Pond Stratification / Turnovers

In the spring, water temperatures are nearly equal at all pond depths. Because of this, dissolved gases and nutrients are equally distributed throughout the pond. As spring turns to summer, the surface water warms up more rapidly than deeper water. As the difference in temperature increases between water surface and cool bottom water, two layers are formed. The temperature difference causes density differences. The density difference creates a "physical" barrier that results in no mixing with the warm water at the surface and the cool water at the bottom. The pond has stratified. This stratification can last for weeks at a time especially in deep ponds.

This density barrier can break down due to heavy winds and rain. When the barrier breaks down, the cool bottom water will mix with the warm surface water.

Pond stratification with turnovers can become a problem in ponds because the deep bottom water has no to low oxygen levels. When the barrier breaks, the oxygen-rich surface water mixes with the oxygen-poor or even oxygen-deficient bottom water. This can result in severe oxygen depletion with a fish kill. Pond turnovers happen quickly, and you should be prepared to handle a low oxygen situation.

(See Figure)

NCCE Aquaculture Home Page

By: Molly Sandfoss, Area Specialized Agent - Aquaculture
Last Updated: October, 2000