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 (salinity for example) that increases the density beyond this value will cause the water to `sink' or
displace more buoyant water. Temperature alone, then, would be insufficient to cause further
displacement or mixing of that water mass by an overlying mass of different chemical content (less
salinity).
The bonding characteristics of water molecules and temperature also affect the property called
"viscosity". Viscosity decreases in a slightly nonlinear manner with increasing temperature. This is
important for any process involving kinetic motion because viscosity is a measure of the internal friction,
the resistance to free, independent motion of water molecules.
1.2.5.4 Thermal Energy and the Ecosystem
Thermal energy is very important to ecosystems. We sense thermal energy by comparison (as
in hot water versus cold) or by direct measurement with a thermometer. By knowing the effect energy
has on water temperature, we can calculate the amount of energy in a mass of water by measuring its
temperature. Often the resultant amount of energy is erroneously referred to heat. All water contains
thermal energy, even water that we think is quite cold.
The intensity of thermal energy is sensed by us as temperature , but it also changes the water.
Colder water usually has greater density than warmer water. This means that a volume of cold water
usually weighs more than the same volume of warmer water, although very cold water can weigh less
than slightly warmer water. Figure 1.2.3 illustrates this relationship graphically.
Because warmer waters are less dense they are more buoyant and tend to remain at or near the
surface of a lake. In early March 1997, many southeastern lakes were already beginning to warm in
response to increased inputs of light energy. In Lake Hartwell, for example, surface waters had already
warmed mensurably more than waters deeper in the lake. This was true for all of the other large
reservoirs in the region. Soon thereafter, these lakes were warm enough to remain stratified. From the
end of March to the end of April, the temperature of the lake surface increased and the surface
temperatures were warmer than the deeper waters.
In summer, surface temperatures are very warm, perhaps 27-30EC. At that time, a swimmer
will still be able to easily reach colder water by submerging a few feet below the surface. Most of us are
already aware of this. However, the way that the temperatures are distributed are important to all
ecological processes in the lake.
During the summer, temperatures in large monomictic lakes decrease from surface to bottom.
In this condition the lake is said to be stratified. That is, the lake is divided into a series of temperature
layers or strata from surface to bottom. Each temperature layer or strata can be identified by locating
the depth for each part of the lake at which a certain temperature occurs. Some of these thermal layers
or strata are similar to each other in temperature and quite different from other layers or strata.
1.2-10
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