October 2017 Discover

October 2017^ DISCOVER^59

ERNIE MASTROIANNI/DISCOVER

to attach to, means they can blanket vast swaths of lake

bottom inaccessible to zebra mussels. Zebras also only

feed during the warmer months. Quaggas filter nutrients

out of the water year-round.

The public can comprehend the devastation of a cata-

strophic wildfire that torches vast stands of trees, leaves

a scorched forest floor littered with wildlife carcasses and

turns dancing streams into oozes of mud and ash. But

forests grow back. The quagga mussel destruction is so

profound it is hard to fathom.

“People look at the lake and don’t think of it as hav-

ing a geography. It’s just a flat surface from above,” says

University of Wisconsin-Milwaukee ecologist Harvey

Bootsma. “From there, it looks pretty much the same

as it did 30 years ago, but underwater, everything has

changed.”

The mollusks now stretch across Lake Michigan almost

from shore to shore. People might still think of Lake

Michigan as an inland sea full of fish. It’s more accurate

to think of it as an exotic mussel bed sprawling across

thousands of square miles. Lake Michigan’s quagga mass

in one recent year was estimated to be about seven times

greater than the schools of prey fish that sustain the lake’s

salmon and trout. Under some conditions, the plankton-

feasting mussels can now “filter” all of Lake Michigan in

less than two weeks, sucking up the life that is the base of

the food web and making its waters some of the clearest

freshwater in the world.

This nearly vodka-clear water is not the sign of a

healthy lake; it’s the sign of one in which the bottom of

the food web is collapsing. One study on southeastern

Lake Michigan revealed that by 2009, phytoplankton

levels in springtime — the prime plankton-growing

time of year — had dropped nearly 90 percent since

the mussels took over the lake bottom. It’s probably

not a coincidence that the lake’s fish populations have

dropped at the same time.

SHIFTING BASELINE

Not all fish are struggling, though. Take the invasive

round goby, another Seaway interloper that arrived just a

couple of years after the mussels and is also native to the

Caspian and Black Sea region. It evolved to feast on the

flesh of quagga and zebra mussels by cracking their shells

with molar-like teeth. Now this bug-eyed, thumb-sized

fish is thriving across the Great Lakes.

“People really don’t grasp what has happened here,”

Bootsma explains to me on a frigid early November day

as he straps on a scuba tank, climbs over the back of the

boat and plunges to the lake bottom 30 feet below. He

was only about 800 yards off the beach of a popular park

in the leafy Milwaukee suburb of Shorewood. But he

might as well have landed on another continent. Under

the surface, Lake Michigan bears little resemblance to

the freshwater wonder that left early European explorers

awestruck with its teeming herring, trout, sturgeon, perch

and whitefish. Down below, the lake has pretty much

become just a goby show.

Bootsma finds the changes professionally interesting,

but personally distressing. He attributes

his whole career to summer days he spent

as a child on Georgian Bay in northern

Lake Huron, fishing for native bass and

perch and snorkeling to the rocky bottom

to capture crayfish. “I still remember tell-

ing myself that when I grow up, I’m going

to get a job that will keep me on these

lakes all the time,” he tells me.

Outside his office window at UW-

Milwaukee’s School of Freshwater Sci-

ences are the grain elevators and coal piles

that define the city’s inner harbor. That

harbor is connected to Lake Michigan,

which is connected to Lake Huron, which

connects to Georgian Bay. They all have

different names, but in actuality they are

the same lake, and the largest lake by

surface area on the globe. It is no longer

the lake Bootsma fell in love with. He’s

known this for years because of his almost

weekly trips out to his research station on the bottom

of Lake Michigan.

What’s even more distressing for him is the idea that his

children don’t even know what they’re missing. Ecologists

call it the “shifting baseline phenomenon” — a fancy way

of saying that kids are getting cheated out of the lakes

their moms and dads loved. “This isn’t the lake it was 25

years ago, and it’s probably not the same lake it’s going to

be in 10 years,” says Bootsma.

It’s not just native fish species and summertime beach-

goers that have been hit by this biological pollution.

Invasive species can have effects just as toxic as the nasti-

est chemicals concocted in a lab. A textbook example is

the botulism outbreaks that have killed tens of thousands

of birds on Lakes Michigan, Erie and Ontario. Mussels

increase water clarity, which helps water plants bloom.

Harvey Bootsma studies

the Great Lakes from his

home base in Milwaukee’s

harbor. His team recently

showed invasive mussels

reduce Lake Michigan

zooplankton — a key

food — by half each year.