Climate Change and Lake Mendota

By: Pamanisha Gross

This story explores how climate change impacts Lake Mendota’s ice cover, fish species, and water quality. It also looks at how these effects contribute to recreation and the way people interact with the lake.

Lake Mendota in the winter. Photo by Bryce Richter.

Lake Mendota in the winter. Photo by Bryce Richter.

Limnologist and climate change expert John Magnuson has contributed a large amount of research to the University of Wisconsin-Madison on climate change and its impact on Lake Mendota. Aside from heavily studying the effects of climate change, he has also witnessed its impact first hand on the community.

“The first time I published a paper on the ice data,” Magnuson said, “I got a phone call from somebody that said ‘that’s really interesting because I noticed it’s harder and harder for us to have a good date for an ice regatta.’”

Ice regattas are water sporting events that include boat and yacht races They are one of many things affected by climate change in the waters of Lake Mendota.

Lake Mendota’s water quality, the abundance and extinction of fish species and less time of having a frozen lake are also at the mercy of climate change. These factors directly affect how Madison residents and visitors interact with and perceive the lake. From not being able to skate on the ice in the winter to swimming in a pile of green in the summer, the effects of climate change and global warming are becoming more and more visible in our lifetime.

Average temperatures are rising and extreme temperature conditions are becoming more common according to the United States Environmental Protection Agency. The EPA reported that the warmest year on record for the United States occurred in 2012.

Wisconsin, specifically, has become much warmer over the past 60 years according to data from the Wisconsin Initiative on Climate Change Impacts. These warmer temperatures contribute to Lake Mendota’s warmer surface water.

As a result, the length of time that Lake Mendota is frozen continues to decrease. For example, Lake Mendota was covered in ice for about four months each winter in the 1850s as noted in Seeing Climate Change in Water by Jeff Miller and based on a presentation by Magnuson.

However, by the early 2000s, that number decreased to an average of three months a year, which is a month less of ice cover.


“In other words, the amount of time the lake is ice-covered is nearly 25 percent less than it was 150 years ago. If this trend continues, the time will come when Lake Mendota will be ice-free all winter long,” Miller writes.

Young angler ice fishing. Photo taken from

Young angler ice fishing. Photo taken from

Ice fisherman. Photo taken from

Ice fisherman. Photo taken from

As Lake Mendota freezes later and later in the year, winter recreation is stalled. Magnuson believes that “winter is part of our sense of place” and that “loss of winter impacts recreation

and economy.” There is less time for people who use the frozen lake as a resource for winter activity.

“I’ve never understood in a political sense why people like ice fishers, ice boaters and ice skaters aren’t marching in the street that you’re taking away this resource,” Magnuson said, noting how big of an impact climate change is having on their community.

Also because of the more extreme weather conditions, the duration of ice cover on Lake Mendota is highly variable within a year, creating uncertainty for what days the lake will completely freeze over and just how thick the ice will be. This may also be the reason as to why it was getting harder for those boaters to set a good date for the ice regatta.

Magnuson also said that this uncertainly can be dangerous. For example, someone may veer off into a seemingly frozen Mendota only to realize the ice is very thin and perhaps fall through and into the ice.

The illustrations below show just how quickly the amount of ice cover can change within a given week on Lake Mendota.

Lake Mendota on January 19, 2007

Lake Mendota on January 19, 2007


Lake Mendota on January 20, 2007

Lake Mendota on January 20, 2007


Lake Mendota on January 21, 2007

Lake Mendota on January 21, 2007


Lake Mendota on January 22, 2007

Lake Mendota on January 22, 2007

Along with ice cover, climate change also impacts the water quality of the lake.

“Cyanobacteria blooms get worse as lakes get warmer,” said Steve Carpenter, director of the UW-Madison Center for Limnology.

As noted by the United States Environmental Protection Agency, “the average surface temperature across the contiguous 48 states has risen at an average rate of 0.14 degrees Fahrenheit per decade” since 1901.

Cyanobacteria, also known as blue-green algae, are a group of photosynthetic

Cyanobacteria. Photo taken from Wikipedia.

Cyanobacteria. Photo taken from Wikipedia.

bacteria capable of producing deadly toxins and causing water discoloration, according to the Wisconsin Department of Natural Resources. If you’ve ever been near the lake and noticed thick green substances near the surface or greenness in general, it may have been the result of cyanobacteria blooms.

“I like swimming but I always hesitated to swim in a lot of the parts of Lake Mendota, just because of how green and dirty it looks sometimes,” said University of Wisconsin-Madison student Egal Warsame.

However, the horrid green color that deters away many potential swimmers in Lake Mendota is at the bottom of concerns when it comes to cyanobacteria blooms. They also pose serious health risks for humans and pets who come into contact with the bacteria.

