Time for Nature

Unwelcome arrivals

May 14, 2007

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Zebra & quagga mussels at Fathom Five National Marine Park of Canada

Parks Canada biologist Scott Parker was on the lookout. Zebra mussels and their relative the quagga mussel had invaded the Great Lakes some years ago, and it was probably inevitable they would show up at Fathom Five National Marine Park of Canada of Canada. Then in July 1991 Scott and his colleagues found a zebra mussel in one of their study areas. With resignation Scott said, "we knew the ecosystem was about to experience a significant change and that we could do very little stop it."

Invasive species like zebra and quagga mussels commonly travel to new areas as stowaways. They hitch a ride with people, goods and vehicles moving from one country or region to another. The zebra mussel was probably transported from its native Asia to the Great Lakes in ballast water dumped by a ship in the late 1980s.

Moving in and taking over

Quagga mussels, like their relative the zebra mussel, have invaded Fathom Five and much of the Great Lakes
© Robyn Korn, 2006

New arrivals can be very disruptive to an ecosystem. Many, like quagga and zebra mussels, can be incredibly prolific. A female zebra mussel can produce a million eggs a year, and her young can colonize most any surface: boats, water-intake pipes, buoys, docks and plants. Zebra mussel communities can be fantastically dense - as much as 700,000 per square metre!

Also consider the fact that a single mussel can filter all the microscopic food from a litre of water each day. This has a huge effect, and people like Scott Parker have witnessed a major shift in the food web, from an efficient pelagic (open water) system where plankton feeds fish to a lake bottom system where plankton feeds mussels. These invasives are essentially sucking life and nutrients to the lakebed.

Some natives can't compete

A female zebra mussel can produce a million eggs a year, and her young can colonize most any surface
© Environment Canada

For some species, the zebra mussel's arrival has been disastrous. At Fathom Five and throughout much of the Great Lakes , the tiny arthropod, Diporeia, is at the base of the food chain. This species lives in the lake bottom and feeds on settling algae and other organic material. But zebra mussels seem to be out-competing Diporeia for this food. Since Diporeia can make up 70 percent of the living matter in a healthy lake bottom, their decline is a problem for fish species that depend on them for food. Smaller fish like lake herring, whitefish, yellow perch and sculpin feed on the Diporeia, and larger salmon and trout in turn feed upon them.

Scott Parker has seen Diporeia decline from 1,000 per m2 to zero in some areas. "We think it has everything to do with zebra mussels," he says. Some species have shifted their diet to eat mussels, however as Scott says " this is junk food, low in calories and nutrients".

More to come?

Researchers have witnessed a major shift in the Great Lakes food web due to invasive species.
© Wilkes, D. A., 1992

Currently there are about 154 non-native species in the Great Lakes, 30 of which have been confirmed in Fathom Five. According to Scott Parker, given the rate of introductions it is only matter of months before we add yet another to our list.

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