Didymosphenia geminata — didymo for short — also has another, much less scientific, nickname: rock snot. Researchers say it's native to North America, and may have been in Michigan longer than previously thought, but now it seems to be spreading in the state's waterways.
Didymo is a single-celled alga that mostly lives on the bottom of rivers and streams.
Jo Latimore, an aquatic ecologist at Michigan State University’s Department of Fisheries and Wildlife, said it forms strings waving in the water that are far from appealing. “It looks gross. It's like snot in ropes coming off of rocks.”
Scott Tiegs, a professor in the Department of Biological Sciences at Oakland University, said the rock snot nickname is a bit of a misnomer because the alga feels coarse, like wet wool, when touched.
Tiegs said didymo was first spotted in the St. Mary’s River in 2015, and researchers have since seen it in abundance on the Manistee River in the Lower Peninsula of Michigan.
Didymo can cause a lot of environmental problems, Tiegs said. “It smothers out the habitat, it makes it difficult for native invertebrates to thrive there, and it has consequences for recreational fisheries and the overall health of riverine ecosystems,” he said.
Latimore said scientists are treating didymo as an invasive species, but there’s evidence that it may be native, especially to Northern Michigan. “People are even looking back at historical samples and say, you know, maybe this stuff's been around a while. But regardless, it's behaving differently than it did before.”
Julianne Heinlein is an aquatic ecologist with the Great Lakes Environmental Center in Traverse City. She said the stalks that didymo creates to attach to rocks and wood in a moving stream or parts of lake beds can form mats that cover the floors of waterways.
Heinlein said that so far, the effects in Michigan have been fairly minimal compared to places like New Zealand, but there is a worry — and a possibility — that the algae can affect fish populations. She said the rock snot mats occupy places where fish populations would usually feed on insect larvae, potentially disrupting the food web.
Latimore said this may result in seeing fewer insects and fish in those areas. “They may move to other parts of the river system,” she said.
Heinlein said didymo stalks form mats when there are low phosphorus or other nutrient levels in the water. The amount of nitrogen the water will also affect whether algae blooms or not, Heinlein added.
Heinlein said there’s evidence to suggest that didymo is becoming a nuisance species due to human-caused environmental changes, including the burning of fossil fuels, which creates more nitrogen in the atmosphere.
Latimore said one hypothesis for why didymo blooms is that there isn’t enough food in the area, so it gets stressed and blooms to adapt or deal with low-nutrient conditions.
Tiegs said cold, clear waters are the conditions that tend to trigger the blooms, but researchers are still studying didymo’s behavior.
The speices can spread easily by attaching to recreational fishers’ equipment, Tiegs said.
“Fly fishermen might go wade in a river, a few cells of didymo get attached to their boots. The next day, the next week, that same angler using the same waders fishes a different river, and then those algal cells can become dislodged and spread into a new habitat," said Tiegs.
Officials recommend washing off fishing gear or other recreational gear to kill off any living cells that may have been picked up.
Tiegs said the ability to detect didymo has improved with technology called Environmental DNA, or eDNA, which analyzes genetic material shed in the natural environment.
Latimore predicts the species will be seen in more places around Michigan moving forward. “It’s an opportunity for us to teach people about invasive species and the impacts they can have on the ecosystems,” she said.
Didymo sightings can be reported to the Department of Natural Resources' Eyes in the Field program.
