This is what an invasion looks like.
Last fall, students in a UW-Madison undergraduate limnology lab found invasive zebra mussels living in Lake Mendota for the first time. Later that year, when we pulled the Hasler Lab pier out of the water for the winter, we only found two, maybe three, mussels per leg of each pier section. While the mussels were undoubtedly in the lake, no one would refer to it as an “invasion.”
Even earlier this July, Center for Limnology graduate student, Mike Spear, wasn’t seeing a boom in the mussel population. Spear is leading research on an aquatic invasive species grant from the Wisconsin Department of Natural Resources to understand population growth and document impacts of zebra mussels in Madison’s lakes,
The prime habitat for zebra mussels is medium-sized rocks with nooks and crannies that let them escape predation and that are sitting in one meter of water, or pretty much as shallow as it can be without freezing over and, “In July [of 2016],” Spear says, “densities of adult zebra mussels in those spots were about 10 to 20 individuals per square meter, which by standards of the Great Lakes region, is extremely low – about as low as you’ll find them in systems where they’re known to exist.”
That was four months ago. Now, Spear says, his team is finding zebra mussels everywhere and congregated in much larger numbers. “We saw a very strong recruitment event in mid August,” Spear says, “which boosted densities to upwards of 200 per square meter in some places. If any decent proportion of these young mussels are left by reproduction time next year, I think we will definitely be seeing continued exponential growth of the population.”
And it’s not just Spear’s crew of scuba diving students noticing the uptick in mussel numbers. For the past couple of months, he has received e-mails and phone calls reporting zebra mussel sightings from local citizens. One even sent a letter with the invasive species in question taped to it!
As more and more lakefront homeowners pull their piers out of the water this fall and as boaters decide it’s time to get their boats back on land and into storage for the winter, zebra mussel sightings are only going to go up. We thought it’d be helpful to answer a few questions we’ve been getting about what we can do and what the invasion means for our lakes.
Is There Any Stopping Them?
“Unfortunately, no,” says Jake Vander Zanden, a professor at the Center for Limnology and expert on aquatic invasive species. “The question now becomes ‘How big will the zebra mussel population in Madison get?'” Zebra mussels need hard surfaces like rock, cobble and piers to attach to to grow, Vander Zanden says. The fact that the bottom of Madison’s lakes are primarily covered with silt and other soft sediments means that it’s possible there’s not enough available habitat for the mussels to reach numbers that will dramatically impact our lakes.
However, he warns, it’s not wise to underestimate one of the most successful invasive species to ever work its way into the Great Lakes. Since the species was first discovered in the Great Lakes in 1988, zebra mussels have spread to lakes and rivers throughout all five Great Lakes and as far away as Louisiana and California.
Vander Zanden says he’s been contacted by concerned citizens who want to organize zebra mussel removal efforts, but that’s just not feasible. There are already too many mussels and countless more “veligers” or mussel larvae, to do much about it. An experimental control effort by Minnesota’s Department of Natural Resources had initially promising results – until the mussel population rebounded. And, just this year, a new Minnesota-based effort to use a copper-based pesticide was launched, although “control” and not “eradication” was the goal.
What’s Going to Happen to Madison Lakes?
No one can know for sure how a zebra mussel invasion is going to play out in a particular ecosystem, Vander Zanden says, but we have documented the impacts of enough such invasions to have a good idea of what’s in store. In fact, Vander Zanden and Scott Higgins authored a 2010 paper on this very subject, reviewing published studies of zebra mussel invasions and finding similar impacts in most lakes and rivers.
The main effect a zebra mussel invasion has on an ecosystem is a restructuring of the food web – moving production from the open waters of a lake down to the bottom. Zebra mussels are amazingly effective filter feeders and they essentially suck all of the phytoplankton and algae (or tiny plants) out of the water column. These tiny plants are important food for zooplankton (tiny crustaceans) that are, in turn, crucial to the diet of a lot of little fish. Which, of course, are then eaten by bigger fish.
According to Vander Zanden’s 2010 report in Ecological Monographs, zebra mussel invasions can lead to a nearly 80% reduction in the amount of both phytoplankton and zooplankton living in an invaded ecosystem. This reduction in plankton leads to dramatic changes in water quality. In other words, it gets a lot clearer. But, there is a price to this.
By pulling food down to the bottom and excreting waste, zebra mussels concentrate nutrients at the bottom of a lake. And, since the water is then clearer, sunlight filters down further. This leads to an explosion of algae and aquatic plant growth – a 170% increase, according to the study. In Lake Michigan, for example, large mats of an algae called cladophora grow on the tops of zebra mussel beds at the bottom, before tearing away and washing up onshore where they decay and foul beaches. What’s more, Vander Zanden says, the reduction of plankton and algae in the open water of a lake means less competition for surface-floating cyanobacteria (or blue-green algae) which can take advantage of these altered conditions to form more frequent and potentially toxic blooms.
While these are the primary impacts of a zebra mussel invasion, other changes may also be in our future. Populations of species of fish, like yellow perch, that eat zooplankton in their younger years, may suffer as that food source goes away. Swimmers or beach walkers may find that bare feet and zebra mussel shells don’t mix. Whatever happens, changes are in store.
I’ve Found Zebra Mussels, Should I Report Them?
That depends on where you find them. We know the mussels can be found in many locations across Lake Mendota and Lake Monona, but they haven’t yet been detected in Lakes Waubesa, Kegonsa or Wingra.
According to an announcement sent out by the Clean Lakes Alliance, the Wisconsin Department of Natural Resources is asking area citizens to report any zebra mussel sightings in those three bodies of water. They are asking for a photo (it’s helpful to use a coin or ruler in the shot for scale), location, and estimate of the number of mussels. If you’re up for taking a sample, you can preserve some mussels in a container filled with clear alcohol at room temperature, or a refrigerated container with water. Gina LaLiberte, the DNR’s water resources management specialist is the main contact – Gina.LaLiberte@wisconsin.gov or 608-221-5377.
What Can We Do?
The most important players in this part of the equation are boaters. Boaters should look for stowaways on their boat hulls, motors, anchors and trailers and remove them by scraping or using a pressure washer. Also, zebra mussel larvae can sneak into any water aboard your boat, so boaters need to be sure to drain the water from motors, boat bilges, live wells and bait wells, and clean the weeds from the boat, motor and trailer before leaving the boat ramp. Likewise, anglers should make sure to check minnow traps for mussels and make sure not to transport bait or water from bait buckets across different waterways.
While we’re not about to get rid of zebra mussels in Madison lakes any time soon, we can at least slow their spread into other lakes and rivers and do our best to protect our waters.
PRESS CONTACT: Jake Vander Zanden – mjvanderzand [at] wisc [dot] edu, 608-770-5891
