Re-posted from UW-Madison Center for Limnology
In the fall of 2009, a group of University of Wisconsin-Madison undergraduates made a startling discovery in the waters off the campus shoreline. Spiny water fleas, a type of invasive zooplankton believed to be suited only to cooler lakes in more northern climates, turned up in their nets as they collected samples for their lab session.
Zoology 315, the popular undergraduate limnology course, has struck again: Zebra mussels, the Great Lakes’ most infamous invasive species, have arrived in Lake Mendota.
While the discovery of the spiny water flea left scientists wondering how a cold-water animal was thriving in a warm-water lake, the big question surrounding zebra mussels is “what took them so long?”
“I would have predicted that they would have gotten here earlier,” says Jake Vander Zanden, a professor at UW-Madison’s Center for Limnology (CFL) and expert on aquatic invasive species. “It’s not that it’s inevitable our lakes get invaded, but we’ve known that Mendota is a good candidate for a long time.”
Aside from being suitable zebra mussel habitat, Lake Mendota is close to many lakes that are already invaded, like Lake Wisconsin, Lake Ripley and several lakes just west of Milwaukee in Waukesha County. Lake Mendota is also popular with anglers, water skiers and anyone with a boat, which made the chance of someone accidently bringing in the invasive mussel high.
Still, says, Vander Zanden, public awareness campaigns may have bought the lake some time. Efforts by the Wisconsin Department of Natural Resources (DNR) and other organizations urging people to clean their boats and not transport live bait from lake to lake may have helped keep incidents of zebra mussel introductions down.
Whatever the reason for their slower-than-expected arrival, the prolific mussel now calls Lake Mendota home and appears to be here to stay.
Not a Typical Day on the Lake
Josie Kolbeck, a UW-Madison senior majoring in zoology and conservation biology, was part of the undergraduate lab group that made the find. She and several classmates had strapped on waders and were out in the cold, hip-deep water along the start of the Lakeshore Path, using D-shaped nets attached to long, wooden poles to catch aquatic insects living near the lake bottom. When Vince Butitta, the CFL graduate student leading the lab, came over to help, they stumbled over something in the water.
“We ended up walking right into a pole,” Kolbeck recalls. “Like the kind you’d use to put up a snow fence.” When Butitta suggested they get the large piece of litter out of the lake and up on shore, the students wanted to first check it for insects.
“At the top was a zebra mussel,” Kolbeck says. “None of us students knew what it was, but Vince was pretty interested.”
When they returned to the lab, Butitta took pictures of the specimen and referenced a digital mussel identification guide. Even though he was fairly sure he recognized the telltale striped pattern, he says: “I thought, ‘Wait a minute, I didn’t think these were supposed to be in the lake.’”
While Kolbeck was sad to hear the news, the class will be one she long remembers. “It was kind of neat once [Butitta] said ‘You guys are part of this discovery,’ “she recalls. “It’s cool to know we did something more than just a regular lab session.”
Here, But Just Hanging Out – For Now
Vander Zanden confirmed from Butitta’s photos that the class had found zebra mussels and the next thing he wanted to know was where else in Lake Mendota zebra mussels were found and how abundant they were.
Soon, CFL graduate student Colin Smith found himself in the water alongside Vander Zanden’s boat, wearing his scuba gear and a dry suit, searching rocks, piers and other hard habitat for signs of the aquatic invaders. The well-hidden, thumbnail-sized mollusks can be difficult to find in the cold and murky Mendota water.
In five of the 10 sites sampled so far, the researchers have found the animals at low abundances, roughly a half dozen or so per square meter. While the zebra mussels aren’t overrunning the lake bottom, they are firmly established, with individuals ranging from one to three years old tucked into crevasses or under the base of rocks in near-shore waters from places like Picnic Point to Tenney Park and Maple Bluff.
Some scientific studies also suggest that lakes like Mendota, with soft, silty bottoms, aren’t the ideal habitat for zebra mussels, because they lack the hard substrates their larvae need to attach to and grow. While it may help moderate the population, Vander Zanden says that doesn’t mean we can count them out. Invasive species seem to have a knack for exceeding expectations.
Future of Mendota Is Murky – Or Clear?
