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 In Southeastern lakes, this condition occurs in the winter and continues through additional
cooling and mixing until spring. These lakes experience one season of mixing each year and are
termed monomictic lakes.
Water density is not only determined by temperature but also by dissolved substances in the
water. The best example of this is salinity which increases water density. In lakes, salinity usually
results from activities in the watershed such as winter salt treatment of roadways. But other dissolved
substances can contribute to density. When the density of deeper waters of a lake is great enough that
complete mixing cannot occur, this condition is classified as meromixis. Carters Lake in Georgia and
Laurel River Lake in Kentucky are examples of meromictic reservoirs. Neither of them ever
experience complete mixing. And the deepest waters of these lakes remain isolated from the
atmosphere. This condition can result from chemical and biological processes or from external addition
of materials. For many of these lakes, however, their morphometry is such that their depth (especially
their mean depth) is large compared to their surface area. There is a morphometric measurement that
quantifies this relationship and it is called relative depth. Lakes with greater relative depth can achieve
meromixis with greater ease.
Many lakes farther north have thermal stratification during the winter. Their surface
temperatures will be at or near freezing and often the lakes will be covered with ice. This is because, as
mentioned earlier, at very cold temperatures the water becomes a little less dense. In those lakes at
those times, the bottom waters will be slightly warmer (and more dense) than the surface.
Because these lakes mix in the fall until they experience winter stratification and then mix again
during the spring thaw until they restratify, they have two seasons of mixing each year and are
termed dimictic lakes.
Although this condition may occur infrequently during the winter in small ponds or in shallow
isolated areas of coves of larger lakes in our area, it occurs only temporarily and is not a dominant
characteristic of Southeastern lakes.
1.2.6.1 Stability and Heat Budgets
Exchanges of energy are represented by such processes as light absorbance, backradiation, as
well as the energy exchanges due to inflows and outflows. These exchanges are greatly influenced by
lake morphometry and hydrology. Limnologists have incorporated the major features of both of these
influences into two measures derived from these incorporated factors. These are stability, a measure
related to the effort needed to mix the lake as discussed previously, and the heat budget, a conceptual
approach to accounting for all inputs and losses of thermal energy.
Stability is derived from knowledge of the morphometry of a lake and the thermal (actually
density) distribution. Simply, positive stability indicates that external force must be applied in order to
1.2-16
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