Fishers, kayakers, tubers, and other recreational users will soon see signs posted along the Boardman River cautioning them to clean their equipment after didymo – a dreaded invasive algae also known as “rock snot” – was identified this week in a popular stretch of river at Shumsky’s Canoe Launch and East River Road. Unconfirmed homeowner reports indicate didymo may be present in other sections of the Boardman, posing potential habitat and aesthetic risks to the waterway.

Sarah LeSage, aquatic invasive species program coordinator with the Michigan Department of Environment, Great Lakes, and Energy (EGLE), sampled suspected didymo clumps in the Boardman River Monday after a user posted photos of the algae on a Michigan Sportsman forum. The samples were verified to be didymo Tuesday by the Great Lakes Environmental Center. The Boardman is now the second river in the Lower Peninsula to have a confirmed presence of rock snot after it was found in the Upper Manistee late last year. Didymo also been found in the St. Marys River in the Upper Peninsula, where it’s been studied extensively since 2015.

Unlike other harmful algal blooms that occur in parts of the Great Lakes and inland lakes due to warm temperatures and excess nutrients, didymo blooms “form in cold, low-nutrient streams generally considered pristine – the same streams prized for their sport fisheries,” according to the Michigan DNR. “Didymo mats can cover streambeds and reduce habitat for macroinvertebrates including mayfly and caddisfly nymphs, which are important food for fish.” LeSage tells The Ticker that didymo is difficult to eradicate because a targeted tool hasn’t yet been developed to remove the algae without impacting the surrounding environment.

“You could dredge the river or scrape it out, but there would be unacceptable detrimental impacts to the habitat and other native organisms that would exceed the harm from didymo,” she says. “If you think of an herbicide used for aquatic plants, that has some specificity to it. It targets one particular species for control. We don’t have any tools to target didymo. Any tools that we would use could be destructive to other native plants and animals and habitat.”

Because rock snot is relatively new to Michigan, researchers are still trying to understand its causes and impacts. When parts of the Boardman River – including Shumsky’s launch – were surveyed in May, didymo was not visibly present. EGLE fisheries staff conducting fish surveys in the Boardman in July also didn’t see any blooms. Anecdotal reports from homeowners along the river indicate didymo may have only emerged within the last few weeks, LeSage says. Researchers aren’t yet clear on whether didymo appears because it’s been transported into the river – such as on boats or waders or fishing equipment – or if it’s already present in low numbers and then suddenly blooms due to environmental changes, developing long stalks that make previously undetected alga cells visible on hard surfaces in the streambed.

“We don’t know what’s going on in the Boardman,” says LeSage. “Was it there and something happened to make it bloom?” Factors like changes in water chemistry or quality could contribute to didymo’s emergence, but researchers are still working to confirm if that’s the case. The exact extent and location of didymo in the Boardman is also still being determined. “We haven’t had biologists go out yet and delineate the reach,” says LeSage. “We have reports from land owners that have seen it from the Brown Bridge Quiet Area to Beitner, but those are still unconfirmed.”

Additional surveys could soon follow on the Boardman, but in the meantime, state officials are working on putting up signage along the river – including at DNR access points and some private waterfront parcels where owners have volunteered to display them – asking river users to “clean, drain, and dry.” Those preventive steps – used to deter the spread of not only didymo but other aquatic invasives – call for cleaning/removing mud and debris from all surfaces, draining water from all bilges, wells, and tanks, and drying equipment for at least five days or disinfecting it with hot water or a diluted bleach solution. That applies not only to boats and fishing gear but recreational equipment like tubes and kayaks, LeSage says. Even without knowing exactly how didymo emerges, LeSage says the “conservative” approach is to assume the algae can be spread by humans and take steps to avoid contaminating other waterways.

Exactly what the appearance of didymo in the Boardman River means for the future of the watershed is still unclear. Enough buildup of rock snot can impact insect levels, which then “has the potential to scale up to fish populations,” LeSage says. Blooms can also make riverbeds slippery, which can be dangerous for waders and swimmers. “There can also be an aesthetic impact, if people are used to seeing the river look a certain way,” LeSage says. However, she notes that pictures online of dramatic didymo growth – such as in New Zealand, where blooms can be inches thick and cover extensive areas – don’t reflect the current reality in Michigan.

“We aren’t observing growths like that in the Boardman, Manistee, or St. Marys,” she says. “It tends to be a film that’s maybe a few millimeters thick. We don’t know what changes might cascade after this (in the Boardman), if any. It remains to be seen.”

In the meantime, in addition to following “clean, drain, and dry” steps, users of the Boardman River (and other Michigan waterways) are asked to report any observances of didymo to the state. Despite its nickname, rock snot is not slimy but has a coarse texture resembling wet wool. It can appear as small, cotton ball-sized patches or thick blankets with rope-like strings that flow in currents. Didymo findings can be reported by by using the Midwest Invasive Species Information Network, available online or as a downloadable smartphone app. The MISIN smartphone app will take a GPS location point if a report is made at the site; it also allows users to upload photos with a report. More information on identifying didymo and preventing its spread is available online here.