re: Pond Update

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The primary source of oxygen in ponds is the atmosphere, the air above the pond. Oxygen diffuses into the water from the air. As long as the water can be exposed to the air, then an ample supply of oxygen can enter the pond. One of the leading obstacles for the diffusion of oxygen into the water is stratification of the water column. Stratification occurs when the sun heats the surface water while deeper water remains cool. The warm and cool layers do not mix. This disrupts the circulation of oxygenated water from the surface of the pond to the bottom. While the warm surface water receives oxygen from the atmosphere as well as from photosynthesis of phytoplankton, the cooler bottom layer becomes depleted of oxygen. As the summer progresses, oxygen levels drop severely in the bottom layer. Stratification can be a significant threat to fish health in small ponds because it increases the potential for a "turnover."

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How do I prevent a turnover?

Proper pond construction is important. The ratio of surface water (epilimnion) to deep water (hypolimnion) is a major determining factor for the frequency and severity of turnovers. Deep ponds with small surface areas often are more prone to severe turnovers because there is a larger volume of deep, low-oxygen water relative to the amount of oxygenated surface water. Broad, shallow ponds have fewer turnovers because less of the pond's volume is tied up in low oxygen deep water. Most stormwater ponds are shallow (6ft deep or less), but some are deeper. It is important to know the topography of the pond's bottom (bathymetry) as well as the average depth of the pond.

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