THERMAL STRATIFICATION refers to a temperature layering effect that occurs in water. Stratification is due to differences in water density: warm water is less dense than cool water and therefore tends to float on top of the cooler heavier water. (Ice is the exception. See Cool Factoids about Temperature, below.)
Stratification is a term usually reserved for discussions about waterbodies found "up north". While Florida lakes sometimes become stratified, northern lakes frequently are. This is because: a) northern lakes are deeper, and b) northern lakes tend to experience extreme differences in temperature over the course of a year. Other factors that contribute to thermal stratification include annual weather patterns, the color of the water, and the presence or absence of aquatic plants.
The usual 3-tiered stratification (see graph at right) happens less frequently in Florida lakes because so many of them are shallow. However, some form of stratification does occur; it’s just not as “strong” or stable: the differing temperature layers are not as deep and the stratification doesn’t last as long. When the 3-tiered version does occur in Florida, it’s usually in deeper spring-fed or sinkhole lakes, or in lakes with an abundnce of floating and/or submersed plants.Another reason that Florida lakes are less stratified: because Florida lakes are shallow, they are more susceptible to the effects of wind and wave action. This constant mixing of the water keeps temperatures fairly uniform throughout the waterbody and prevents stratification. In shallow lakes, just a bit of wave action is enough to thoroughly mix the water temperature.
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COOL FACTOIDS ABOUT TEMPERATURE As water cools from 35ºC (95ºF), it becomes more dense until it reaches its maximum density at 4ºC (39.2 ºF). After that, things get interesting: as water becomes colder than 4ºC, the density actually begins to decrease. Finally at 0ºC, water becomes ice and is less dense (lighter) than its liquid counterpart. At this point, newly formed ice is able to float on the surface, even though it is a solid. This is a good thing. Otherwise, ice would form on the bottom of lakes, increasing in volume and eventually displacing all of the water. If this were to happen, there would be no habitat for fish and other organisms during the winter. Also, floating layers of ice act as thermal barriers, helping to prevent further loss of heat from the waterbody. From Florida LAKEWATCH Information Circular 109 A Beginner’s Guide to Water Management — Oxygen and Temperature |
For more about temperature and water density, see the web site Water on the Web.
Vic Ramey is the editor.
DEP review is by Jeff Schardt and Judy Ludlow.
This project is a collaboration of
the Center for Aquatic and
Invasive Plants, University of Florida,
and the Bureau of Invasive Plant
Management, Florida Department of Environmental Protection
