Zebra mussels, the tiny, bottom-dwelling clam and scourge to Wisconsin's freshwater lakes, are quickly increasing in Lake Mendota.

University of Wisconsin-Madison lake researchers discovered the invasive mussel species in Mendota in 2015 and since then the mussels have spread rapidly.  

"It is the most successful invasive species on the history of the planet," said Jake Vander Zanden, a professor at the UW's Center for Limnology during an event for reporters Thursday. Zebra mussels have hard outer shells, protecting them from predators, they're easily transplanted and they reproduce fast, he said.

"Things have changed so dramatically, we're struggling to keep up because they've reproduced so quickly," Vander Zanden said. "It is a transformation of the ecosystem ... we've never seen anything like this in Mendota." 

The mussels are already making an appearance along shorelines and docks and will likely become more visible this summer. The lake will smell and algae will congregate near the shoreline and under the water. The water has become clearer this year and will continue to clear as the zebra mussels feed by pulling microscopic plants, animals and debris out of the water, Vander Zanden said.

Why does this happen and how do zebra mussels operate? Here are some question and answers.

Can we get rid of the zebra mussels?

No. They are here to stay and will only increase. According to the state DNR website, "once zebra mussels are established in a water body, very little can be done to control them." 

What do zebra mussels look like?

They are small clams, up to two inches long, but usually under one inch. They have a hard, D-shaped shell that is yellow or brown. They cling to piers, shoreline rocks and anything they can find at the bottom of lakes. They are sharp and will cut your foot. 

How do zebra mussels affect lakes? 

Zebra mussels act like tiny water filters. They suck up water, eat the microscopic animals, plants and other debris out of it, then spit the water back out. They also ooze a mucus excrement that drops to the bottom of the lake and becomes a food source for other bottom-dwelling creatures.

Right now, Mendota's zebra mussels can collectively suck up and spit out all of the water in the lake over the course of two to three weeks, Vander Zanden said.

This whole process results in nutrients being concentrated at the bottom of the lake. This makes the lake clearer and allows for more sunlight to filter into it, which then leads to a surge in plant growth at the bottom of the lake. This is good news for bottom-feeding fish like carp. But it is bad news for other fish that live nearer to the surface of the lake like yellow perch because it clears the rest of the lake of food. 

It also leads to more growth in blue-green algae which depend on sunlight. This kind of algae can release toxins into the water which can lead to fish kills. 

How does this infestation affect people? Is the water still safe?

The water is still safe for swimming and boating, but zebra mussels will spread along the bottom of the lakes and beaches. They are sharp and will cut feet. Swimmers are advised to wear footwear. 

The mussels can accumulate to clog pipes and attach to boats, which can affect their performance. 

How do zebra mussels reproduce?

Zebra mussels reproduce in the middle of the summer. One female zebra mussel can release as many as 1 million fertilized eggs into the water. Within two days, eggs hatch and free-swimming larvae attach themselves to any solid thing they can find and then grow. 

How did zebra mussels get here? 

According to the DNR, zebra mussels were introduced into the Great Lakes in 1985 or 1986, and have spread throughout Wisconsin lakes since then.  

"They were most likely brought to North America as larvae in ballast water of ships that traveled from freshwater Eurasian ports to the Great Lakes," according to the DNR website.

A recreational boat used in other lakes infested with mussels likely transferred them to Lake Mendota, according to UW researchers.

How can I learn more about zebra mussels?

UW's Center for Limnology has a variety of resources about the Mendota zebra mussel invasion. The center has a buoy on Lake Mendota where people can get real-time lake data including information on wind, clarity, air temperature, dissolved oxygen and chlorophyll. 

The DNR also operates a Citizens Lake Monitoring Network to share and report information about lake quality in each county statewide. 


Katelyn Ferral is The Cap Times' public affairs and investigative reporter. She joined the paper in 2015 and previously covered the energy industry for the Pittsburgh Tribune Review. She's also covered state politics and government in North Carolina.