'Substantial' number of zebra mussel larvae found along U.S. shore of Lake of the Woods

Zebra mussel larvae have been found in large numbers in another large Canadian body of water — Lake of the Woods, the world's 30th-largest freshwater lake.

The Minnesota Department of Natural Resources announced Friday it confirmed the presence of the invasive species at three locations along the U.S. shore of Lake of the Woods, an international body of water with basins in Ontario, Minnesota and Manitoba.

Only the larvae, known as veligers, were found.

"While no adult or juvenile zebra mussels have been reported, the number of larvae is substantial," the department said in statement.

Zebra mussels are an invasive species that has colonized the Great Lakes, as well as the Red River/Lake Winnipeg drainage system.

• Invasive zebra mussel larva found in lake that supplies Winnipeg drinking water

The bivalves coat surfaces and can clog water pipes. They also have choked out native species in some places, and their shells litter beaches.

The spread of zebra mussels into Lake of the Woods could have huge ramifications for homeowners, cottagers, fishing and tourism in the area. In addition to potential environmental effects, the invasive species also competes with native mussels for food, growing space and calcium.

"This is a wake-up call to Ontario, in particular, and to all those who love Lake of the Woods," said Todd Sellers, the director of the Lake of the Woods Water Sustainability Foundation.

"This is very unfortunate news," he said, though he added the Minnesota natural resources department's findings aren't entirely unexpected, since the basin is near other lakes infested with the mussels.

The discovery of zebra mussels will, though, "kick in a whole suite of enhanced regulations in Minnesota," he said.

All jurisdictions need to be more vigilant and ramp up their efforts to prevent the invasive species from spreading, Sellers said.

His group works closely with the states and provinces that share the Lake of the Woods bodies of water.

He said the different parties are trying to find a common response to the issue of zebra mussels through the International Multi-Agency Arrangement — which involves several organizations co-operating to research and manage the basin — and the International Joint Commission, a bi-national organization that works to prevent and resolve water disputes between the neighbouring countries.

The next step is to watch for evidence of larvae developing into adults and attaching to docks and rocks, he said.

Zebra mussels can change water quality by sucking nutrients out of the main body of water and redistributing them to areas near shorelines, which reduces potential for oxygen and walleye egg survival.

But scientists are not certain the species can survive in the lake.

Natural defence

Compared to Lake Winnipeg, which has been infested with zebra mussels since 2013, Lake of the Woods could be less hospitable to the species. 

All the lakes in the Canadian Shield typically have low calcium concentrations — and zebra mussels need calcium to build their shells.

"We don't know if the lake's water chemistry is conducive to zebra mussel survival," research scientist Gary Montz said in the Minnesota department of natural resources statement. "It is possible that calcium levels or other factors might prevent propagation."

Lake of the Woods flows through the Winnipeg River into Lake Winnipeg. The lake's Minnesota waters are shallower and less rocky than the Canadian portions of the lake. The Canadian Lake of the Woods area includes the city of Kenora and most of the lake's developed cottage areas.

A single zebra mussel veliger was found in 2018 in Shoal Lake, a bay of Lake of the Woods that serves as the source of Winnipeg's drinking water.

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Unlike sandy Lake Winnipeg, Shoal Lake and Lake of the Woods have a lot of exposed granite that could serve as zebra mussel habitat.

The City of Winnipeg has been preparing for the prospect of zebra mussels in Shoal Lake since the 1990s. Zebra mussels can grow on screens, fittings and pipes at the aqueduct intake, Winnipeg water services manager Tim Shanks said in 2018.

The city has taken steps to protect its water supply and has set aside $1.2 million to protect the Winnipeg aqueduct intake pipe and other aspects of its facility at Indian Bay on Shoal Lake, a city engineer told council in 2017.

The city could add chlorine to the aqueduct intake to prevent mussels from growing in the pipe and it also has plans to scrape out the mollusks if need be, he said.

An owner of a cabin on Lake of the Woods said the evidence of larvae is concerning for him.

Joël Marcoux said the sharp shells of zebra mussels would pose a hazard for families walking along the shorelines, and the mussels would also pose a danger to the lake's biodiversity.

"It's an unfortunate discovery," Marcoux said.

"What will happen to the walleye that I'm such a fan of fishing for?"