“Cyanobacterial blooms in the United States have been associated with the death of wildlife and domestic animals,” the EPA reports.

They can be extremely harmful to aquatic ecosystems and anyone or anything that comes in contact with it. The EPA warns that exposure to the dangerous toxins produced by the algae can have numerous heath effects on humans such as mild skin rashes, abdominal pain, vomiting and diarrhea, kidney damage and even respiratory paralysis leading to death.

“Wow, that’s definitely something I didn’t know,” said UW-Madison student Bobbie Briggs, expressing her surprise at the health risks of cyanobacteria blooms. “One time, I saw a sign over on the Lakeshore path saying do not enter the dock going out to the lake but I didn’t think too much of it…I’ll definitely pay more attention to those signs now that I know what they could be warning us about,” Briggs said.

Lake Mendota’s water quality is also impacted by climate change in another way. According to Magnuson, increasing temperatures stemming from global warming speeds up the water cycle which can create extreme weather events such as droughts, floods and rainstorms.

“Wisconsin’s annual average precipitation has increased by about 10 percent since 1950. That means that on average we receive about three percent more rain in a year than we might have expected during the 1950s,” said Daniel Vimont member of the Science Council and co-chair of Wisconsin Initiative on Climate Change Impacts.

During big rainstorms, runoff events occur. This is a problem because runoff events tend to transport more nutrients, such as phosphorus, into the lakes from fields, construction sites, streams and barnyards. A major concern in terms of improving Lake Mendota’s water quality is lowering the amount of phosphorous pollution.

“Phosphorus has long been recognized as the controlling factor in plant and algae growth in Wisconsin lakes and streams. Small increases in phosphorus can fuel substantial increases in aquatic plant and algae growth, which in turn can reduce recreational use, property values, and public health,” the Wisconsin DNR reports.

Data show that the two years with the most extreme rain events were the same twoGRAPH years when the maximum amount of nutrients flowed into Lake Mendota. The table on the shows the total amount of phosphorus that flowed into Lake Mendota during the biggest rain events (indicated by the blue circles).

“This tells us that the thing responsible for eutrophication in the lake and the lake getting really green in the summer time is an overdose of nutrients – and that overdose of nutrients is going to be greater in storm years,” Magnuson said.

Carpenter predicts that there will be an increase in big storms as a result of global warming.

Fish species in Lake Mendota are also being affected by the increasing global temperatures.

“If the air temperature were warmer, you might not observe specific species that you would if the water was cooler,” said Katie Van Gheem, watershed engagement coordinator of the Clean Lakes Alliance.

Cool water fish are most at risk of death and extinction as a result. This is largely related to a process known as summer stratification.

During summer stratification, warm water floats on top of cooler water. While warm water fish survive in warmer temperatures, cool water fish survive in cooler temperatures. The same analysis can be applied to cold water fish. If the water is warmer, it makes it harder for cool or cold water species to exist in that zone, so as a result of summer stratification, the cool water fish lose habitat space closer to the surface. However, there is vital resource cool water fish gain from being in those zones – oxygen. Cool water fish need oxygen just as much as cooler temperatures to survive.

Because Lake Mendota is a very productive lake, oxygen is limited. When critters, plants, planktons, algae, and other microscopic life die off, they sink toward the bottom of Lake Mendota and bacteria decompose them using a large amount of oxygen which leaves less oxygen for the cool water fish species. By late summer, there is almost no oxygen further down within Lake Mendota.

As a result of global warming, the period of summer stratification lasts longer making it harder for cool water fish in Lake Mendota to sustain where temperatures have increased. Stratification has been occurring as early as May in Lake Mendota rather than July.

Magnuson believes the fish that are maintaining are doing so by staying within the short margin of water where there is enough oxygen coming from the surface and cool water from below but that margin continues to sharpen.

Lake Mendota is currently seeing the warming climate have its biggest effect on the cool water fish known as the Cisco.

“They die in mass numbers and float up on the shore,” Magnuson said.

According to Magnuson and Carpenter, data suggests that the Cisco will be the first fish species to go extinct in Lake Mendota due to climate change and this extinction is projected to occur within a UW-Madison student’s lifetime.

“As we have more pollution in the lake but also warmer summers, the zone of water that has no oxygen in it is pushing upwards further and further and is beginning to engorge on the cold water layer where the cisco live,” Carpenter said.

Over the years, Carpenter says he also expects to see a greater abundance of warm water fish in Lake Mendota such as the largemouth bass, smallmouth bass, bluegill and carp and a decrease in the amount of cool water fish such as the northern pike, walleye and yellow perch.

Only time can truly tell what will happen to the different fish species and Lake Mendota in the years to come. Although if one thing is for certain, it is that we should all be taking a closer look at climate change is impacting the community.