Didymo algae found in Poplar River near Lutsen

It took years of work by several local and state governmental agencies to get Poplar River off of the MPCA impaired waters list for having too much turbidity, and now this: a new form of algae has invaded the river. Under the right conditions this algae can form a dense mat of brown slime that can hurt the invertebrates that are food for the fish. File photo

Anglers, hikers and other users of the Poplar River should be aware that a species of freshwater algae commonly called didymo (or “rock snot”) has been identified in upstream waters of the Poplar River near Lutsen, beyond the Lake Superior splash zone where it has previously been documented.

Algae experts at the Science Museum of Minnesota and University of Wisconsin – Oshkosh have confirmed the identification. DNR fisheries and invasive species biologists will be searching other nearby streams and rivers this fall to verify the extent of its distribution.

“It’s important for people using the Poplar River to know it’s there and use good cleaning practices, so they don’t spread it,” said Chris Kavanaugh, Minnesota Department of Natural Resources regional fisheries manager. “This species of algae is not well-studied in Minnesota and additional surveys are needed to determine if this new find is an expansion to new waters or if didymo was already present and the conditions were right for mats to form.”

Didymo is present in low densities in Lake Superior, but was not previously documented in the upper watersheds. Once considered invasive throughout the United States, recent research has shown this species to be native in parts of North America.

The freshwater algae lives in low nutrient, low temperature environments that are common in North Shore streams and Lake Superior. 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.

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

Recreational users should follow these simple steps whenever moving between streams and rivers along the North Shore:

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

. If possible, 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.