Mysterious alga threatens rivers

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

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