Zebra mussels are filter feeders – they suck in water, filter out anything edible, then “spit” out the rest. A single adult animal can go through a liter of water each day. As a result, once invaded, water bodies as large as the Great Lakes often experience dramatic increase in water clarity.
While this may be welcome news for anyone who has bemoaned a Lake Mendota algae bloom over the years, zebra mussels can ultimately make it more difficult to enjoy a lake, says Susan Graham, south-central lakes management coordinator for the Wisconsin DNR. For starters, Graham says, zebra mussel shells coat the bottom of some invaded Wisconsin lakes and if that happens in Lake Mendota, “everyone’s going to start buying water shoes and waders because you’re not going to want to cut your feet,” she says.
Lakefront homeowners are also sometimes forced to store their boats on out-of-water lifts to keep mussels from growing damaging colonies on the hulls, Graham says.
What’s more, large mats of algae can grow along the bottom of zebra mussel-infested lakes and eventually break free, creating noxious odors as they wash up on shore and decompose. This is phenomenon is all-too-common along the Milwaukee lakeshore.
Zebra mussels also concentrate a lake’s nutrients along the bottom, dramatically changing ecosystem dynamics. For example, says Vander Zanden, fish species, like carp, that prefer the bottom of the lake and can actually eat zebra mussels, do quite well while others, like walleye, which prefer open-water habitat, can
Across the country, zebra mussels also clog water intake valves for power plants and water treatment facilities, costing hundreds of millions of dollars to clean up. They lead to more frequent blue green algae blooms and incidents of avian botulism, which can kill waterfowl. The list goes on.
What’s unclear, says Vander Zanden is when and how impacts will be felt in Lake Mendota. From a scientific standpoint, the Zoology 315 lab’s discovery presents a unique opportunity for scientists to follow along. “We usually only notice an exotic species once it’s become abundant,” he says. “We rarely get a chance to observe what happens at such low densities. I would say that we’re on the front end of this and that they haven’t had any real impacts on the lake yet.”
We Can’t Stop Them, But Can We Hope to Contain Them?
While researchers wait for impacts from the mussels, another question also gnaws at them, how can we stop these unwanted invasions? No one knows how, exactly, the mussels made their way into Lake Mendota, but human assistance is the most likely route.
For example, on a snowy day in the fall of 2011, a local company pulled a pontoon boat out of Lake Mendota to bring it to a winter storage facility. In the process, someone spotted a suspicious cluster of shells growing on the stern of the boat and called the DNR. Graham was one of the officials who responded, helping confirm the shells were zebra mussels. The boat had spent a month moored in Lake Mendota after the owner had purchased it from someone living on Lake Ripley, which is infested with zebra mussels.
Graham and her colleagues searched the pier for other mussels and, the next spring also looked for larvae in the water or adults growing on nearby piers or rocks, but came up empty. No mussels were found actually growing in the lake.
While it’s possible the single “exposure” event brought in the current crop of mussels, “there are probably lots of other such events we don’t know about,”Graham says, also noting that it highlights the importance of the DNR’s consistent reminders that people exercise vigilance when moving any sort of gear from one lake to another.
“I know it’s really easy to get caught up in the excitement of getting a new boat, or getting out on the lake,” she says, “but you still have to pay attention to invasives.” Whether it’s bait buckets, the wheel wells of boat trailers, or the hulls of boats, we need to be better at cleaning gear and checking them for hitchhikers.
And now, Graham, says, people need to be aware that when they move a boat from Lake Mendota to somewhere else they take a chance that zebra mussels are coming with them. The microscopic larvae can survive in any water left in the boat, or hard-shelled stowaways can lurk on hulls and engines. The take home message is one resource managers and state agencies deliver again and again: Stop moving plants and animals around.
In the meantime, Vander Zanden says, he and his students will continue to take advantage of this mild fall weather and survey as many sites as possible in the Madison lakes, looking for the presence of mussels and any patterns of their distribution. Lakefront homeowners can help as they pull their piers out of the water for the winter by checking for signs of mussels and reporting them to the DNR.
Whatever this newest resident of Lake Mendota brings, Vander Zanden says, it will be interesting to watch how the ecosystem responds. The impacts of a potential population explosion could take many forms, he says. “It’s not necessarily a doom and gloom story, but it certainly knocks the lake in a different direction.”
Where that takes Lake Mendota, we’ll just have to wait and see.