In the late 1980s, a freshwater alga began mysteriously blooming in the rivers of Vancouver, British Columbia, covering once-pristine riverbeds with a thick, woolly mat. Dubbed "rock snot" for its yellowish color and globular form, the sudden dominance by a previously benign alga presented something of a puzzle. Thought of as native to the area – and to many rivers and streams throughout the northern hemisphere – this particular alga was acting as if it had just been introduced.
"This is the mystery," says Max Bothwell, a research scientist with Environment Canada who studied the Vancouver blooms. "How could an endemic species invade?"
Scientists, who often refer to Vancouver's experience as the "epicenter" of an ongoing global epidemic, are still not quite sure. Known as didymosphenia geminata, or "didymo" for short, the alga (algae is the plural form) has since bloomed in the Ozarks, the Rockies, Iceland, and Eastern Europe. And its worldwide spread seems to be accelerating. In 2002, didymo appeared in South Dakota, causing a near collapse of the Rapid Creek brown trout fishery. In 2004, it jumped hemispheres, covering New Zealand's famously scenic rivers with mats the likes of which scientists had seen nowhere else. And just last year, the alga appeared in Quebec's Matapedia River, an important East Coast salmon fishery.
Scientists warn that the blooms could spread throughout cold rivers in both hemispheres if not kept in check. Utility companies eye didymo nervously as a costly fouler of intake grates. Anglers and ecologists worry about its potential effects on a river's food web. Its major effect so far seems to be aesthetic – the algal mats are often compared to shag carpeting or fiberglass insulation. But the larger question keeps scientists musing: What could have led to such rampant blooms? In today's well-traveled world, suspicion fell squarely on humans. But inadvertent dispersal by globe-trotting citizens doesn't totally account for its rapid spread. And how could it "invade" places where it already lives? These contradictions have led some to suggest an evolutionary event: an innocuous alga mutating into a superstrain.
Hard proof is lacking, "but it's the only guess that's consistent with all the observations we have," Mr. Bothwell says.
Didymo is paradoxical. It is the largest diatom, a family of single-celled algae defined by a silica encasement. It favors cold, clear water flowing over a relatively hard bottom. Unlike other algal species prone to blooming, didymo favors nutrient-poor waters. And while algal matter often forms the base of any food chain, didymo's stalk – the bulk of a "rock snot" bloom – is curiously immune to grazers. It is often found wrapped around river foliage a year after dying – dried, bleached, and undecomposed. (Those unacquainted with didymo mistake it for toilet paper, and assume didymo-choked streams are open sewers.)
"The nature of the stalk – that hasn't changed," says Sarah Spaulding, an ecologist with the US Geological Survey in Denver. "Something about the production of the stalk has."
This speaks to one of the most frustrating – or fascinating, depending on whom you ask – aspects of didymo: It continues to surprise. After concluding it lived only in cold, nutrient-poor streams, for example, scientists found it in the warmer rivers of the eastern United States and the phosphorus-rich rivers of Eastern Europe.
The hypothesis of a newly virulent strain already faces considerable hurdles. History describes didymo blooms in the Faroe Islands north of Scotland dating back nearly 200 years. Norway has experienced occasional blooms for at least 100 years, as has northwestern China.
The jury is still out on didymo's long-term effects on river ecology, but some think that they may be considerable.
"It's gobs of carbon, and that stuff has to go someplace," says Craig Cary, a professor of biotechnology at the University of Waikato in New Zealand. "You can't just load a system with that much carbon without its being degraded."
While it apparently had no effect on Vancouver's steelhead trout population, its arrival in Rapid Creek, S.D., coincided with the near-collapse of the brown trout fishery. A possible explanation: Scientists note that didymo mats led to a shift in the size and diversity of larvae species. In the case of Rapid Creek, maybe the less-nutritious food starved the trout.
But it wasn't until didymo invaded New Zealand in 2004 that there was a big break in the case. Unlike Vancouver, where the species was native, rock snot was foreign to New Zealand. Its arrival strongly hinted at a human culprit. Was that true of Vancouver?
After poring over fish and game data, Bothwell found a possible link: The number of fishermen visiting Vancouver Island had increased tenfold in the early 1980s, says Bothwell. Anglers were now hopping on planes to visit faraway, world-class fishing sites. Also in the mid-1980s, felt-soled waders – which provide a superior grip on rocky river bottoms – came into widespread use. The felt stays wet long after use and could easily harbor freshwater organisms.
Armed with this information, the Kiwis launched an aggressive didymo-awareness campaign, urging anglers and water enthusiasts to disinfect their gear. Their efforts seem to have paid off: Didymo has yet to arrive on New Zealand's North Island.
Nature, meanwhile, has hinted at its own way to control the diatom. The Vancouver blooms subsided on their own, leading Bothwell to guess that a virus or bacterium brought the alga back in line.
After extensive testing, scientists in New Zealand have deemed an algaecide, copper chelate, as most effective at curbing didymo growths. Eradication isn't the hope, says Christina Vieglais, head of the country's Didymo Science Program, but pushing didymo into the background is. "The impacts of this organism are corresponding to the amount of biomass it produces," she says. "If we can reduce the biomass, we'll reduce the impact."
In the meantime, seeking to prevent infestation of its North Island, New Zealand has launched an aggressive didymo-awareness campaign. Summarized as "check, clean, dry," here are its recommendations for outdoor enthusiasts:
Check: Before leaving a waterway, check items for clumps of algae. Do not take debris found at the waterway.
Clean: Clean all items for at least one minute with one of the following:
• Hot water (60° C; 140° F).
• A 2 percent solution of bleach (200 ml. or 7 oz. of bleach, added to water to make 10 liters or 2.6 gallons).
• A 5 percent solution of salt, dishwashing liquid, or antiseptic hand cleaner, (500 ml. or 2 cups of product, with water added to make 10 liters).
Dry: If cleaning is not practical, make sure the item is dry to the touch, and then dry for an additional 48 hours.
For more information, visit: www.biosecurity.govt.nz/didymo