On the gently curving beach at Knife River, just north of Duluth, a cluster of wave-lapped boulders is a good place to find Didymosphenia geminata, a sometimes-slippery single-celled alga.
Jo Thompson, a researcher at the Environmental Protection Agency lab in Duluth, is collecting samples of the algae that is causing big problems in some parts of the world -- notably New Zealand and New England.
"It's not highly technical," Thompson said. "I've brought a pocket knife and a bottle and I'm just going to scrape a few rocks so we can take a couple of samples back to the lab."
She scrapes something off a rock just under the clear water. It looks like khaki-colored felt.
"You can see it's really not slimy; it's more like a wet piece of wool," she said. "If you pull it apart, you can see these little fibers."
The fibers are produced by Didymo, the shorthand name for Didymosphenia geminata. Lake Superior is home to the microscopic organism that lives peacefully on the rocky shore. While Didymo isn't causing problems in Lake Superior yet, just why it's not a problem is a mystery that a handful of scientists are racing to solve.
Thompson said finding the algae here is a sign of good water quality, because Didymo usually lives in cold, clear water.
But if someone fishing here were to pack up their boots and travel to New Zealand to fish, or to some streams in Colorado, or Vermont, they might be carrying along a very big problem. In all of those places, Didymo is threatening once-pristine streams.
"It was thought that there might have been somebody dumping sewage in there, because it looked like wet toilet paper going down the stream," said Jerry Wilhite, a biologist with the South Dakota Department of Game, Fish and Parks. He's describing what it was like eight years ago, when Didymo took over in Rapid Creek, a popular trout stream near Rapid City.
"It's kind of slimy looking, grows in clumps," he said. "A lot of times you hear it referred to as rock snot, because that's kind of what it looks like on the rocks."
What looks like wet wool on the shore of Lake Superior in another setting turns into something gross enough to be called rock snot. And it's not just the appearance that causes a problem. Didymo can smother the entire stream.
"If it's completely covering the bottom, the gravel, it could essentially choke out lots of the aquatic insects, because there's no water flowing through those little spaces between the rocks, and it could potentially choke out any spawning habitat and stuff like that," Wilhite said.
So far the trout seem to be doing okay.
But South Dakota has launched a public awareness program to get people to check, clean, and dry their equipment before moving from one water body to another. Didymo can survive more than two months on felt-soled waders. Similar efforts are underway wherever Didymo has caused problems -- including New England and New Zealand.
Meanwhile, a small group of biologists has stepped up to try to figure out why Didymo is not a problem in Lake Superior, but is a problem in places like Rapid Creek.
Sarah Spaulding, a researcher with the U.S. Geological Survey in Denver, is the unofficial leader of these scientific sleuths.
Spaulding said the experience in Rapid Creek is typical -- when Didymo gets out of control, it can actually change the stream it's living in.
"Starting from the primary producers, so starting from the algae, going all the way up the food chain and including the fish," she said.
And this is just one of possibly thousands of small organisms people are spreading to fragile places as we jet around the world to play. Spaulding said the results are predictable.
"As this happens, we lose the biodiversity of these different places, and they start looking just like one another, like the same shops in the strip mall," she said.
For anglers and hikers along Lake Superior, it means the boots you wear here could spread Didymo to places where it might cause a problem.
So far Didymo hasn't been found in other Minnesota waters, but the DNR recommends you wash boots and other gear in hot water and let it dry for 48 hours before moving to another lake.
The silver lining in this story is that manufacturers are now making boots that will dry quicker and be less likely to carry hitchhikers to vulnerable places.