What Lies Beneath: Sudden Invasion of a Wisconsin Lake Wasn’t So Sudden After All

Jake Walsh (right), graduate researcher for the Center for Limnology, and undergraduate Carly Broshat (left) use plankton nets to collect water samples of spiny water flea, an invasive species of zooplankton, in Lake Mendota. (Photo by Bryce Richter / UW-Madison)

In the fall of 2009, a tiny aquatic creature known as the spiny water flea showed up in a lake where it had never before been seen. At first, students in the UW-Madison undergraduate limnology class didn’t know what they were pulling up from Lake Mendota in their plankton nets.
A mass of spiny water fleas attached to a fishing line. Photo: Jeff Gunderson, Minnesota Sea grant Program

But professor Jake Vander Zanden did. It was a zooplankton with a comically long spike protruding behind it like a tail. And there were thousands of them in each tow of the net, samples piling upon one another and building into a zooplankton mound as they were strained from the water. Looking, Vander Zanden has since remarked, like “spiny water flea applesauce.”
Since that time, Vander Zanden and other researchers at the UW-Madison Center for Limnology have pondered a question this accidental detection raised. The Limnology course runs the same sampling trip at about the same time each fall. Students use the same plankton nets off the sides of the same boat in the same place in a lake that is often touted as “the most studied lake in the world.” So why didn’t we find the spiny water flea sooner?
“I think it’s surprising, going from undetected to the highest densities we’ve seen in any lake anywhere at any time. That’s what sparked the whole study,” says Jake Walsh, a post doctoral researcher at the CFL and lead author of a recent report that has shed some light on this story of spiny water flea (or SWF) in Lake Mendota. And what they’ve found is a tale fit for a spy thriller – SWF have likely been living in the waters of Lake Mendota for decades just waiting for the right conditions to make their move.
Spiny water flea under the microscope. These voracious non-native zooplankton eat native zooplankton that graze on algae and help keep water clear. Photo: Jake Walsh

In the aftermath of detecting the invasion in 2009, says Walsh, the working hypothesis was that SWF must have been recently introduced to the lake. Whether they had arrived in bait buckets or their eggs had been carried in on sediment still attached to something like a boat anchor, “the common narrative is that an invasive species invades a lake and then takes off relatively soon thereafter. Our first assumption wasn’t ‘They must have been here for a while just biding their time,’” he says.
But when Walsh looked back at old samples, he discovered that that’s exactly what they had been doing. His findings were published in the journal, Ecosphere, late last year.
Walsh first got his hands on old samples from the Long-Term Ecological Research project, a program funded by the National Science Foundation to monitor long-term changes to ecosystems across the U.S. The CFL runs LTER’s North Temperate Lakes program and has been studying a group of lakes in Wisconsin since the 1980s. Walsh combed meticulously through past LTER zooplankton samples from Lake Mendota and found evidence of SWF from 2008, the year before limnology students started catching them in their nets.
Jake Walsh holds a sediment core taken from the bottom of a northern Wisconsin lake. Photo: Carol Warden.

Then Walsh turned to taking samples, or cores, from the sediment at the bottom of Lake Mendota. As sediment builds up on a lakebed over time, it preserves the hard parts of animals – like bones, shells or, in this case, spines. Each layer preserves an annual record of life in the lake. Digging down through the sediment cores, Walsh found evidence of SWF that suggested that, for more than two-decades, they had made Lake Mendota their home and no one had a clue.
If that is the case, it doesn’t exactly make the tale of SWF all that unusual. In fact, most invasive species get discovered only after their population densities reach high numbers. To understand how that’s possible, says Jake Vander Zanden, a professor at the Center for Limnology and co-author of the study, one must first appreciate the sheer scale of Lake Mendota.
“Lake Mendota contains roughly 500 million cubic meters of water,” says Vander Zanden. “Now imagine there are 500 million spiny water fleas in the lake. That may sound like a lot, but that’s only one tiny flea per cubic meter of water.” The odds of catching that flea on a routine sampling run, Vander Zanden says, simply aren’t good.
But, in the fall of 2009, he says, estimates put densities of SWF in Lake Mendota at 300 individuals per cubic meter of water – or 150 billion in the lake.
Vander Zanden and Walsh suspect that such an unusually high number of SWF was due to the unseasonably cool weather that summer. Like many species of zooplankton, SWF prefer milder water temperatures and, in 2009, cooler-than-average air temperatures led to cooler-than-average water temperatures. This, combined with the usual late summer growth of algae in a lake heavily polluted by agricultural fertilizer, led to ideal conditions for a boom.
A plankton net being lowered into Lake Mendota. Since spiny water flea populations exploded, the lake has lost a meter of water clarity. Photo: Adam Hinterthuer

Vander Zanden says the study points to the need for more sophisticated tools for determining the presence of species in an ecosystem. If we are only ever finding a non-native species once it’s already taken off, there is not a lot we can do to manage the invasion. But early detection, potentially with new technologies like “eDNA,” or chunks of an organism’s DNA filtered out of a water sample and then identified in the lab, may signal important progress.
A microscopic view of the spine fragments Jake Walsh detected in his lake sediment core samples. Photo: Jake Walsh

“Having an understanding of where invasive species are and what they’re doing in an ecosystem is important,” he says. “If they are in a lot more systems than we think and they’re not always going to stay at those low densities, being able to anticipate [a boom] is useful.”
Walsh agrees. “Ecologists are surprisingly limited in our ability to detect organisms and I think it’s sort of biased our thinking or our narrative of how invasions happen,” he says. He hopes his study can help correct this misperception and lead to invasive species management that is both better at detecting invasions and better at managing the invaded ecosystems.
”Managing invasive species is hard,” Walsh says, “but I think we have a lot of control over making sure that our ecosystems are in good shape. If we didn’t have the same water quality issues that we have in Lake Mendota now, maybe we wouldn’t see such astronomical spiny water flea populations.”
The success of the spiny water flea, in other words, may say more about our lake than the species itself.
And, if invasive species are in more systems than we think – simply persisting at low densities and biding their time for the right conditions – one of the best things we can do is make sure our lakes aren’t stressed by other factors and are in overall “good health.” It may not stop the next invasion, but it may help us keep the next invasion in check.

To Contact the author about this paper: Jake Walsh, jrwalsh2@wisc.edu; Jake Vander Zanden, mjvanderzand@wisc.edu

The journal article in Ecosphere can be found here.