the spring with compounds containing phosphorus. So it is usually best to apply lime in fall or winter when productivity is unlikely to be affected. The pond will equilibrate within several weeks and then fertilizer can be applied to adjust productivity. Limestone dissolves slowly over time. Alkalinity and hardness are washed out of the pond with overflow and drainage water. Ponds that require lime usually need repeat treatments every 3 to 5 years. Alternatively, annual lime applications can be made using one-fourth the original recommendation to maintain alkalinity, hardness and pH at acceptable levels. If a pond needs lime, it will not respond well to fertilizer.
Managing Calcium Hardness
If the alkalinity concentration is below 50 mg/L, agricultural limestone can be used to increase alkalinity and hardness. If total alkalinity is above 50 mg/L, adding agricultural limestone will not be effective. Similarly, if pond pH is stable at 8.3 or greater, limestone will not dissolve. For several aquaculture species the preferred concentration of calcium hardness is above 50 mg/L.
Liming with agricultural limestone, using recommendations based on soil analysis, will usually increase alkalinity and hardness to the minimum required concentration of 20 mg/L. A low total hardness value is a reliable indication that the calcium concentration is low. However, a high hardness value does not necessarily mean that the calcium concentration is high. Where hardness is caused by dolomitic limestone, the total hardness value reflects a mixture of calcium and magnesium. Magnesium can represent as much as 50 percent of the hardness produced. Other magnesium-containing compounds, such as magnesium sulfate, may be the source of hardness in high alkalinity environments.
Therefore, agricultural limestone may not always raise calcium to the required or minimum desired concentrations. Agricultural gypsum (calcium sulfate) or food grade calcium chloride may be needed to raise calcium hardness in waters with alkalinities greater than 50 mg/L and low hardness. Where alkalinity is high and hardness is caused by magnesium, adding agricultural gypsum or calcium chloride is also an effective way to raise the calcium concentration.
Alternative Materials for Raising Calcium Hardness
Agricultural gypsum (calcium sulfate): Calcium hardness and total hardness can be increased about 1 mg/L by applying 5 pounds of agricultural gypsum per acre-foot. Adding 125 pounds of agricultural gypsum per acre-foot would raise hardness approximately 25 ppm.
Calcium chloride: Calcium hardness and total hardness can be increased about 1 mg/L by applying 4 pounds of calcium chloride per acre-foot. Adding 100 pounds of calcium chloride per acre-foot would raise hardness roughly 25 ppm. It is important to note that if phosphorus is added to ponds immediately before or shortly after applying gypsum or calcium chloride, the phosphorus may combine with calcium. This may cause both elements to drop out of solution as calcium phosphate. Phosphorus based fertilizers should not be added for several weeks before or after the application of compounds that increase calcium hardness.
If high volumes of water regularly flush through a pond, the agricultural limestone, agricultural gypsum or calcium chloride that have been added can be washed out. Often more than the recommended amount of limestone or gypsum is added so the materials will not have to be applied as often. These chemicals will not cause problems in a pond if added at two or three times the calculated amount.
Culturists often overlook the importance of hardness and alkalinity. The pond environment and aquatic animals benefit from water that has the desired levels of alkalinity and hardness. The minimum concentration for both is 20 mg/L. Managing these two components of pond water stabilizes or buffers pH fluctuations, improves the availability of phosphorus for phytoplankton, increases the natural food in ponds, and provides calcium for osmoregulation, egg hardening and other metabolic needs. Water should be tested periodically so that hardness and alkalinity can be managed properly. Apply liming materials as needed and keep good records to improve water quality and overall pond productivity.
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| Product and company names mentioned in this publication are for informational purposes only. It does not imply endorsement by the MTAN or the U.S. Government. |
