It’s a Jungle Down There: Zebra Mussels Transforming Depths of Lake Mendota

Zebra mussels encrust sections of the UW Hoofers sailing pier pulled out of the water fall, 2016. The invasion has only gotten worse. Photo: A. Hinterthuer

by Mike Spear

In September 2015, a single zebra mussel about the size of a fingernail sent alarm through the Center for Limnology. (CFL). It signaled the arrival of one of the world’s most notorious invasive species in our own Lake Mendota, and it was big news.

Today, zebra mussels carpet the rocky bottom of nearshore Mendota, transforming the landscape to a jagged matrix of invasive biology and reaching densities in the tens of thousands per square meter.

That transformation has been rapid. In June 2016, I led a team of SCUBA diving researchers at the CFL on surveys of the lake’s nearshore habitat. We were finding maybe four or five mussels at each dive site. Just a year later, they now count upwards of 30,000 below the dock of the Hasler Laboratory, just west of the Memorial Union Terrace.

To help the public track the invasion in space and time, the CFL has put together an online tool that provides interactive maps and figures for curious Madisonians (or freshwater enthusiasts anywhere) to see the transformation happening just beneath the waves.

This new interactive map shows how quickly zebra mussel populations exploded. Users can click through time and watch the incredible growth unfold, or pan around the map to get a detailed look at what’s happening in their neck of Mendota. Courtesy: Mike Spear

The bottom of the lake looks very different since the outbreak of mussels. Most hard surfaces are now encrusted in the critters, their sharp shells creating a hazard for bare feet but a new world of nooks and crannies for all sorts of algae and invertebrates to inhabit.

Unfortunately, the zebra mussels have also colonized the lake’s native mussels, called Unionids, latching on to their openings, weighing them down, sucking away their food and, eventually, killing them.

Me (Mike Spear) showing off a rock encrusted with zebra mussels from the waters under our pier. Photo: UW Communications

Faculty, staff and students at the CFL are intensely studying the growth of the lake’s new zebra mussel population. They’re also keeping their fingers on the pulse of the rest of Mendota to detect any changes brought on by this newly abundant species. Though it’s difficult to pin impacts on zebra mussels this early, their continued growth should bring more nutrients to the lake bottom, upsetting the balance between the open water and nearshore habitats. In fact, thick, stringy mats of algae already coated Mendota’s rocks early this summer as they found nice, hard shells to grow on and a buffet of nutrients at the bottom of the lake.

Between vine-like weeds, algal underbrush, and a stampede of hungry zebra mussels, it’s truly a jungle down there!

This exponential growth can’t go on forever, but zebra mussel populations in the Great Lakes reached the hundreds of thousands per square meter before they leveled off. Such densities are sure to have impacts on Lake Mendota. Their growth here is limited to the nearshore, as their need for rock and wood make most of Mendota’s mucky bottom an inhospitable desert of silt.

Monitoring indicates zebra mussels spread to Lake Monona last year, and they now seem widespread there. WDNR scientists detected zebra mussels in Lake Waubesa this summer, as well, so it appears the invasion is headed downstream.

Zebra mussels are likely a permanent resident of the Yahara chain of lakes going forward. However, it’s important to limit exposure of Wisconsin’s other lakes, the vast majority of which are still zebra mussel-free. Following DNR guidelines to clean and dry boating and angling equipment could prevent these changes in more waters across the state.

6 thoughts on “It’s a Jungle Down There: Zebra Mussels Transforming Depths of Lake Mendota

  1. Dumb question, but can’t we “Eat them” out? In Europe and the UK, many mussel beds have been stripped out and made barren through overfishing. This has resulted in beds having to be resewn, reseeded artificially Can’t you do the same with them?

    1. Hi Tom,
      Unfortunately, zebra mussels are too small to be even really much of a bite. More problematic is the fact that millions of tiny veligers (larvae) and just-getting-started mussels are in every nook and cranny in the lake, so a mussel fishery is unlikely to truly get rid of them!

      1. I guess it’s time for the nuclear option!!!! Cheers, Tom Brown P.S.
        Read that the adults reach 50mm and hang out in the mud in the middle of the lake. remove the adults by dredging then ther’s no larvae???

    1. Hi Ken, sorry for the delay, I’ve been out of office! The zebra mussels will likely (as long as their numbers continue to go up) have big impacts on water clarity AND quality. For clarity, they will most likely make the water much clearer as they filter feed so much algae and other tiny particles out of the water column. However, they do not eat cyanobacteria (or blue-green) algae and in other lakes with zebra mussels, it’s been documented that, once the “good” algae has been eaten, blue-green algae has less competition and more blooms occur. Zebra mussels also concentrate nutrients at the bottom of the lake and provide substrate for mats of algae called cladophora to grow – which then breaks free and washes up on beaches and makes everything pretty stinky!

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