Somewhere along the way, someone lost a pair of Oakley sunglasses. Maybe they were blown off on a full-throttle boat ride. Or slipped from a shirt pocket while wrestling a fish on board. Whatever the cause, the sleek, black glasses with the reflective lenses sank eight meters (about 24 feet) and settled on top of the soft, silty floor of Lake Mendota.
Which is where CFL graduate student, Mike Spear, found them. Except something else had gotten there first.
Zebra mussels.
And the fact that the little, invasive mollusks had colonized a pair of sunglasses in what, Spear says, was otherwise a “desert of silt,” shows just how much their population has grown since zebra mussels were first detected in the lake in the fall of 2015.
Full-Scale Invasion
It’s one thing to know that zebra mussels are prolific invaders that can overrun an ecosystem in a short amount of time. It’s another thing entirely to see it in action.
Since zebra mussels were first discovered by students in a UW-Madison limnology class in the fall of 2015, Center for Limnology researchers, led by Mike, a graduate student working with Jake Vander Zanden, have been documenting their invasion. This summer, like last summer, Mike and a team of undergraduate students don scuba gear and routinely head to three sites in Lake Mendota, where they take transects at different depths – laying a quadrat (essentially a large, square hula hoop) on the lake bed and counting any zebra mussels within its meter-squared radius.
“Last June,”says Mike, “zebra mussel density off the Hasler Lab dock was five per square meter. We haven’t quantified this year’s numbers yet, but it’s got to be thousands [per square meter]. I mean, you pick up a rock and there’s hundreds on that single rock.”
The fact that the pair of orphaned Oakleys managed to become zebra mussel habitat tells us a lot about populations in the lake. Zebra mussel larvae, called veligers, can only grow on hard surfaces and they don’t really swim, they sort of drift along with currents and hope to eventually settle on something hard enough to latch on to. And Lake Mendota doesn’t offer a whole lot of hard substrate too far from shore.
Only a year ago, says Mike, ” if you put the quadrat down [further from shore], there would be nothing in the muck. But now, if there’s a snail shell that’s big enough, it’ll be colonized, A leaf with hard veins its colonized, even among this desert of silt. So you have to think, if these veligers are settling randomly, for them to colonize these little tiny refuges among huge deserts of unsuitable substrate, they must just be raining down everywhere.”
Based on data Mike and his crew collected last year, a high number of tiny little zebra mussel larvae isn’t that surprising, but seeing it in action is still almost hard to believe.
“We’re having to re-imagine how we’re going to sample,” Mike says. “That’s how much this changes things. We have to throw our sampling routines out the window because it’s changed so much, it’s not feasible to do the same thing anymore. Now we’re going to have to get paint scrapers and underwater suction to try and get the samples we need. It’s crazy.”
We’ll be posting updates of Mike’s crew all summer long as they document the zebra mussel invasion. Stay tuned next week for one zebra mussel impact – algae.