madison wisconsin balances on on isthmus between mendota and monona, the two vibrant blue lakes that the city shares. but underneath their surfaces, a time bomb has been set off by unforeseen circumstances. the zebra mussel population has exploded in the past two years. their invasion is harmful to the lake, and worse is that it wasn’t expected to happen when it did.

researchers at uw madison are shedding light on this lake-bottom mystery. underneath over 30 feet of murky water, mike spear and his team scuba dive for living data. 

equipment must be inventive for a science like this. sometimes they tow in nets of zooplankton that graze on green particles in the lake. some days they dive down with a mesh bag of petri dishes to scoop up samples of sediment-hugging algae. other days they lug a contraption of tubing, mesh and an oxygen tank to the lake-bottom with them. it works as a vacuum, sucking up bottom sediment and shooting it through the filter to collect squirming water mites, blood-worms, and other lake-bottom invertebrates.  

spear’s research project sets out to understand the impact that the zebra mussel population has on lake mendota’s ecosystem as they invade. that’s why he’s sampling just about everything. the mussels may be small- about the size of a quarter- but they do have a bite. in just two years they’ve multiplied greatly, and as they grow they do damage in every sector.

spear thinks that zebra mussels will continue to alter lake mendota like they have in other lakes hit by them. like lake michigan, where they’ve not only hurt ecosystems but have also had economic impact on things ranging from game fishing to city sewage. maintenance on pipes clogged by zebra mussels can cost $60 million a year for the power industry alone. 

“the zebra mussel shouldn’t be here,” spear says. “they upset the balance in a lake’s system that’s evolved in this balance for thousands of years.”

zebra mussels have competed with lakes’ native food webs by eating up nutrients in the food column that small fish rely on. less small fish means less big fish too. as they filter feed they clear the lake water. this lets more sunlight penetrate to the bottom than usual, which affects which plants grow. eventually, mats of gooey algae dominate, which wash onto shore in rotting piles whose smell can close down beaches. recent outbreaks of toxic blue-green algae may be linked to the ways that zebra mussels are altering lake mendota. 

“changes have been very drastic… seeing it in person is dramatic”

it’s clear that the zebra mussels are well established in lake mendota now, and will continue to thrive and grow.

what’s unclear, though, is why they showed up now. they were found in the great lakes in the 1990’s. researchers predicted that lake mendota, being high in boat traffic, would be hit by the invasion at least a decade ago. was there something special about madison’s lakes that kept them away? and if so, what changed two years ago?

“unchecked growth we see now should have happened a decade ago,” spear says. “what controls were in place, what changed and why?” 

lake mendota has been a centerpiece for city life since the state capitol was founded here in 1848. it’s been a hub for research, too. it’s known for being one of the best studied lakes in the world. the dramatic zebra mussel invasion seems like bad news, but it’s also a rare opportunity to learn about the dynamics of an aquatic invasion from the very beginning.

“we don’t know much about what happens about any species at low densities,” spear says. “usually by the time we realize something’s invaded and gear up in time to study it, it’s already exploded. here we get to watch the explosion play out in real time.”

spear was in the right place at the right time when zebra mussels were discovered in their earliest stages here. he’s a pd.d. candidate at a world renown limnology center, with decades worth of detailed data about the lake at his fingertips. the data he collects as the zebra mussels spread will become important pieces in this puzzle. it will help us understand exactly how ecosystem changes in the lake relate to the biological invasion. 

“we’ll learn a lot about zebra mussels in particular, and about invasive species in a broader scale,” spear says. “i think there are some real opportunities for contributing to basic frameworks of invasion biology.”

when asked how the progress is going, mike laughs. “we’ve got some ideas, but we’re mostly scrambling to document change that happens.”

the zebra mussels’ have moved quickly, and spear has to follow suit. his summer days are booked with deadlines and dive sites. he and his team scuba dive throughout lake mendota to make careful assessments of what is happening to the lake-bottom communities over time.

last year he could only find one or two in a square meter. now he’s charted hundreds in the same space. the razor-sharp mussels are coating docks and shorelines. so are their tiny immature offspring, which will reach their full size by next summer and make for an even thicker coat of zebra mussels on everything.

under the iconic surface of lake mendota, in murky waters where the zebra mussels thrive, they also provide us with a lesson we should learn from. this could have been prevented, as is the case with all invasive species. 

“it’s a ‘you don’t know what you got ’til it’s gone’ kinda scenario,” says spear.

on top of contributing to a scientific understanding of how and why this invasion is occurring, he and other scientists at the uw center for limnology engage in public outreach events to keep locals in touch with their beloved lakes. they create frequent social media posts and news letters to keep the public aware of what’s happening underwater, including news about research, and how to prevent the next invasive species from moving in.

as the zebra mussel invasion proceeds, this could open opportunities for more invasive species to become established. winters in the area have become increasingly warm, making it easier for non-native species to survive, reproduce and outcompete native ones.  

“hopefully people will see the changes that happened elsewhere, and what’s happening here, and will have a greater appreciation for balanced ecosystems.”