The case of the snotty streams

A mysterious algae known as “rock snot” is smothering wild rivers — and may hold clues to their future.


On a cool, wet July morning at the Rocky Mountain Biological Lab in Gothic, Colorado, thick clouds erased the hulking mountains from view. The former mining town is a working summer camp for scientists. Brad Taylor, a Dartmouth professor, met his wife here. She studies bees, and he studies fish, the bugs they eat and the algae that set the bug buffet.

These days, he’s especially preoccupied with one kind of algae — an enigma that haunts wild rivers worldwide. Wearing a blue rain jacket and an orange baseball cap over salt-and-pepper hair, Taylor stood in a bubbling stream on lab property, his eyes searching the rocks for Didymosphenia geminata. Didymo (pronounced “Did-i-mo”) is a single-celled algae or “diatom.” Diatoms are among Earth’s most common life forms and the foundation of aquatic food webs. They’re only visible through microscopes, but if you’ve ever slipped on a slimy river rock, you’ve encountered them.

Brad Taylor stuns fish to observe the effects of whirling disease, which increases in response to Didymo blooms, in Copper Creek, Colorado.
Courtesy Crystal Edmunds/Coal Creek Watershed Coalition

Less than a decade ago, however, Didymo became much easier to spot here. The cells began sprouting stalks about the thickness of human hairs that coalesced into sprawling underwater manes, which felt like wet, dirty wool. They did the same in Colorado, Idaho, Wyoming, Montana, South Dakota, Connecticut, West Virgina, Canada, Chile, New Zealand, Poland. The algal mats smothered streambeds for miles. They were inconvenient, threatening and gross — described, in scientific journals, as “mucilaginous.” In the popular press, Didymo was dubbed “rock snot.”

The mats mucked up fishing in popular rivers, like Montana’s Kootenai. Anglers’ casts got lodged in gobs of snot. They seemed capable of clogging water intakes, and, in the Rockies, were also bad for fish. The worms that carry whirling disease, an exotic killer of native trout, thrive in Didymo blooms. The mats even change the food web.

To demonstrate, Taylor grabbed a flat rock from the stream, and scraped pimples of Didymo into a dish. A mess of fidgeting midges emerged, but few large mayflies. Blooms favor small insects, Taylor explained; they can take refuge in the mats, while the larger ones become entangled and more vulnerable to predators. Here, that’s stunted the growth of some trout. Taylor next scraped a Didymo-free rock, revealing a diverse mix of bigger bugs. “You don’t need a Ph.D. to be able to say something’s going on,” he said.

Something — but what? Puzzlingly, the algae colonize rivers that are virtually devoid of phosphorus, the nutrient from farms and septic systems that often stimulates nuisance algae. So what’s behind Didymo’s advance? And can anything be done to make it stop?


Tufts of Didymo adhere to a rock.
Brad Taylor

If you fly-fish, you might have heard that Didymo is an invasive species. That may not, in fact, be true.

Canadian freshwater researcher Max Bothwell is one of the people most responsible for popularizing this idea. Now, he and Taylor are its most vocal critics. Bothwell, whom Taylor lauds as the “Yoda” of rock snot, spent much of the 1990s trying to finger its cause on Vancouver Island. Didymo cells were native there, but the blooms were new. Bothwell looked for evidence that rivers’ phosphorus levels had increased, but found none. Their hydrology hadn’t shifted significantly. Experiments also ruled out ultraviolet radiation from the waning ozone layer as the culprit.-

In 2004, Bothwell got a new clue when blooms were discovered in New Zealand, where Didymo cells had never been seen, despite thorough diatom surveys. Here, scientist Cathy Kilroy and others argued, Didymo was a recent immigrant, transplanted by humans. After a research trip to New Zealand in 2006, Bothwell started to wonder if Vancouver Island had suffered an introduction, too — perhaps of a new genetic strain more prone to stalk production. Combing historic records, he noticed that the blooms coincided with an uptick in fishing and the popularity of felt-soled waders, which prevent anglers from slipping in streams, but, if not properly dried and cleaned, can transport living cells. Plus, the blooms seemed to occur at popular fishing spots. The evidence for introduction of a genetic super-stalker was circumstantial, Bothwell wrote in his 2009 On the Boots of Fishermen paper, but it seemed convincing.-

The paper was a hit: Didymo was -declared invasive the world over. In some places, felt-soled waders were banned. Public agencies and fishing groups launched educational campaigns, -encouraging anglers to “check, clean and dry” their gear between rivers and switch to rubber-soled boots. In the past, says Dave Kumlien, who works on invasive species for Trout Unlimited, the stream-fishing community resisted the idea that they helped spread whirling disease, which, like a lot of aquatic invaders, was mostly invisible. “This is one that people could see, and it screwed up your fishing,” Kumlien says. “Didymo got stream anglers thinking about what they were doing. It began to shift the paradigm of -behavior.”

