Limnologists often divide these strata into three general thermal regions of the lake. Figure 1.2.4
illustrates a theoretical stratification pattern for such a lake.
Near the surface of the lake in the summer, temperatures decrease slowly at first with depth.
This part of the lake usually is related to the depth to which most light penetrates. This top region of a
stratified lake is called the epilimnion. Although water temperatures in this layer decrease slowly with
depth and sometimes are similar to each other (after a storm, for example) the temperatures of strata at
deeper depths decrease more rapidly. The region of rapid temperature change is called the
thermocline . The depth region of the lake encompassed by the thermocline is often called the
metalimnion. The thermocline or metalimnion separates the epilimnion from the deepest region in
which temperatures again change slowly with depth. The region of the lake deeper than the metalimnion
in which there is slow change of temperature is called the hypolimnion. Regardless of what else might
happen in these regions, they are classically defined by temperature.
The distribution of thermal energy is the consequence of the absorbance of solar energy and
any water movements responsible for its further distribution. Figure 1.2.5 shows the theoretical
extinction of light in pure water. This pattern is not dependent on wavelength although different
wavelengths are absorbed more rapidly than others. This fact is illustrated in Figure 1.2.6.
Actually, we use temperature to define these regions of the lake because temperature is easy to
measure. The real reason that these layers are stratified is because of the density differences of the
water that are imparted from temperature differences. Under these conditions of stratification, the water
in the lake tends not to mix between layers and the lake is said to be