Spread of river algae “Rock Snot” annoys biologists and anglers in Michigan
The boulders beneath the Sault Ste. Marie International Bridge were once clean and smooth, clearly visible through the clear waters of Lake Superior that flowed over them.
They don’t look like that anymore.
This stretch of the St. Marys River has become “spooky” because the boulders are covered with dead algae that entered the river in 2015 and is increasingly appearing in Michigan’s precious waterways, said Ashley Moerke, dean of Lake Superior State University’s College of Great Lakes Ecology and Education.
“The Didymus now covers pretty much everything, like a shaggy carpet,” said Moerke. “If you were to pick up a stone, there would just be this thick mat, and maybe a few fibers would come loose from it.”
Didymo algae, now widespread in the St. Marys River, has become a mysterious threat to Michigan’s rivers over the past five years, appearing in some of Michigan’s coldest and cleanest rivers and clogging them with silt before disappearing again.
Scientists are at a loss. They don’t know where the algae comes from – whether it’s a new arrival or a native species that’s now causing a stir – or how to control its rampant growth.
“We really don’t know what’s going on here,” said William Keiper, an aquatic biologist with the Michigan Department of Environment, Great Lakes and Energy’s Division of Water Resources. “We have a lot of questions. I wish I had more answers. It’s pretty frustrating for a lot of us trying to figure this out.”
“Rock snot” is spreading
Didymo is a tiny, single-celled diatom that can go unnoticed in rivers unless a scientist looks for it with a microscope. But when the algae explodes, it’s hard to miss. When that happens, each cell shoots a long stalk toward the water’s surface, clogging rivers with sticky threads.
The algae is nicknamed “rock snot” because when it is in bloom it looks like what you would find in the crook of a sick child’s arm after a productive sneeze. Others said it looked like toilet paper. Despite its appearance, didymo does not feel like mucus. It is dense. When you squeeze it, it feels like squeezing a wet wool scarf.
Nor does it behave like other problematic algae species, such as blue-green algae, which produce toxins and harmful algal blooms. Unlike the algae that thrive in warm, nutrient-rich waters like western Lake Erie, Didymo appears to prefer cold, clean water with steady currents.
Didymo’s occasional explosive growth spurts aren’t technically algal blooms, although that’s how they’re described in layman’s terms. A true algal bloom occurs when algae multiply; Didymo forms stalks in search of nutrients, not offspring. Scientists believe Didymo forms these long, sticky stalks when there are few nutrients in a river—they’re just not sure what levels of phosphorus, nitrogen, or other compounds trigger the growth.
This is not a phenomenon unique to Michigan. Over the past 20 years, unexpected didymo blooms have flooded rivers in Montana, as well as New Zealand, Canada’s Vancouver Island and Quebec province, remote northern tributaries of Lake Superior, and rivers all over the world. And no one understands why.
The Michigan EGLE aquatic invasive species team’s concerns about didymo intensified in 2021 when the algae grew thickly in the Upper Manistee River in Kalkaska County. The department warned that the algae could become a major problem for trout streams.
The sighting in Upper Manistee was the first didymo discovery in the Lower Peninsula. The algae had been blooming in the St. Marys River since 2015 but did not appear to be spreading beyond the river, which connects Lakes Superior and Huron.
At least until it appeared in the Upper Manistee. Then in 2022, Didymo flourished in the Boardman River in Grand Traverse County.
Researchers discovered didymocells in the Rapid River in Kalkaska County in 2023 and earlier this year in the Au Sable River in Oscoda County, although the cells did not bloom in those rivers.
“Our historical knowledge of Didymo is really unknown,” Keiper said. “That’s unfortunate. We don’t know if it’s always been in Michigan’s rivers and we’re only seeing changes in recent years due to climate or changing river conditions that are causing it to now produce more cells, or if it was brought in from somewhere else and is now spreading in Michigan. That’s what we’re really struggling with.”
Has the “Rock Snot” always been here?
These rivers have something in common, said Moerke of Lake Superior State. They are popular with anglers.
That’s the main reason why Moerke is among those who believe Didymo is spreading in new currents. She says she is one of the few. Most believe it has been there all along and is now flourishing because something in the environment has changed.
Didymo is proven to be native to the Northern Hemisphere and is considered a true invasive species in more southern areas such as New Zealand.
