It’s a Jungle Down There

Zebra Mussels Transform Depths of Lake Mendota

by Mike Spear

Zebra Mussel Density June 2016

In September 2015, a single zebra mussel about the size of a fingernail signaled the arrival of one of the world’s most notorious invasive species in one of the world’s most studied lakes.

In June 2016, I led a team of SCUBA diving researchers at the CFL on surveys of the lake’s nearshore habitat. We found maybe four or five mussels at each dive site, needing little more than our fingers and a tiny plastic baggie to take samples. Just a year later, the mussels now count upwards of 30,000 below the dock of the Hasler Laboratory and now we use paint scrapers and underwater vacuums to chisel our countless specimens off rocks, logs and piers and get them back to the lab.

To help the public track the invasion, the CFL put together an online tool that provides interactive maps and figures for freshwater enthusiasts anywhere to see the transformation happening beneath the waves.

Zebra Mussel Density June 2017

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

Here at the CFL, we’re intensely studying the growth of the lake’s newest invasive population and keeping our finger on the pulse of the rest of Mendota to detect any changes they trigger. 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 this summer as they found nice, hard shells to grow on and a buffet of nutrients at the bottom of the lake.

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

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 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.