Bothwell thought he’d solved the mystery. “Finally, this monkey was off my back,” he says. “It seemed to make sense.”

A microscopic image shows the silica cell wall of the Didymo diatom.
Sarah Spaulding/USGS

Except it didn’t — not entirely, anyway. By then, Brad Taylor was separately studying Didymo in Colorado, where invasion seemed an unlikely explanation. The cells had long been present in streams, but Taylor and others never saw long, thick mats around the field lab until 2006 and 2007. In 2008, however, the mats didn’t appear. They smothered streambeds again the next year, but didn’t in 2011. And Didymo erupted only in certain streams, even though the cells lived in many others. If the blooms were caused by an invasion of a genetic mutant, shouldn’t they grow every year, in every stream?  

Taylor suspected something in the environment had shifted. The blooms followed unusually warm springs and rapid snowmelt. In non-bloom years, on the other hand, substantial snowpacks melted gradually, more as they had in previous decades, when Didymo cells remained benign. Taylor measured flows in unaffected and affected streams; the latter peaked about two weeks earlier, because they drained smaller watersheds, or were more exposed to the sun. 

Around the same time, Bothwell and Kilroy — who had been running experiments in New Zealand — made an unexpected discovery: Didymo bloomed when levels of the nutrient phosphorus dropped. Typically, the opposite is true. Most algal blooms — the toxic blue-green variety in Lake Erie, for instance — are caused by spikes in phosphorus.

The drop Didymo responded to was so small it wasn’t even measureable with traditional tools. That’s why changes in the nutrient’s levels hadn’t registered in other streams despite testing. The scientists think cells produce stalks to allow the cells to move up into the water column, where they have a better shot of accessing traces of phosphorus, which they need to divide and reproduce. 

Epeorus deceptivus and other flat mayflies decrease in abundance in response to Didymo blooms.
Bob Henricks

Taylor and Bothwell began brainstorming potential causes of phosphorus declines. Last spring, they published a paper detailing their leading theories — and arguing that Didymo, rather than being invasive, is probably native to most places it’s bloomed.

If they’re right, Didymo may ultimately be a lesson in how minor environmental changes can have outsized effects on ecosystems. Taylor now believes the Colorado blooms are linked to changes in the timing of spring, a sign of the warming climate. When spring comes early to the Rocky Mountain Lab — as it does with increasing regularity — rapid snowmelt could be flushing phosphorus from soil in one big pulse, rather than delivering it gradually, depriving streams of the nutrient after the snow is gone. Or, when plants “turn on” early, they might use more of the available phosphorus, leaving less for streams. Most likely, some combination of these factors is at work. Elsewhere, phosphorus declines could be a symptom of nitrogen pollution, from the atmosphere or fertilizer- applications.

U.S. Geological Survey diatom expert Sarah Spaulding agrees that the evidence that phosphorus regulates blooms is compelling, but thinks Taylor and Bothwell have been too quick to dismiss the human role. The idea that human-caused changes in phosphorus and nitrogen are driving the blooms “might be true,” she says, “but it needs to be shown with data.” 

Cathy Kilroy, in New Zealand, tends to agree with Taylor and Bothwell that in much of the world blooms are likely to be the result of environmental changes. But many questions remain. There is still no historic evidence, she says, of the cells in New Zealand streams, and it’s even possible that a boom in environmentally triggered blooms “facilitated (the) spread of cells, possibly to places where Didymo wasn’t present previously.”

Taylor acknowledges that more research is needed. But if Didymo was introduced to some rivers, he believes, it will continue to spread, whether on the boots of fishermen or the hooves of livestock and wildlife. “If environmental factors are causing the blooms, that’s the main issue,” he said. “We need to nail down if Didymo is a good sentinel for impending changes” — the canary-in-a-cage for rivers, warning of important shifts most of us can’t yet perceive.