For decades, aquatic invaders have been plaguing the Great Lakes. They’ve changed the way the ecosystems work and affected the balance of life in the lakes. Most of them didn’t just wander in. They hitchhiked a ride into the Lakes in the ballast water of ships from across the Atlantic. The Great Lakes Radio Consortium’s Carolyn Gramling reports… now the combination of these invasive species is causing changes that concern scientists:
For decades, aquatic invaders have been plaguing the Great Lakes. They’ve changed the way the
ecosystems work and affected the balance of life in the lakes. Most of them didn’t just wander in.
They’ve hitchhiked a ride into the Lakes in the ballast water of ships from across the Atlantic. The
Great Lakes Radio Consortium’s Carolyn Gramling reports…now the combination of these invasive
species is causing changes that concern scientists:
Zebra mussels were one of those species that hitched a ride in the ballast of a ship. They first
appeared in the Lakes in the mid-1980s. Zebras and their cousins the quagga mussels compete for
food needed by aquatic animals native to the lakes.
Researchers say now these mussels are part of another problem. They’re changing the food web.
The food web is made up of organisms that feed on each other. Usually it’s a chain of small, even
microscopic species that are food for ever larger species. Zebra mussels are near the bottom. For
their food, they filter large volumes of water containing contaminant-laden algae and sediment. In
the process they ingest PCBs and other toxins.
Gene Kim is a researcher in the Ohio State University’s Aquatic Ecology Laboratory. He says that
zebra mussels and a non-native fish called the round goby have helped to form a new food chain
within Lake Erie – a chain that can connect harmful chemicals buried in lake mud to humans.
“A lot of the exotic species, these alien species, have incorporated themselves into the Lake Erie
food web, and there’s a lot of ramifications, in terms of, will they change the cycling of historical
contaminants that right now are in the sediments, but they could be redirected back into sport fish
and eventually, humans.”
Zebra mussels have few natural predators in North America, and they reproduce rapidly. As a
result, they’ve been wiping out native mussels and clogging up water intake pipes in the lake. So
the arrival of the round goby, which likes to eat zebra mussels, would seem to be good news.
Instead, it has proven to be a double-edged sword.
Roy Stein is a professor in Ohio State’s Aquatic Ecology Laboratory. He says the PCBs and other
contaminants, once held captive in the sediment at the bottom of Lake Erie are taken up by zebra
mussels, and then the zebras are eaten by the round goby.
“And then, interestingly enough, round gobies are important prey for smallmouth bass that people
eat, and all of a sudden we have the opportunity for those PCBs that were stored in the sediments
to come up through the food chain and influence humans.”
So, Stein says, those contaminants that were trapped in the sediment now have a pathway up the
Gene Kim’s research is confirming the link between smallmouth bass and round gobies. He says
it’s clear round gobies like to eat zebra mussels. But it’s less clear whether bass prefer to eat gobies
over other prey fish. So, Kim devised a laboratory behavior study that let the smallmouth bass
choose between several types of prey, including gobies, emerald shiners, and crayfish.
“The interesting thing is that they actually target these emerald shiners more often than round
gobies, but emerald shiners have superior escape abilities.”
Round gobies, Kim says, just don’t swim away as fast – and so get eaten the most. He adds that
when compared with the stomach contents of Lake Erie bass, this laboratory result is borne out –
more gobies were consumed than any other prey.
Roy Stein says that this puts the system in a kind of double jeopardy.
“The combination of PCBs plus being a slow prey causes perhaps more PCBs to move up through
the food web than otherwise might be the case.”
PCBs have been linked to cancer and birth defects in humans – and they’re not the only
contaminants in the lake.
Other research indicates this new food chain might be helping other pollutants in the sediment find
their way to humans. For example, another Ohio State study finds methylmercury is also getting
into the food web through invasive species. Methylmercury in fish can cause neurological problems
for expectant mothers and other health problems.
Doug Haffner is the Canada Research Chair for Great Lakes Environmental Health and a professor
of Biological Sciences at the University of Windsor. He agrees a zebra mussel – round goby –
smallmouth bass food chain has created a route that exposes humans to harmful chemicals in lake
“For a chemical to be of concern to us, it has to be biologically available, it has to be able to enter a
human being or a fish or whatever it might be. Some chemicals may be out there but not available;
we can measure them, but they’re not really a risk to the ecosystem per se. But processes can
change, which make them available.”
Martin Berg is a professor of Aquatic Ecology at Loyola University Chicago. He says the non-
native species have had a similar impact on PCB transfer from Lake Michigan sediment.
“You can think of it almost like a conduit, like a pipe. Now we have a direct link, as you move up
the food web, to organisms that are going to be directly consumed by humans.”
And the problem spreads as the non-native species expand their range. Researcher Gene Kim says
that the implications are far-reaching.
“Not only are we just talking about a Great Lakes phenomenon – zebra mussels have already
escaped into the Mississippi drainage, and right now round gobies – we’re spending a lot of money
to prevent round gobies from entering that same drainage.”
Scientists’ concerns about toxins in the Lakes are not limited to how invasive species are changing
the food web. Researchers say that other changes caused by people can help harmful chemicals
trapped in sediments to return to the ecosystem. Ultimately, they say, each of these issues is part
of a much larger concern: the overall health of the environment.
For the Great Lakes Radio Consortium, I’m Carolyn Gramling.