liming ponds aquaculture

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, especially in the southern and southeastern U.S., 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 (as CaCO3) pH and productivity are 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 effect 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

It is important to understand the difference between alkalinity and hardness. These two aspects of water chemistry are often confused. The misunderstanding relates to the term used to report them – as ppm CaCO3 (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, etc.) but does not identify which of these elements is/are 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 absorb CO2 for photosynthesis, causing pH to rise. Large, daily changes in pH can stress aquatic animals (Fig. 1). 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 (Fig. 2).