Scientists trying to solve mystery of ‘rock snot’ in Michigan streams

Didymo — also called “rock snot” — first bloomed in Michigan in 2015 and again in 2021 and cells have been found elsewhere
Scientists don’t know whether the algae is native to Michigan or invasive
Scientists are also trying to figure out why rock snot blooms happen in the first place

In the summer of 2021, fly-fishing guide Brian “Koz” Kozminski headed to his homewaters of the Upper Manistee River for a typical day of fishing alongside Sam Day, a fellow angler and an aquatic biologist for the Little Traverse Bay Band of Odawa Indians.

When the two arrived at the stream, however, they were met with a strange sight. What was usually a cold, babbling trout river was now a waterway filled a mysterious white algae.

“It literally looked like someone flushed a toilet,” Kozminski said.

Didymosphenia geminata — otherwise known as “didymo” or “rock snot,” because of its snotty appearance — is a microscopic, single-celled algae, or “diatom,” that clings to the rocks lining riverbeds and lakebeds.

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It is unknown when didymo first arrived in Michigan, or whether it’s invasive at all. The organism is something of an enigma scientists are working hard to understand because rock snot can quickly overtake a stream, occupying habitat used by the insects on which fish feed, meaning hungry fish and anglers who strike less often.

Scientists remain stumped over the question of where didymo came from or if it has been here all along. Further, if it has been here all along, why has it suddenly started blooming in some Michigan rivers only to disappear once more into its microscopic form?

“We’re worried about it because, even if it was here all along, it sure is acting differently than it was before,” said Jo Latimore, aquatic ecologist, outreach specialist and director for Michigan State University’s Extension Center for Lakes and Streams. “Even if it is a native species, it may be an indicator that something is out of whack in these waterways.”

The first Michigan bloom was found in the St. Mary’s River in 2015, and later the Upper Manistee River in 2021. Cells have been found in the Au Sable River and Jordan River, though full blooms have not occurred at either spot. Because didymo can reproduce asexually, a colony can begin from a single microscopic cell.

Billy Keiper, an aquatic biologist for the Michigan Department of Environment, Great Lakes, and Energy’s Water Resources Division, explained that, if didymo is native, it would have existed in its typical microscopic form in Michigan lakes and streams well before it was first found as a more obvious bloom in 2015.

Likewise, since the blooms died down in the past few years, didymo has lurked in its microscopic form, forgotten but not gone.

“When Koz and I went out in 2021, it was a huge concern, because that was the first time we had found it in the Lower Peninsula,” Day said. “With the Manistee being one of the top trout fishing destinations in Michigan, even across the US, it was definitely a real major concern.”

Some scientists wonder if it is in fact native to Michigan streams but just requires a new set of conditions to bloom.

It’s thought by Keiper that didymo thrives in rivers with low nutrients — particularly low levels of phosphorus and nitrogen — and in cold water with stable flow.

Michigan’s streams hit all those conditions, particularly in tailwaters where didymo has thrived in past blooms.

Michigan river and fishing guide Ed McCoy said that, as invasive species like zebra mussels and quagga mussels have increased in the Great Lakes, phosphorous levels in Michigan’s waterways have decreased. The mussels’ consumption of nutrients could be impacting the odd behavior of didymo.

“Sulfate is a really important nutrient parameter, as well,” Day said.

That is because, in didymo blooms, the visible filament or stalks of algae aren’t living cells but rather carbohydrate excrement called sulfated polysaccharides. Sulfur is a crucial building block of those stalks. Therefore, water rich with sulfate can further support didymo growth.

The element is introduced to rivers when rocks with sulfide minerals go through weathering and sulfate ions dissolve into the water. Scientists think didymo forms those longer stalks in low-nutrient waters as a last-ditch attempt at life, as opposed to its usual existence as microscopic cells.

However, those are still only theorized reasons for the rapid growth of didymo in blooms.

“What’s the magic, the soup that makes it happen?” said Kozminski.

“We have a lot more questions with didymo than we have answers,” Keiper said.

Keiper is a lead on a joint team of scientists from EGLE and the Michigan Department of Natural Resources researching didymo and its impacts. Michigan’s DNR also works closely with the Native American tribes across Michigan.

One of the primary questions all those scientists are working to answer is why — if didymo is indeed native — it is suddenly exhibiting different behavior than normal.

“I was fishing regularly on a lot of our rivers by the late 80s, 90s … I don’t think there was ever a chance of that stuff growing back then,” said McCoy, the fishing guide. “Fast-forward 30 years of zebra and quagga mussel invasion … what else can you attribute it to? If it’s been here, and it’s conditions-based, why the sudden change?”

As Lake Superior faces an increasing abundance of invasive species that gobble up nutrients, it raises the question of how that decrease in dissolved phosphorus impacts the flourishing of flora and fauna downstream.

When rock snot blooms, it takes up crucial real estate on rocks and logs in the water that aquatic insects would otherwise use for their hatches.

“It covered everything but sand,” McCoy said of the 2021 bloom. “Wood, vegetation … anything it could attach to. It was like a blanket.”

Because of that algae-induced habitat loss, anglers reported a decrease in the number of insects in the water when the blooms took over, and, similarly, a decrease in their catch rates.

The full extent of didymo’s effects on Michigan’s rivers is still a topic of research for the DNR and of curiosity for fly-fishers.

Beyond ecological impacts on insect hatches, didymo impacts water tourism — a $2 billion industry in Michigan, according to the US Fish and Wildlife Service — because of its unsightly appearance and interference with the fishing experience. When the blooms were peaking, fly-fishing guides had to find new waters for clients to have a successful day on the water as didymo severely limited potential stomping grounds.

A sign warns people about spreading invasive species — Signs warning recreationalists about didymo and New Zealand mudsnails are posted at every major boat launch site along the Au Sable River. (Courtesy of Grace Lahti/University of Michigan)

While fishers are the people most greatly affected by didymo, they are also believed to be the primary mode of transit for the microorganism. Didymo spreads by clinging onto recreationalists’ gear, particularly the felt-soled waders of anglers, according to the US Department of Agriculture’s National Invasive Species Information Center.

The Michigan DNR recommends cleaning gear with the common household cleaning product Formula 409 between uses and before moving from one waterway to the next to help prevent the spread of both didymo and New Zealand snails, an invasive species.

Anglers have been the most vocal reporters of didymo to Michigan’s DNR because of its effects on their activity.

“The places where you find didymo tend to be in really great trout streams,” said Day.

For fishing guides who rely heavily on the health of trout and insect populations in these rivers, any effect on catch rates can be detrimental to livelihoods. Scientists and fly-fishing guides alike agree that the primary way to prevent damage from didymo to Michigan’s aquatic ecosystems is to prevent the initial spread of the algae.

However, on the same day that a fly-fisher launches his sterilized boat into the Au Sable and trods along the waterbed in his deep-cleaned waders, a crew of casual recreationalists could decide to enjoy the river with inflatable tubes that have never seen a drop of Formula 409.

Rock snot will not discriminate against any gear it wants to latch onto.

“It’s so easy to transfer didymo,” Kozminski said. “We’ve just got to get the message across to everybody who uses the river.”

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