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Mendota's Zooplankton

In 1986, after decades of excessive nutrient runoff from farmland and urban areas marked Lake Mendota with murky algal blooms, people unified to do something drastic: clear up Lake Mendota’s waters. The Wisconsin Department of Natural Resources teamed up with UW-Madison’s Center for Limnology for a massive project that would span more than a decade. 12 years, 2,700,000 Walleye and 170,000 Northern Pike later, Lake Mendota had undergone its largest biomanipulation experiment yet.

To understand the changes that these managers and scientists catalyzed, we have to first delve into Lake Mendota’s food web. By the late 1980s, Cisco and Yellow Perch were the dominant fish in Lake Mendota’s waters. These fish are planktivores: they feed on the zooplankton, tiny “wanderers” that float and swim in the waters. Some zooplankton, particularly a kind called Daphnia, spend their lives grazing on algae.

By introducing Walleye and Northern Pike who prey on Cisco and Perch, managers hoped to lower the populations of fish that eat the algae-regulating zooplankton. With fewer fish consuming Daphnia, that meant there were more zooplankton to graze on the prolific algae in Lake Mendota.

The above illustration depicts Lake Mendota's food web—at the surface of the water there is green algae, with tiny microscopic zooplankton grazing on it. By manipulating the types of fish in the lake, the rates of grazing by zooplankton can be increased or decreased.

The cascading effects of the food web biomanipulation worked—the 12-year, millions-of-fish (and dollars) project successfully raised the populations of Walleye and Northern Pike, reducing planktivores and raising zooplankton populations. With huge populations of zooplankton grazing on algae, Lake Mendota began to see some of its clearest waters since colonization began.

A 2002 paper, written by the scientists who were involved in the project, synthesized the biomanipulation and called it a success. They even suggested more improvements were in the lake’s future, if the gains in water clarity from the project could be compounded by lowering Phosphorus runoff. And then came the Spiny Water Flea.

“[Spiny Water Fleas] are bigger, faster, and more defensive than any of our native zooplankton,” says Ben Martin, a PhD student in UW-Madison's Freshwater and Marine Science program who studies the Spiny Water Flea. This invasive zooplankton species is originally from Europe and Asia, but was accidentally brought to North America in the 1980s on the ballasts of ships. They are aptly named: their tiny bodies are accentuated by a comically long “tail,” the spine of the animal that has three protruding barbs. What’s bizarre to look at is also the problem—most native fish are deterred from eating these zooplankton because of their uncomfortably pokey shape.

With no fish to keep their numbers in check, the Spiny Water Flea began devouring the local Daphnia populations in Lake Mendota. With fewer Daphnia, Lake Mendota quickly descended back into the algal greeness that had been in control for the past few decades.

“It can be really easy to change a system,” says Martin. “All it took was one boater who didn’t clean their boat to bring [Spiny Water Flea] into the Madison lakes,” he tells me. “And now they’re changed forever.”

Lake Mendota has seen some of the strongest impacts from Spiny Water Flea of any infected lake on the continent. “And that goes back to all those nutrients in the lake and the biomanipulation,” Martin says. “That biomanipulation was maximizing the amount of Daphnia we had, all the grazers. So when Spiny Water Flea get here, they’ve got the densest food that they’ve ever had anywhere.” This abundance of zooplankton supported high levels of the invasive, and ultimately decreased Lake Mendota’s water clarity by three feet. “And Mendota doesn’t have much water clarity to lose,” Martin laughs.

His research has been focused on managing the Spiny Water Fleas, research that sounds, in many ways, rather similar to the original biomanipulation of the 80s and 90s. “Who’s eating Spiny Water Flea?” he’s asking, and “How can we maximize those populations?”

Martin has found that Perch are the best consumers of Spinys. But they’re also what are currently being managed for low populations, Ben explains. “It’s going to become a question of stakeholders—it’s a classic natural resource management dilemma.” In Martin's eyes, increasing Perch to decrease Spiny’s is the way to go. “I’m definitely biased,” he laughs, “But I don’t think it’d be much of a trade off.” Managers would have to stop stocking as many Walleye to get Perch levels up, which would definitely be met with resistance from certain anglers. “But Perch are an easier fish to catch,” Martin points out, which would be great for encouraging younger and less experienced anglers to enjoy fishing in Mendota. It would also increase water clarity, which is a positive for boaters, kayakers, and other water users.

Taken in 1948, hundreds of people are gathered on Lake Mendota to go ice fishing for Perch, the dominant fish in the waters at the time. Photo from the Wisconsin Historical Society.

The tensions between ecosystem health and the interests of humans come to the forefront in resource management—What do we prioritize? Who do we prioritize? “The more we love a natural place, the more we oftentimes hurt it,” Martin says. “People like the [Department of Natural Resources] are trying to improve and maximize these natural resources so we can continue to enjoy them.”

So which strings of the food web do we want to pull on, snip, knit? What cascading effects do our actions catalyze, from the accidents to the scientifically planned? And what's happening to our food webs in human's greatest experiment of all?

We will continue to explore these ideas in my next blog as we delve into another recent newcomer to the Madison Lakes. Stay tuned!

About the Author & Project

Julia Buskirk has always loved lakes, but after doing research for the Center for Limnology at Trout Lake Station last summer, she is particularly interested in the stories science tells about them. As a Conservation Biology and English double major, she looks forward to bringing these stories outside of a scientific realm and sharing them with anyone who cares about our water.

Thanks to UW-Madison's Hilldale Undergraduate/Faculty Fellowship for funding this project, to Joshua Calhoun (Assoc. Prof., English Department; Affiliate Faculty, Nelson Institute for Environmental Studies) for advising this project, & to the Holding History program for hosting this bookish, bloggish conversation about waterways.

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