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Turning Lakes

© Keith C. Heidorn

We generally think of weather processes as occurring from the surface upward into the atmosphere, but weather has its influences downward into the soil and water bodies as well. Many atmosphere-ocean interactions are well known, particularly those siblings El Niño and La Niña. Atmosphere-lake interactions produce lake-effect snow squalls. Seasonal weather interactions play major roles in the processes and ecology of even the smallest freshwater lakes. One of the more important is the semi-annual turnover of a lake's water mass.

I'll begin this look at the turnover cycle in late summer when lake surface waters have reached their annual maximum temperatures. At this time in a sufficiently deep lake, you will find a definite stratification or layering of water temperatures. If a lake is too shallow, wind blowing over the lake surface thoroughly mixes the waters and prevents the development of significant stratification. Warmest, and therefore least dense, waters lie on top, and the water temperature decreases with depth, reaching its minimum temperature at the greatest lake depths. How cold the lake bottom water becomes depends on the lake depth and other characteristics (I assume for this discussion no major rivers or streams enter the lake to affect the stratification), but it will never fall below 4^oC (39^oF).

Three Water Layers

With stratification in place in a "deep" lake, we can define three major layers within the lake.

• Epilimnion, an upper layer of circulating warm water, usually no more than 6 m (20 ft) deep, where dissolved oxygen concentrations are moderate to high.
• Thermocline, a layer of rapid temperature and oxygen decrease with depth, often quite thin, separating the upper and lower layers.
• Hypolimnion, a cold, deep-water, non-circulating layer in which oxygen is low or absent.

Winds blowing over the lake generally keep the epilimnion stirred by pushing a quantity of surface water downwind. This draws a flow of deeper water upward (upwelling) along the lee shore to replace the pushed waters. As a result, epilimnion waters mix, producing generally warm temperatures and high oxygen content (important to fish and other creatures) throughout the layer. Atmospheric oxygen is added by the air-water interaction to the oxygen produced within the water by aquatic plants.

The thermocline layer has minimal mixing, and what does occur is slow, thus isolating the bottom waters from the surface zone. Because this region lies between the epilimnion and hypolimnion, it is also called the mesolimnion (meso- meaning "middle").

The hypolimnion is a deep-water stagnant layer where plants alone cannot produce enough oxygen for the layer's demands. As a result, the hypolimnion is not only depleted or devoid of

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