VPP 322: Aquatic Animal Diseases, Health Care and Management (1+1)

TEMPERATURE STRATIFICATION

• Temperature stratification is not a problem in itself but instead it cause changes in pond water quality, which can be lethal. Temperature stratification refers to the development of a distinct temperature zone in a pond. It occurs when the surface water of a pond warms up, while the bottom water remains cooler. Temperature stratification is a common problem in farm and water shed-type ponds later often over 1.5 m depth, but it is rarely a problem in commercial fish ponds which are usually less than 1.5 m (5ft) deep.

Causes of temperature stratification

• Stratification is most likely to develop during hot, calm summer days when little water mixing occurs by wind action. As the temperature difference between surface water and bottom water increases, the pond stratify into two layers of water that are separated by the metalimnion or thermo-cline, where water temperature changes rapidly from the warm surface temperature to the cool bottom temperatures. Warm water is lighter, thus the thermocline acts as physical barrier between epilimnion and hypolimnion, and considerable amount of energy is required to mix or “turn over” the pond.
• The DO in the hypolimnion is rapidly deleted by pond metabolism and oxygen demand builds up an anaerobic reaction which are not sufficient to form final degradation products of pond metabolites. Toxic substances such as hydrogen, sulphide and methane may accumulate under these reduction condition.