Mix-up over Lake Superior’s Mixing
by Sharon Moen
Older limnology textbooks and even some newer resources (try Googling "Lake Superior monomictic") need a bit of updating. Lake Superior is a dimictic system. In other words, Lake Superior has two distinct periods of stratification and mixing each year. Reports that Lake Superior is monomictic – that it has just one cycle of mixing and thermally stratifying per year – are missing interesting winter data showing the formation of thermal layers from January through May.
The graph below demonstrates Lake Superior's dimictic cycle. Thermal layers begin to form in July as warm water rests on top of cooler water (positive stratification). The layers become more pronounced and deeper as summer moves into autumn. As the surface waters cool, winds mix the water to deeper depths until the lake becomes a uniform 4°C (39°F). The lake is thought to mix rather completely until the surface waters cool enough to show negative stratification, a condition in which positively frigid water lies above very cold water. This period of stratification breaks down in June, after the returning sun has warmed the surface water back to 4°C.
To read more about Lake Superior's stratification, turnover, and other natural processes, see: www.seagrant.umn.edu/superior/processes.