Lake Superior's shipwrecks aren't immune to invasive species in the Great Lakes
Zebra mussels, quagga mussels causing shipwrecks to collapse, say divers
Beneath the cold blue waters of Lake Superior lie hundreds of shipwrecks, which serve as underwater time capsules of travel, trade and turbulence dating back more than a century.
While the fate of these structures is largely subject to the hands of time, new foes have crept on board: zebra and quagga mussels.
These invasive species have stirred up concern among archaeologists, historians and divers, who are reporting increased cases of the mussels coating wrecks across the Great Lakes, causing accelerated deterioration.
Ken Merryman, a shipwreck hunter and diver from Duluth, Minn., has been documenting Great Lakes naval relics for 50 years and believes 1,400 shipwrecks known to be in the Great Lakes are collapsing beneath the weight of the mussels – but cites a lack of data as one reason why it's hard to keep track of their decay.
Merryman is using 3D scanning technology called photogrammetry to document the wrecks before they disappear, in hopes of preserving their stories while also making the case to save them before it's too late.
Although most of the damage has been seen within the depths of Lake Michigan, Lake Huron, Lake Erie and Lake Ontario, experts say Lake Superior is no longer immune to the meddlesome mollusks.
Michael Rennie, an associate professor at Lakehead University's biology department in Thunder Bay, has done extensive research on the impact of invasive species on the Great Lakes. He's a Canada research chair in aquatic ecology and fisheries and is also a research fellow with the IISD Experimental Lakes Area.
While zebra mussels have a flat surface along the hinge of their shells, quagga mussels are larger and rounder.
"Wherever zebra mussels have showed up on the Great Lakes, quagga mussels have come in shortly after," Rennie said. "What we find is that zebra mussels are almost entirely replaced by quaggas once they move in."
Zebra mussels were first detected in the Great Lakes in the mid-to-late 1980s, followed by the quaggas. Since their arrival, these invasive species have intercepted a lot of the nutrients that would naturally flow through the Lakes, resulting in a loss of offshore fish, large salmonid species and organisms that can exist in sediments, Rennie explained.
"In some ways, we think that these mussels are actually reshaping the food web of the Great Lakes," he said. "With these mussels moving in, we're actually seeing what seems to be a sort of a wholesale change in terms of how the lake functions."
The Superior exception
Northwestern Ontario's lakes tend to have softer water, meaning there's lower levels of calcium, the nutrient necessary for the production of zebra and quagga mussels' shells.
Zebra mussels invaded Lake Winnipeg about a decade ago and efforts to eradicate the species have been in vain, Rennie said.
Now, the mussels are beginning to move into Lake Superior's systems, from Nipigon Bay to the Apostle Islands.
"Today on Lake Superior, you can do a dive on a wreck and you can see all the pieces really clearly and you can make out the attributes of the wrecks really well," said Rennie. "What's going to end up happening is that's all just going to be obscured because it's under a crust of mussels."
Lake Superior's fresh water and frigid temperatures have always made it "a real attraction" for divers, offering pristine conditions for viewing the wrecks, said underwater archaeologist Richard Harvey.
"It allows us to really get [a] look through that window of history into what actually it was like back when this wreck took place," Harvey said. "It's almost like being a detective."
Harvey, who is chair of the Save Ontario Shipwrecks Superior Chapter, has concentrated much of his work along Lake Superior's northern shore. He'd always considered himself lucky that the zebra mussels couldn't survive the cool waters.
But not anymore.
"We actually noticed zebra mussels as deep as three deep in spots, which is a concern. They are here now, they've adapted, they are able to tolerate that colder water," Harvey said.
The wreck he's most familiar with is the Mary E McLachlan, which sank in November 1921 in Mountain Bay.
"Believe it or not, it was at anchor in the protection of the bay when a massive storm blew up," he said. "She went straight down, stern first."
About four years ago, Harvey and his team began to see the invasive mussels take hold of the Mary E McLachlan. But around the corner in Cavers Cove, which is usually about five degrees colder, "there's not a single zebra mussel that we found in that location yet."
The fear is that in the next couple decades, these mussels will speed up shipwrecks' decay, meaning once and for all, they will be lost at sea.
There isn't much divers can do about the mussels besides report their findings to researchers and conservation authorities, said Harvey. In the next few years, he hopes funding becomes available for more extensive archaeological surveys on the impact on shipwrecks, and on the aquatic life that now call them home.
"We want to have this information so that other divers can go there where we can tell the story, so that we can learn the richness of our heritage, so that we can understand how the lakes change," Harvey said.
As for boat users, Rennie reminds them to clean out their livewells and ballasts to prevent the spread of invasive species and, when possible, to air out their vessels for 48 hours before moving them from one body of water to another.
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