Nature Note – Lake Stratification Begins
Read this article to learn what 40°F means for Lake Champlain. Photo by bucklava - CC-BY.
On April, 1, 2016 the water temperature at the USGS gage in Burlington hit 40°F for the first time this year. Each year we eagerly await this milestone. This temperature is significant because it marks the point at which the lake begins to stratify for the summer. Liquid water is more dense at 39° than it is at 40°, so once the top layer of water warms to 40° or above it begins to float over the colder water. As the days lengthen and the water warms further, the warm top layer gets thicker.
A stratified lake presents different environments between the upper and lower layer for fish. In addition to temperature differences, the cool bottom layer holds more oxygen. Additionally, detritus from the upper layer can sink to the lower layer and get stuck at the interface. Many fish will feed right at the boundary.
LCC Staff Scientist Mike Winslow has been tracking daily water temperatures since 2002. April 1 marks the earliest date at which the lake has reached this milestone in that time – no surprise given the extraordinarily warm winter we had. The previous earliest mark was April 10 in 2010. The latest date was May 9 in 2007.
During the early season, lake stratification is weak. A slight breeze can still mix the upper and lower water layers. Not only is the upper layer fairly thin, but the density difference between water at 40° and 39° is very little. Later in the summer, the upper water layer can get into the high 70°s. The lower, colder water layer however remains close to 39° degrees so the density difference between the two layers is much greater.
The summer temperature of the cool lower layer is dependent upon the number of spring storms that occurred. If there were numerous spring storms, lots of mixing occurred and the lower layer is warmer. Few spring storms mean less mixing and a cooler layer.
In the summer when stratification is strong, the difference in density between the upper and lower water layers is high enough that waves, known as an internal seiche, propagate along this interface. The seiche can displace the thermocline as much as 145 feet. Once the seiche begins it passes up and down the Main Lake in four to six days following strong forcing winds along the north-south axis of the lake. The seiche sometimes brings the cool lower layer closer to the surface so water temperatures can go from bathwater warm to bone-chilling cold overnight!
