Maine Lakes Society and Lakes Alive!
Have you ever jumped into the lake and felt the water get colder as you sink further and further down? That’s because varying amounts of sunlight and mixing patterns affect the lake in a way that makes it get colder as you reach the bottom (in the summer, at least).
In deep water, students drop a temperature probe on a line to measure water temperature at various levels of lake depth. They record and plot the data to observe changes or patterns in the temperature as a function of depth.
- Does this lake stratify?
- If so, how deep is that separating layer (the thermocline)?
- What is the temperature of the water above and below that layer?
- If it does not stratify, why does the temperature of this lake stay the same from top to bottom?
- How deep is this lake?
- Deep lakes often become stratified in the summer and develop three distinct layers. The topmost layer is sun-warmed, uniform in temperature, and well mixed and oxygenated by wind, wave action, and photosynthesis. The middle layer, the thermocline, is characterized by a temperature gradient because it separates the warmer top layer from the cooler bottom layer. This occurs because the density of water depends on the temperature, so the cold water below does not easily mix with the warmer water at the surface directly. The thermocline is where the two layers mix. The bottom layer is very cold and has very little oxygen. When there are a lot of decaying materials it is possible for this layer to lose most or all of its dissolved oxygen (which is a necessity for fish!)
- Shallow lakes may lack a thermocline, which means that warm, highly oxygenated water at the surface can directly mix with the cold, anoxic (lacking oxygen) water at the bottom. Wind contributes to this process, and it causes nutrients in the lake sediment to be recycled. As a result, phytoplankton can grow faster and possibly produce a bloom.
- Oxygen gets mixed or dissolved into water by wind at the lake’s surface and by plant photosynthesis.
- Maine’s seasonal variations in air temperature cause water on the surface to freeze in winter and melt in spring.
- The density of water varies with temperature; water is densest at 39 degrees Fahrenheit.
- If a lake stratifies, it will “turn over” when the temperature of the lake is the same from top to bottom, typically around 39 degrees Fahrenheit. This happens biannually in stratified lakes: spring (after ice out) and fall (before ice in). When the temperature is uniform throughout the lake, wind blowing across the surface will cause the entire lake to mix by driving the oxygen-rich surface water down to all depths, simultaneously carrying materials found on the lake bottom up through the entire water column. This often makes lakes less transparent during the “turn over” period.
- Coldwater fish that can only survive hot summer weather by going to the deepest parts of the lake (refugia) depend on “turn over” to spread oxygenated water throughout the lake, even into its deepest holes. At the time of “turn over”, the oxygen content of the water is maximized and is uniform throughout the lake.
- Taking the Lake’s Temperature to “See” Lake Stratification (PDF)
- Aboard the Melinda Ann: On the Water Activities (Full Version as PDF)
Why is lake temperature important?
Discussion of the causes of thermal stratification, including reverse stratification under ice (uses degrees Celsium only; 1 Fahrenheit = (Celsius x 9/5) + 32)
Lab activity using collected data
Why Lakes Stratify (a much more technical description can be found here)