Fly fishing

Rock snot algae threaten Michigan’s precious freshwater trout streams

A mysterious algae with the potential to devastate Michigan’s waterways was first spotted on the Lower Peninsula last month, a worrying development for biologists trying to learn more about the causes of its voracious blooms .

The Michigan Department of Natural Resources alerted the public to the algae sighting on Monday and warned anglers to wash their gear after getting out of the water to prevent further spread.

“We take this very seriously,” said Bill Keiper, aquatic biologist with the Michigan Department of the Environment, Great Lakes and Energy.“Our feeling is that it could take over any cold water stream we have in the state.”

The seaweed – didymo, or “rock snot” – was in bloom on the Upper Manistee River when Sam Day saw it in part of Kalkaska County from the Upper Manistee River on November 14.

Day, a water quality biologist for the Odawa Indian Bands of Little Traverse Bay, studied didymo as an undergraduate student at Lake Superior State University and as part of his masters program at Tennessee Tech University. He was fishing with friends when he saw a tuft of stringy algae in mid-November. They anchored so he could inspect further.

In this photo from July 27, 2007, Mary Russ, executive director of the White River Partnership, holds a boulder covered with aquatic algae Didymosphenia geminata, known as the didymo or rock snot, in the White River in Stockbridge, Vermont.

The seaweed had the texture of didymo – oddly dry, like wet wool. Day took a sample to take to work the next day. Using a microscope, he confirmed his suspicions: it was a didymo in bloom, the first sighting in Michigan’s Lower Peninsula.

This could be a problem for the Manistee and other popular Michigan waterways. Didymo, when in bloom, can cover stream beds and dramatically reduce the types of insects that can survive, Day said. This would mean that fish, including trout and others popular for fishing, would have less food available.

Growth of dark brown didymo on gravel is visible in the Manistee River.  The areas where the thick shoots break off have a woolly appearance and a light tan, with the clean, exposed creek bottom below.  Didymo blooms could pose a threat to Michigan's cold rivers and streams, which are popular for fishing.

“If the fishery is affected by didymo, it could put a pretty decent dent in the portfolios of many local fly shops and fly fishing guides,” Day said.

Didymo is not considered an invasive species in Michigan, Keipler said. Biologists have reported finding algae cells in Lake Superior a century ago, although he said they did not indicate whether they were in bloom.

Algae is not toxic or dangerous to humans, but it can certainly be a nuisance. When it blooms, the didymo turns into long strands of rope that line rocks and logs with filth.

The first reported bloom of didymo in Michigan occurred on the St. Mary’s River near Sault Ste. Marie in 2015. Algae continue to bloom there every year, Keipler said.

Didymo flowers remain a mystery to biologists. They are not sure what causes algae to change from an invisible and inconspicuous algae to a thick, stringy mass.

A view of didymo cells and stems through a microscope.

“He could be around for a long time and for some reason decide to bloom,” Keipler said. “It’s a really complex organism. A lot of people are quite confused by this.”

Didymo is a unique algae, Day said. It blooms in cold water which does not contain a lot of nutrients like nitrogen or phosphorus. Other algae, including cyanobacteria that cause harmful algal blooms, thrive in warm, nutrient-laden waters from sources such as broken septic tanks and runoff from farms.

Michigan has many cold water streams that are popular with anglers and likely vulnerable to didymo blooms.

“That’s a lot of your very high quality trout fisheries like, say, Manistee or Au Sable (rivers),” Day said. “These are naturally cold, nutrient-poor streams and are excellent trout fisheries for that reason. Conversely, they happen to be a perfect habitat for didymo to form these flowers.”

It is possible that didymo was widespread in Michigan streams, but did not flower outside of the St. Mary’s or Upper Manistee rivers. Day said he hopes to start scanning northern Michigan streams for algae next year through his work with the Little Traverse Band.

There’s no right strategy for controlling didymo when it blooms, which is why state biologists urge people to always clean and dry their gear after boating, fishing, or wading in Michigan streams. .

“There is huge potential for disseminating didymo,” Keiper said. “It’s so microscopic and it can stick to anything it comes in contact with, like boots, laces, fishing gear, and waders.”

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