Madison winters are often centered around the ice on Lakes Mendota and Monona. From hockey games to ice skating to Lady Liberty and festivals, Madisonians form a community around the cold days and thick ice.
This year, though, Lady Liberty sat on Library Mall, the Clean Lakes Alliance Frozen Assets Festival canceled their ice events and university police urged the public to stay off the ice due to unsafe conditions.
However, these conditions aren’t a surprise. Data shows this ecosystem has been changing.
For 170 years, the Wisconsin Climatology Office has kept records of ice cover on Lake Mendota and neighboring lakes. These records show Lake Mendota sees fewer days of ice cover than before, with later freeze dates and earlier thaw dates.
Constituting if a lake is “frozen” varies between each body of water. A “frozen” Lake Mendota means there is too much ice to travel from Picnic Point to Maple Bluff by boat.
Lake Mendota officially froze on Jan. 15, the third latest date on record, almost a month after the typical freeze around Dec. 20.
“We're seeing it statewide. We’re seeing it continent-wide, across the globe: Lake ice durations are becoming shorter and more variable,” said Tyler Butts, a postdoctoral researcher at the UW-Madison Center for Limnology. “This is driven by a changing climate.”
December 2023 was one of the warmest on record for Wisconsin, according to the Wisconsin State Climatology Office. Overall, the state’s winters are becoming milder, with warmer temperatures, less snowfall and more rainfall.
These factors, along with fluctuating temperatures, play a role in ice formation and ice safety, Butts said.
“With all these swings, where it is above freezing during the day and below freezing at night, [the ice] is a lot less reliable and could impact a lot of the good winter sports that are important for culture and society here in Madison, as well as across the state,” he said.
James Tye, founder and executive director of the Clean Lakes Alliance, said Madison’s lakes hold both ecological and cultural significance for the community.
“Our lakes are truly our number one asset for our community. It is the thing that makes Madison special. Wisconsin's a land grant university, and it was specifically chosen to be on Lake Mendota, ” Tye said.
Decreasing ice cover impacts ecosystem wellbeing, particularly the lake’s phenology, Butts said. “There's this term that a lot of people use to describe the timing of events over the course of the season called phenology,” Butts said. “The animals have been around for a long, long time, and they sort of developed these rhythms.”
When snow on the lake melts, more solar radiation hits the lake, melting the ice and warming the water. This triggers a bloom of zooplankton, a small animal that eats the phytoplankton, Butts explained. In a typical season, by the time zooplankton bloom, fish larvae are hatching and eating the zooplankton, maintaining an ecosystem balance.
When phytoplankton bloom early, zooplankton miss that food source at its peak, creating less zooplankton available for fish when they hatch. Butts said this disrupts the fish population and decreases algae control in the spring.
“It sort of throws off all these things in the lake when we see these really early ice-off dates,” he added.
In addition to food chain disruption, warmer waters affect which fish are favored, Butts said, with decreased habitat for cold water fish like walleye and an increased warm habitat for fish like largemouth bass and invasive species.
Warming ice and melting snow pose another problem for Madison’s lakes: increased flash runoff. Runoff pollution, like phosphorus and chloride, occurs whenever snow melts. But how much runs into waterways depends on the soil — if the soil is frozen when snow melts or it rains, the water cannot be absorbed by the ground, and more flows to the lakes.
This pollution impacts the lake's ecosystems, especially in the summer. Chloride from road salt can be toxic to aquatic life, according to Wisconsin SaltWise. Phosphorus from fertilizer and cow manure causes more harmful algae blooms like cyanobacteria, according to the Clean Lakes Alliance.
“Everything you put in land eventually washes into lakes,” Tye said. “If you get a lot of rain or a flash melt in the winter, the ground is still frozen, so more of it is going to flush into the lake.”
Tye emphasizes that Wisconsin has taken great strides to reduce the amount of phosphorus pollution and runoff, like adding buffer strips and retention ponds or reducing manure spread in the winter. With more water running into the lake, though, more pollution runs in as well, creating more algae blooms and disrupting water quality later in the summer.
The full effects of a changing climate and changing winter ecosystems are still being discovered. The UW-Madison Center for Limnology is working to expand its research to understand more about changes to wintertime lakes. Looking forward, Butts is unsure what the future holds.
“It's hard to say. It really depends on a lot of factors that are difficult to predict,” Butts said. “But I can say if we continue on this trend, we'll just see a continuous decrease in that freeze-thaw cycle… It's difficult to say what the future might hold.”
