The early stages of a Didymo bloom. Also known as “rock snot,” Didymo disappeared as quickly as it had appeared in Maryland. Photograph, Matthew Shank, Susquehanna River Basin Commission

In early spring 2008, fishermen in the Gunpowder River found something they had not seen before in the stream.

A fibrous mat of grey-green algae was covering the rocks and boulders. Nicknamed “rock snot” for the way it stuck to rock boulders along the bottom of the stream channel, the algae blanketed the stream’s rocks. Fisheries managers worried it would spread quickly throughout one of the area’s remaining pristine fishing areas. By April, the Maryland Department of Natural Resources (DNR) had identified it: rock snot’s scientific name was Didymosphenia geminata, or Didymo for short.

Unlike many other algae that have invaded the Chesapeake Bay and its tributaries, Didymo thrives in cold, clear waters with especially low phosphorus concentrations. Given these preferences, the prevailing hypothesis has been that Didymo originated in Scotland and other areas in the far northern latitudes of Europe and Asia. Until 2008, scientists had not observed Didymo on the East Coast of the United States. So, how did it get into the Gunpowder?

In 2014, scientists at Dartmouth College compiled fossil evidence from the Delaware River suggesting Didymo had been in Pennsylvania for hundreds of years and was native to Maryland too. The researchers used time-series data from the EPA’s Environmental Monitoring and Assessment Program to show that when there was a lot of phosphorus in the water, there wasn’t much Didymo, but with only a little bit of phosphorus, there could be a lot of Didymo. However, new work published in late 2017 led by the University of Maryland Center for Environmental Science’s Appalachian Laboratory in Frostburg, and the University of Vermont uncovered some surprising new results: Didymo is most likely an invasive species, sharing genetic material with other Didymo algae found in such far-flung places as Italy and New Zealand.

Before Bob Hilderbrand, an aquatic ecologist at the Appalachian Laboratory, began his research, there was some evidence Didymo might be invasive to Maryland. The Didymo found in the Gunpowder River watershed in spring 2008 was the first sighting in the state. If it were truly native to the Mid-Atlantic region, Hilderbrand argued, there should have been reports of it before ­—whether among fishermen, or in newspapers, or other historical documents. No such evidence has been found.

As quickly as Didymo appeared in Gunpowder Falls in 2008, it disappeared after a few months. According to the DNR, the second and last confirmed Didymo sighting happened in spring 2009 in the Savage River, in Western Maryland.

Is Didymo truly gone? To determine that, Hilderbrand and colleagues went to 76 different sites in Maryland along with dozens more in Pennsylvania. They were looking for DNA in the water, known as eDNA or environmental DNA as a chemical tool to search for Didymo’s genetic material in the streams. This technique allows even trace amounts of Didymo to be detected. To collect samples, they used a specially designed net with very fine mesh to filter 10,000 liters of water at each site.

Once they had their sample of eDNA from each stream site, they used a technique common in many biology labs called polymerase chain reaction (PCR) to identify the Didymo DNA in the sample and produce many copies of it rapidly. With an abundance of DNA, Hilderbrand and his colleagues then used a “probe” to identify their unknown samples. Hilderbrand described the probe as a kind of lock made from known Didymo DNA. This particular lock would only open with a Didymo DNA key. So each sample was paired with the probe to try and unlock it. The only two keys that unlocked the probe were from DNA samples from the Gunpowder and the Savage Rivers — the two locations where Didymo was found in 2008 and 2009, respectively.

By comparing their DNA results to what was already catalogued in GenBank — the National Center for Biotechnology Information’s collection of publicly accessible genetic data — Hilderbrand and colleagues constructed a family tree for Didymo. They determined that the Didymo found in Maryland’s Gunpowder and Savage Rivers had also been found before in other places around the world.

“Didymo is a world traveler,” says Hilderbrand.

The surprising result is that the Didymo DNA found at the two rivers consists of multiple different strains. Some samples from Gunpowder Falls are more like the Didymo found in Colorado, Italy, and New Zealand. “There is some Didymo diversity,” says Hilderbrand. The Didymo found in Maryland is “not just one weird mutant.”

The timing for the discoveries of Didymo across these different places would not be possible from evolution — it’s just too fast, Hilderbrand said. Rather, Didymo may have made a new home in these different places after people unwittingly introduced it into the ecosystem. In other words, fishermen who love the Gunpowder also tend to love the Savage, and maybe on their trips to Western Maryland, they brought along an unwelcome guest.

After the 2008 outbreak, anglers worked with government and businesses to restrict Didymo’s spread. Theaux Le Gardeur, Gunpowder Riverkeeper and the owner of a local fishing shop, helped lead that effort. They convinced many anglers to carry another pair of shoes, switch to rubber-soled waders, and clean their gear. Felt-soled waders were acting like sponges and absorbing and spreading Didymo. In 2011, with fishermen’s support, the DNR instituted a state-wide felt-soled wader ban.

“In Maryland’s case, we had folks who were biologists who said let’s try to stop the spread of this. It was very progressive,” said Le Gardeur.

Ecologist Robert Hilderbrand is trying to figure out why it came, where it went, and if changing phosphorus conditions might allow for its return. Photograph, Rona Kobell

Maryland has not experienced a nuisance bloom like Gunpowder River’s in 2008 or Savage River’s in 2009. In 2012, there were unconfirmed sightings of Didymo at Big Hunting Creek and the North Branch of the Potomac River in Garrett County.

The last field season for Hilderbrand and his colleagues was in 2014. Out of the 76 Maryland research sites, only two, Gunpowder and Savage, had Didymo present. Even nearby water body sites — streams with cold, clear, low-phosphorus water, the kind of environment Didymo should thrive in — had none of it. But, Hilderbrand also noted that some Maryland streams, like the Gunpowder, are seeing increases in phosphorus from fertilizers and other sources, suggesting low phosphorus levels may not tell the full story. Scientists do not yet know how changing phosphorus concentrations affects Didymo.

Didymo could restrict food sources for fish, and also slow spawning for different fish species. Many “need clean substrate” — rocks without Didymo on them — to lay their eggs, said Hilderbrand. Initial research by Maryland DNR shows that the trout populations seem unaffected by the Didymo outbreak at the Gunpowder River. But Hilderbrand cautioned that a multi-year event could have much more negative consequences by causing sustained changes to light levels and food availability for these fish populations.

Removing Didymo would be difficult and costly. Strands can grow up to two feet long, which then become entangled. This gritty mat of algal material has proven difficult to remove from the rocks it clings to in streams. State biologists could find no successful eradication effort.

Scientists do not know why the bloom hasn’t returned to the Gunpowder. But now that they have linked Didymo genetically with other sightings of this species across the globe, new work may uncover more about its life cycle and possible harm to fish populations.

Alex Lopatka, a former Maryland Sea Grant intern, is an associate editor at Physics Today. He earned his Ph.D. in geology at the University of Maryland, College Park.