The Science Beneath the Surface of Garner Lake
by
Gerrit Verschuur
Garner Lake Association
Three physical properties of the lake water can be monitored relatively easily, with the right equipment. They are:
- Temperature
- Dissolved oxygen content
- Transparency
Garner Lake undergoes annual variations in temperature, which is hardly surprising as the seasons come and go. Also the amount of oxygen dissolved in the water varies during the year and with depth in the lake. The presence of oxygen dissolved in the water is crucial for fish, or they would suffocate.
From 1994 to 1996 I monitored all three quantities on an extremely regular basis especially during 1995.
Measurements were made over the deepest part of the lake.
The most interesting way to plot the data is to use color contour maps that display the results much as a weather map or a geological contour map might be plotted.
The following color plots show the data as a function of day number (because that is what my program allows me to do most readily). Thus:
Day 150 is May 30
Day 180 is June 29
Day 210 is July 29
Day 240 is August 28
Day 270 is September 27
Day 300 is October 27
Day 330 is November 26
1. Temperature data
The simplest measurement is of temperature as a function of date and for 1995, when I took the largest number of measurements, the results look like this:
The temperatures in °C show maxima, yellow shading, from June-September. The edge across the middle of the plot at a depth of about 5 to 6 meters (15 to 18 feet) shows that the lake is stratified and this boundary marks the thermocline, the level at which the temperature drops very rapidly with increasing depth. Anyone who has ever swum in a lake in the early summer and drifted about will have experienced the fact that one’s feet can sense much colder water than one’s chest. That is because you are then sensing the presence of the thermocline.
Temperature Gradients
The thermocline marks the layer where the temperature change with increasing depth is large. This quantity is known as the temperature gradient. I was able to use the measured temperatures to calculate the temperature gradient at different depths and the contour plots that follow show a dramatic pattern that highlights the fact that what goes on beneath the surface is very interesting.
In 1994 I began to take data and the day number continues into 1995.
The data for 1995 show a great deal of structure because I took data so often, essentially every other day. Those little fingers of red at the top mark downward moving impulses of warmer water that were created during very hot spells and they propagate down into the depths quite slowly. They can even create secondary thermoclines such as the yellow patch at a depth of about 2 meters around day 200. When these data are examined more closely those pulses of heat can be followed right down through the main thermocline and below it.
I am not aware that anyone has ever published data on this phenomenon, which would make a great student project if someone had the patience to make measurements every day for about 8 months. Such a high school project would surely win at Science Fairs.
The data for 1996 were not obtained as often as in 1995 (there is a limit to one’s obsession with data!) but nevertheless show an interesting secondary thermocline at the start of the summer. In fact the passage of heat into the depths of the lake was quite complex in 1996. No doubt every year will be slightly different, depending on weather phenomena. For example, the effect of a severe down burst we experienced one year could be followed as a heat pulse traveling downward for weeks afterward.
In all three plots above the thermocline sinks to great depths as the summer wears on until day number 330 or so, depending on the year. Then the thermocline reaches bottom the lake at its deepest levelsand the lake will no longer be stratified.
If conditions are right, the day when the lake is again fully mixed can happen very suddenly and will cause a turn over which brings gasses from the lake bottom suddenly to the top. This sometimes in November. That is the cause of the smell of rotting vegetation we sometimes detect at that time of year. Rest assured, it’s just the lake turning over. That’s Nature in action.