Still, debate continues over its historical range in the Northern Hemisphere. Like Moerke, some say the alga’s appearance is too closely linked to human movement for it to be anything but invasive. Others point to its presence in deep samples of decades-old sediments and the inclusion of Didymo in historical lists of local algae as evidence that it has always been among us.
“I am not convinced,” said Moerke.
She laid out her case for Didymo as an invasive species: It was discovered in the St. Marys Rapids in 2015, but not in the nearby tributaries. If something had changed in the rapids, wouldn’t it have changed in the tributaries too?
“Then it started showing up in very popular fishing spots that you get to by wading,” Moerke said, claiming that felt-soled waders are a major dispersal route for problematic species such as didymo and New Zealand mud snails. “It wasn’t in the sections of the river where there was no fishing or where there was a lot of recreational activity. So it just seems like a coincidence to me.”
She acknowledged that the answer may lie somewhere in the middle. Perhaps Didymo has spread to Michigan’s rivers before, but at a time when those rivers did not provide suitable conditions for it to thrive.
Carole-Anne Gillis, director of research at the Gespe’gewa’gi Institute of Natural Understanding in Quebec City, is in the other corner of the ring. She said Didymo has probably been home to us for decades.
Gillis has been studying Didymo in the province since it appeared in 2006, long before it erupted in St. Marys. It was everywhere and no one understood why, she said.
Gillis and her colleagues dug for clues in the Matapedia River in Quebec province, collecting core samples deep from the riverbed to see if they could find didymocells that may have been present there before visible growth.
They found some. Didymocells were rare, but were present in the sediment as early as the early 1970s. That was the furthest back in the sediment’s history they could trace, Gillis said.
Changes in the river’s water chemistry have caused the Didymo’s rapid growth, Gillis claims. She pointed to changes since the 1970s, including the introduction of wastewater treatment systems that reduce sewage and nutrient pollution, the reduction of saltwater fish that bring nutrients to the freshwater river, and the increase in nitrogen input into the water via the atmosphere, which have upended the previous chemical regime.
Simply put, the river’s water chemistry is now different. In turn, the didymocells that previously went undetected are growing long stalks “like a coping mechanism” that helps them adapt to changes in water chemistry, Gillis said.
“We still don’t rule out that cells can be multiplied and transported,” she said. “I think it’s a mix of both. You can transport living cells to a river and if the conditions there are good, so the acidity is OK, Didymo will re-establish itself and repopulate riverbeds.”
“I guess it’s a bit of both. Rivers change.”
Michigan wants to study rivers
While researchers in Michigan try to learn more about the origins of didymo algae and what causes it to grow, a well-known fishing group is launching an education campaign and baseline survey to find the algae and curb its potential spread in Michigan’s rivers and streams.
The first mission is to find out what is causing Didymo’s explosive growth, said Bryan Burroughs, executive director of Michigan Trout Unlimited.
Because some scientists suspect that water chemistry may have changed, triggering the disease, Michigan Trout Unlimited is coordinating a large-scale sampling program to track water conditions in rivers across Michigan over time. When didymo blooms in a river, the group can determine what water conditions accompany the bloom.
“We actually have an incredible opportunity to figure out what parameters in the water quality caused them to bloom,” Burroughs said. “If we understand that, we’ll get a better picture of what’s causing these things. Until we know that, we really can’t dream bigger.”
Michigan Trout Unlimited will also provide its members with sampling kits that will allow them to collect algae samples from the rivers they fish. Michigan Trout Unlimited will look for didymo in those samples. This is how Michigan Trout Unlimited aquatic biologist Jordyn Stoll identified didymo in the Au Sable River when it wasn’t blooming, Burroughs said, and this is how another local researcher identified it in the Rapid River when it wasn’t blooming.
“People have smart guesses and a lot of hypotheses, but we’re still in the early stages of collecting data and trying to figure it out ourselves,” Burroughs said.
The organization will also launch an awareness campaign to educate anglers about Didymo and encourage them to clean their gear between fishing spots so they do not inadvertently spread the algae to new locations.
Keiper said EGLE, Michigan Trout Unlimited and the Michigan Department of Natural Resources received $154,000 for the project from the Great Lakes Restoration Initiative, a federal program that funds projects to protect, restore and research the Great Lakes.
Ukraine war weakens Russia’s border with China
“Mughal Glass” | Tara Desjardins’ new book traces the history of glassmaking in India
The list of famous birthdays for today, July 7, 2024, includes celebrities Jim Gaffigan and Ringo Starr
About The Author
