Temperature and Thermal Stratification


       Changes in water temperature with depth are largely determined by season, light penetration and pond morphology. Water in very shallow ponds often circulates from top to bottom throughout the year. In deeper ponds, especially if light penetration is low or if the pond is protected from wind-driven mixing, the water column may be “stratified” in summer. Light heats up water at a particular depth in proportion to its intensity; thus, surface waters warm up faster than deeper water. The resulting temperature differences produce differences in water density. Water at 4oC is most dense, and warmer water is progressively less dense; thus, warmer water will sit stably above cooler water, resulting in stratification. Wind activity in stratified ponds is sufficient only to mix the upper portion of the water column (in lakes this is termed the “epilimnion”), while the lower layer below (the “hypolimnion”) remains cool, dense and relatively unmixed. Separating the two layers is a zone of rapid temperature transition, often termed the “thermocline”, although the precise depth where this zone of temperature change begins is often indistinct in ponds.
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Fig. 20. Temperature profiles in shallow vs. deep ponds in summer. In shallow ponds (a), wind-driven mixing circulates water from top to bottom. In deeper ponds (b), wind activity is insufficient to mix the water column completely, and stable density layers develop during the growing season. In the graphs at right, temperature is seen to be relatively uniform from top to bottom; in stratified ponds the epilimnion typically shows little temperature change, but a rapid decline in temperature occurs in the thermocline.


       The upper mixed layer of a stratified pond is warm with sufficient light to support photosynthesis, but may have relatively low nutrient content as phytoplankton take up N and P, and then sink to the bottom. In contrast, the bottom waters have relatively nutrients, but little light. Because photosynthetic organisms require both nutrients and light for rapid growth, stratification in summer (by controlling primary production by nutrients at the surface and by light at the bottom) provides a measure of protection from excessive plant/algal growth.
       Mixing depth varies according to wind activity at the surface, the degree of light penetration and temperature. Thus, larger ponds without riparian trees, because they are more exposed to wind activity, are also mixed more thoroughly. Stratification is also more typical on hot, calm days. At night, the surface layers lose heat to the atmosphere, and the pond often mixes from top to bottom. Such ponds are said to be “polymictic”, meaning that they mix completely with high frequency (and thus resupply nutrients to the surface waters, while resupplying oxygen to the bottom — see Section J). By contrast, very shallow ponds may rarely stratify and, with an abundance of both nutrients and light, are often hyper-eutrophic.
       The extent and duration of stratification can be recorded using temperature dataloggers suspended for long periods near the surface and near the bottom of the water column. As shown from recordings during a 50-day period by two loggers (Fig. 21) placed in a relatively large (5 ha) pond in Chester County during mid-summer, the surface waters (in pink) fluctuated on a daily basis much more than the bottom waters (in blue); at night, however, temperatures at the surface and bottom were indistinguishable for the first 20 days of the recording, indicating mixing. Later in the time period the surface temperatures remained higher than bottom temperatures on most days, signaling longer-term stratification and likely greater differences in oxygen and nutrients as well.
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Fig. 21.Temperature profiles at 0.2 m (in pink) and 1.4 m (in blue) in a local pond during a 50-day period in summer.


       The upper mixed layer of a stratified pond is warm with sufficient light to support photosynthesis, but may have relatively low nutrient content as phytoplankton take up N and P, and then sink to the bottom. In contrast, the bottom waters have relatively nutrients, but little light. Because photosynthetic organisms require both nutrients and light for rapid growth, stratification in summer (by controlling primary production by nutrients at the surface and by light at the bottom) provides a measure of protection from excessive plant/algal growth.