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How to Mix a Lake

If you frequent lakes in the Northwoods, you know that invasive species are a big problem.  Take rainbow smelt – the tiny fish are known for outcompeting native fish and devouring their young.  Once rainbow smelt get into a lake, it can be all but impossible to get rid of.  Some approaches rely on chemicals that wipe out all fish species.  But one project out of UW’s Trout Lake Research Station is experimenting with a new technique that could have many fewer side effects than the chemical method.  

The Crystal Lake Mixing project is driven by one basic fact: rainbow smelt like cold water. In a deep lake like Crystal, warm water rises to the surface, so the smelt stay down below where it’s nice and cool.  But what if you could take away that safe cold zone, so the lake would become the same temperature throughout? 

"The idea is that if we can mix the lake and mix the cold deep waters with the warm surface waters, then we can effectively remove that cold water habitat that smelt need to survive."

That’s Zach Lawson, one of nine researchers on the mixing project.  He thinks stirring up the layers of water could warm the lake and make it very tough for smelt to survive.  But the lake’s native species would be unharmed. 

"We’re trying to come up with a method where we can restore native fish species without having to poison the entire lake and start from scratch. So it’s novel in that sense that we’re trying to eradicate species by a physical manipulation instead of chemically poisoning the lake."

So if you want to mix up a lake, what’s the best way to do it?  No, you don’t use a giant pair of eggbeaters.  The Crystal Lake Mixing Project has built a contraption that’s made up of six discs called GELIs.  They move up and down and stir up the water.  GELI, of course, is short for Gradual Entrainment Lake Inverter.

"It’s really similar to a backyard trampoline. It has a stainless steel metal frame, that’s about 26 feet in diameter. And there’s a geomembrane that spans that frame, with a centralized air bladder."

So a ring of trampolines is moving up and down mixing Crystal Lake?  I’m still not sure what a geomembrane is, so I head out to Crystal Lake with Colin Smith.  He’s a technician on the project, and he helps maintain the GELI system. 

“Good morning…we’re going out to do some work on the lake this morning…”

We’re driving into the Crystal Lake Campground in Vilas County, a popular destination for vacationers - and also for Smith, who comes here to check on the GELIs almost every day.  

From the shore, you can’t see much of the GELIs except a few buoys.  That’s because most of the system is underwater, so we’ll have to get much closer to really understand how it works. 

Smith backs an aluminum john boat into the water.  There are no motors allowed on Crystal Lake, so we hop in the boat with a pair of oars. 

On the way, Smith explains exactly how mixing the lake could affect invasive smelt. 

“That warmer bottom temperature for the rainbow smelt is detrimental in that it increases the metabolism of the fish…and eventually their metabolism is so high that they can’t keep pace with their foraging needs, they can’t consume food and energy fast enough so they can’t keep their body weight up …so they eventually starve.”

Wow, pretty brutal…but then again think of those invasive rainbow smelt with their sharp teeth, eating native fish babies.  Anyway , back in the john boat.  Before we get close to an actual GELI, we head to the opposite shore of the lake to understand how the whole system is powered. 

"We’re on the north shore of crystal lake, and we’re going to go visit the pumphouse, where the air compressors are."

Inside a small shed are eight air compressors.  Air compressor noise   Valves at the on-shore pumphouse send compressed air out to the GELIs through a big underwater hose.  Compressed air fills the GELIS, and they float up.  The air lets out and the GELIs deflate and sink.  They’re up and down all day long.  Every 22 minutes one surfaces.  We head to the middle of the lake and hope for a sighting.

“You can tell where a GELI’s coming up because there’s a little bit of a disturbance at the water – say over there you can see a little bit of bubbles rising to the surface and then a calmer patch out there.”

A large black bulge crests the surface of the water.  It glistens and looks sort of like a seamonster - but let’s stick with the floating trampoline comparison.

“You can kind of see the air bladder in the middle there. The fill hose goes to the frame, and then from the frame to the air pouch there in the middle. That’s what’s inflated, you see the little dome shape on the top of the membrane there.”

The GELI membrane is a plasticky canvas material, stretched over a metal frame.  But the trampoline isn’t exactly taut – it can inflate when air is pumped in.  This is what Smith and the other researchers refer to as an air bladder.  Once it surfaces, it begins to deflate.

So the idea is pretty simple.  These inflated trampolines have a big surface area so they move a lot of water as they travel up and down.  Smith says the project could have tried to mix the lake using a more tried and true method – a simple aeration pump.  But the unorthodox GELI system uses just one tenth of the energy.  The technology has been tested in a nearby bog lake, but this experiment is the first use it to target smelt.  Smith explains Crystal Lake is a good testing site because the ecosystem is relatively uncomplicated. 

“The biological community is relatively simple – there’s the rainbow smelt, and perch…the relatively simple composition of the fish made it ideal for testing this hypothesis, because in a more complex system there’s more variables."

Then there’s the physical shape of the lake.  It’s also pretty straightforward, making it an even better test site.

“The lake is essentially a large bowl…there’s no odd shape to the lake. Having that relatively simple bowl shape allows the physical manipulation to be a little bit more successful.”

And successful it appears to be.  This is year two of the Crystal Lake Mixing project.  And though it’s too early to say whether the project will actually eliminate rainbow smelt, or be a viable model for treating future lakes…the results from the first year were promising.  Researchers saw a significant reduction in the adult smelt population.  They’ll continue to mix the lake through the summer of 2014 and monitor the effects. Zach Lawson says if it works, the GELI system could be another tool to help states turn the tide against invasive species.  

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