More 'rock snot' found in North Shore streams

North Shore anglers are urged to avoid moving invasive species between streams.

Didymo, an invasive algae that can harm trout populations, covers rocks in the Devil Track River on Minnesota's North Shore of Lake Superior. Contributed / Minnesota DNR
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Scientists have found an invasive, cold-water algae in more Lake Superior tributary streams along Minnesota's North Shore.

Researchers first found didymo — nicknamed "rock snot" for the slime it often develops on rocks in streams — along the North Shore in 1965, but then didn’t see it again in any large amounts until 2018, when it showed up in the Polar River at Lutsen.

Didymosphenia geminata has now been confirmed in the Caribou, Devil Track, Kadunce, Kimball and Onion rivers — places it has never been found before.

While the algae blooms don’t necessarily indicate poor water quality, they create significant problems when they cover natural stream habitats and turn in-stream nutrients into a difficult-to-digest food source for other organisms.

Under the right conditions, didymo can form dense mats of brown slime that smother streambeds and may affect stream invertebrates that are food for fish, birds and other animals. Didymo generally doesn't directly kill trout, but smothers trout food.


Researchers Mark Edlund and Bob Pillsbury sample for didymo in the French River. The invasive, cold-water algae has been newly found in several North Shore streams. Contributed / Science Museum of Minnesota

In other regions of the U.S., and as far away as New Zealand, didymo has been devastating to local trout populations, and anglers are being warned to avoid spreading the algae to other Northland waters. In South Dakota's Rapid Creek, for example, didymo was discovered in 2005 and has since been blamed for a major reduction in the stream's brown trout population.

A research team composed of Mark Edlund, Adam Heathcote and David Burge from the Science Museum of Minnesota, Heidi Rantala from the Minnesota Department of Natural Resources and Robert Pillsbury from the University of Wisconsin Oshkosh surveyed 24 streams from the Lester River in Duluth to the Grand Portage River as well as corresponding Lake Superior shore sites. Their plan is to return over the next year and examine each stream to see if the blooms continue, expand or shrink.


  • Lake Superior’s low phosphorus spurs 'rock snot' For many lakes and rivers around the world, too much phosphorus is a crisis, super-fertilizing aquatic ecosystems and spurring unusually high algae blooms, weed growth and other problems.Just look at Lake Erie, where high-phosphorus runoff into t...
  • In Lake Superior, a problem algae lies dormant On the gently curving beach at Knife River, a cluster of wave-lapped boulders is a good place to find Didymosphenia geminata, a sometimes-slippery single-celled algae.

They also will check DNA to see if the didymo is native to Lake Superior or is a new introduction of an invasive strain from elsewhere. The stuff can be transported, like many other aquatic invasive species, by unsuspecting anglers, including trout anglers who unknowingly pick it up on their waders or other gear and move it from stream to stream.
Until then, the scientists are asking anglers not to move from an infested stream to an uninfested stream — called stream-hopping — until after their equipment has thoroughly dried. Felt-soled boots are especially likely to transport didymo.

“Until the origin of these new didymo populations is known, we encourage those who enter streams to avoid river-hopping in a day,” Burge said in announcing the new didymo outbreaks. “As is always our practice during field sampling, if you are going to enter streams, please adopt aquatic invasive species decontamination practices. And as tempting as it is to reduce slipping on slimy rocks, please refrain from the use of felt-soled boots. Wet soles can harbor many AIS, including didymo, for long periods of time.”


Didymo bloom "tufts" on a rock in the Caribou River along Minnesota's North Shore of Lake Superior. In some streams, didymo can form dense mats and choke out ecosystems, in some cases decimating trout populations. Contributed / Science Museum of Minnesota

Didymo was previously reported as a background and uncommon algae in Lake Superior along Minnesota’s North Shore. In 2018, what was described as “cotton candy” growths of the invasive diatom were found in the Poplar River. While diatoms form the base of aquatic food webs and contribute one-fifth of our atmospheric oxygen, some species accidentally relocated to new environments cause nuisance growth, choking out streams.

READ MORE: Trout-snuffing 'rock snot' confirmed in North Shore stream A nasty cold-water algae nicknamed "rock snot" has been confirmed in the Poplar River near Lutsen along the North Shore of Lake Superior, the first such finding in a Minnesota trout stream. The freshwater algae, officially called didymo, lives in...
“The DNR has been watching out for didymo for years, and why it is forming nuisance mats is something of a mystery,” Rantala said in announcing the new findings. “We don’t know what the presence of didymo means for the ecology, including the fish communities, of North Shore streams. We are working with the algae experts at the St. Croix Watershed Research Station to understand the impacts. This is a great partnership, as the DNR doesn’t have that expertise in the agency.”

The species has the potential to be harmful, and many North Shore streams are low-phosphorus, cold-water systems that provide conditions suitable for didymo growth.

Anglers and others using streams should follow a few steps whenever moving between streams and rivers along the North Shore:

  • Remove any mud, plants or other material from recreational gear, and drain water.

  • Wash boots and other gear in hot water and dry for 48 hours or freeze for 48 hours before reusing.

  • Avoid using materials that absorb water or may be difficult to dry, such as felt-soled waders.

  • Anyone who sees the invasive species is asked to report it at

John Myers reports on the outdoors, natural resources and the environment for the Duluth News Tribune. You can reach him at
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