Water Temperature Patterns in Lakes. <<<...return
One of the most significant phenomena of freshwater lakes are the thermal gradients that develop over the seasons. In many lakes, these variations take the form of pronounced stratification of lake water temperature.
How do these thermally stratified conditions come about in a lake?
This happens because the density of water varies inversely with temperature. Water is most dense at approximately 4° C. and becomes progressively less dense as it cools below 4° C. Ice dimensions expand the colder the water gets. (in other words, the water ice becomes less dense thus ice floats in water)
In summer, surface water is warmed by solar radiation causing its density, weight and viscosity decrease. In deep lakes, when the warm water moves along the bottom, it encounters colder and denser waters that resist mixing. The warm water is then diverted and stratifies horizontally. Thus, the lake becomes stratified horizontally into an upper epilimnion where the water circulates and is fairly turbulent and a lower hypolimnion which is relatively undisturbed. These layers are shown in Figure 2.
Figure 2. Water currents and thermal stratifications in deep lakes.
Although the thermal conductivity of water is very low, the temperature is nearly uniform down to the thermocline because of the thorough mixing of the waters in the epilimnion during the summer. The thermocline is the zone of most rapid temperature decrease generally involving a drop of at least 1° C. per meter of depth and occasionally as much as 7° C. per meter. When the thermocline forms early in the season it is close to the surface of the water. As the summer progresses, the developing thermocline sinks lower increasing the volume of the epilimnion and decreasing the volume of the hypolimnion. The temperature of the hypolimnion is fairly uniform, although it declines gradually from the lower edge of the thermocline to the bottom where it is seldom below 4° C. The 4° C minimum is on the bottom because water at that temperature is most dense.
During autumn, the surface water cools and the thermocline sinks. The epilimnion increases in thickness until it includes the entire lake. The waters are then uniform in temperature and density at all depths. At this time, even slight winds produce complete water mixing and water circulation. This is the autumn overturn that may last for several weeks or until ice forms.
As winter approaches and surface waters cool below 4° C., the water no longer sink and ice may begin to form. The ice is less dense than the underlying water so it floats. Immediately below the ice, the temperature of the water is very close to 0° C., but in one or two meters of additional depth it usually rises to 4° C., although in some lakes temperatures below 4° C. occur even at considerable depths.
When the ice melts during the spring and as the surface waters warm up, a spring overturn occurs when the water at all depths is at the same uniform temperature and thus the same density. The time and duration of the spring overturn depends upon weather conditions. The spring overturn often occurs intermittently, but it may also last for several weeks. When a lake has two overturns during the year, it is called a dimictic lake. Lakes undergoing a single overturn are called monomictic.