Algae with potential to harm trout habitat found in North Shore river

Didymo (Minnesota DNR)
Didymo (photo courtesy Minnesota DNR)

A tiny aquatic plant that can cause big problems for fish has been confirmed for the first time in a Lake Superior tributary. Didymosphenia geminata, commonly called didymo, or “rock snot,” is believed to be always present in Lake Superior and other waterbodies at low levels, but in certain conditions, it can cause thick mats that choke out other life.

The slimy globs and strands were witnessed on the Poplar River earlier this year.

This fall, the Department of Natural Resources and algae experts from the St. Croix Watershed Research Station and and University of Wisconsin-Oshkosh confirmed it was in the river, which enters Lake Superior at Lutsen.

There are still many questions about didymo, but experts say river users should know it’s there and take precautions to prevent spreading it.

“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,” Chris Kavanaugh, Minnesota Department of Natural Resources regional fisheries manager, told the Duluth News Tribune. “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.”

Unlike many types of algae, didymo thrives in cold, low-nutrient waters. It’s only when there is very little phosphorus that it causes nuisance blooms. Scientists think it may be that when nutrients are low, the algae create long stalks that extend into the water to gather food for the organism.

Since the 1980s, there has been growing concern that either didymo is spreading to new waterbodies, or conditions are changing to encourage more of the nuisance growth. The organism has been especially harmful to prized trout rivers in New Zealand, since first appearing in 2004.

“Didymo’s fall from grace started about 20 years ago in British Colombia, about the time the algae’s extracellular stalks began covering swaths of streambeds in slime that feels like wet wool and looks like fiberglass insulation or… nasal discharge,” Minnesota SeaGrant explains. “Since then excessive and persistent didymo blooms have been noticed throughout North America.”

How to deter didymo

Anglers and other river users can spread didymo on their gear and clothing. Minnesota SeaGrant asks anybody entering the Poplar River to take the following steps to prevent moving it:

  • Learn to recognize didymo.
  • Avoid using felt-soled boots, the most likely way it spreads.
  • Clean all aquatic plants, animals and mud from watercraft, trailers, docks, lifts, anchors and other recreational equipment before leaving access.
  • Drain water-related equipment (boat, ballast tanks, portable bait containers, motor) and drain bilge, livewell and baitwell by removing drain plugs before leaving water access. Keep drain plugs out while transporting watercraft.
  • Spray watercraft and equipment with high-pressure water, or
  • Rinse waders, hip boots and gear with hot water, or soak in 5% salt (2 cup/3 gal) for 30 minutes followed by a tap water rinse; and/or
  • Dry gear for five days.

Anyone who believes they have seen didymo should note the exact location; place a scrape of suspicious material in a sealed plastic bag; and contact the Minnesota DNR, call 1-888-MINNDNR or (651) 259-5100; or the Minnesota Sea Grant Program in Duluth, (218) 726-8712.

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