Mysterious alga threatens rivers
'Didymo' is perplexing scientists as invasive, ruglike blooms of the stuff snarl waterways in both hemispheres.
By Moises Velasquez-Manoff | Correspondent of The Christian Science Monitorfrom the March 1, 2007 edition
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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."
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