Thermal Stratification occurs because of the differences in density (weight) between warm and cold waters. Density depends on temperature: water is most dense (heaviest) at about 39 degrees and less dense (lighter) at warmer temperatures.
In the Spring, the ice melts and surface water begins to warm up. The increasing density of the warming water along with the wind cause the surface water to sink and mix with the deeper water, this process is called “Spring turnover”. During this time, most of the lake water is at the same temperature, and surface and bottom waters mix freely. Smaller lakes or ponds completely mix for a few days, and large lakes often circulate for weeks.
As the sun continues to warm the lake surface, the temperature differences increase between the surface and deeper waters. In ponds with areas deeper than 10 feet, the temperature differences eventually create a barrier strong enough to resist the wind’s mixing forces (it only takes a difference of a few degrees to prevent mixing).
Warm water holds less oxygen than cold water; therefore, during the summer lakes can be depleted of much of their oxygen, this can lead to fish kills and poor water quality. Since the oxygen cannot be replaced by photosynthesis (too dark) or diffusion with the atmosphere, the bottom of the lake loses all oxygen and becomes toxic. Aerating Ponds and Lakes prevents stratifications and increases the overall oxygen supply in the lake. This forced mixing will reduce natural water turnover and eliminate the possibility of a fish kill which can occur during the time of natural water mixing in the Spring and Summer months.
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