Lakes Mendota, Monona, Waubesa and Kegonsa are prominent features of Dane County. Early residents, the Ho-Chunk Nation, named the area Teejop, or Four Lakes. They recognized the benefits of settling here, abundant food being a primary consideration. They lived all along the shoreline of the lakes.

Edward Birge was a zoology professor at UW-Madison, joining the faculty in 1875. In the late 1890s Birge began conducting research that ultimately led to the university emerging as the birthplace of the discipline of limnology in North America. A few years later, Chancey Juday joined the team, and this dynamic duo provided much of the groundwork for how we study and understand lakes, measuring dissolved oxygen and other critical factors to lake health. This collaboration started in 1905, making Lake Mendota one of the most studied lakes in the world.

In the 1960s, Dr. Arthur Hasler founded the Center for Limnology, located in a tiny Brutalist building just past the racks of Hoofer sailboats near the Memorial Union. It’s an unassuming building given its history of contributions to science.

These days, if you want to know about Madison’s lakes (or any other lake, for that matter), you call Jake Vander Zanden, professor in the Department of Integrative Biology and the director of the Center for Limnology. I had questions. The enthusiasm with which he answered them took us down a limnological rabbit hole.

What happens to lakes in the winter? Do they go to sleep?

Vander Zanden says no, lakes don’t go to sleep in the winter. “Everything that normally goes on, keeps going, just a lot slower.” So they’re just snoozing a little, like my dog Lucy, who always has one eye open. Phytoplankton continue their photosynthesis. “If there’s no snow cover, the light coming into the lake isn’t much different than when there’s no ice.” Add snow cover, and the light diminishes, but even when there’s snow cover, a little light still comes through.

The big difference between ice and open water is the change in water column, the term that describes the water from the surface to the bottom. “In winter, there’s no significant variability in water temperature from just below the ice to the bottom of the lake,” says Vander Zanden. “There’s no movement because the ice protects the lake from the effects of wind, which is what mixes the water.”

What does “frozen over” mean for Lake Mendota?

If there’s continuous ice from Picnic Point to Maple Bluff, Mendota is officially locked up. Since records have been kept (1850!), the average number of days Lake Mendota is frozen over has decreased by a month. “Of course, climate change has played a role, but there is variability,” Vander Zanden says. “But the trend is that ice covers the lake fewer and fewer days.”

What happens when there’s no ice? “The summer algal blooms likely have a running start and we see worse water quality,” Vander Zanden says, because the lake gets a lot more solar energy without ice cover.

When the water opens up, things change. Solar energy and wind across the surface start to stir things up.

On a lake the size of Mendota, the top strata warms up. That’s also known as the epilimnion, which means “top of the lake” in Greek, and it extends down 30 feet or so. Shallow bays warm up quickly, and University Bay may feel almost balmy, 10 or more degrees warmer than the open water. Wind and solar energy churn nutrients around and critters start eating more, and making more critters. A water molecule can move from the surface of the lake to the bottom in a day before water starts to stratify into the aforementioned epilimnion, the middle section or thermocline, and the hypolimnion (“bottom of the lake”) — the layer of cold, dense, low-oxygenated water. The levels stratify much like oil and water: cold water weighs more, warm water less.

The immense amount of solar energy hitting the lakes is mind-blowing. A calorie is the unit of heat required to raise the temperature of a cubic centimeter of water one degree Celsius. Hold that thought.

Lake Mendota contains 505 million cubic meters of water. That’s 505 billion liters of water. That’s a trillion Stanley mugs. Even if only the top ten feet warm up 15 degrees Celsius, the energy required, while certainly calculable, is such an astronomical number it wouldn’t mean anything. It’s like the number of stars in a galaxy. It’s just freaking big.

I like the thought of a lake not going fully to sleep, but snoozing a little, a cycle that’s been going on for millenia.