Environmental change may spur growth of ‘rock snot’

Controversial theory suggests alga is a native species, not an invader

BLIGHTED WATERWAYS  Explosions of rock snot, an algal bloom in rivers around the world, may be due to environmental changes, new research suggests.

B. Taylor

Environmental change could be triggering “rock snot” algal blooms that harm fish and leave pristine riverbeds looking like tattered mats of soggy toilet paper.

Since the mid-2000s the goopy blooms, which look mucuslike up close, have cropped up in rivers worldwide. Researchers assumed that the responsible alga, Didymosphenia geminata, was a foreigner or a mutant aggressively invading clean watersheds. But a new analysis suggests that the alga nicknamed didymo is instead native to much of the globe, and that changes in water conditions are to blame for a recent boom in blooms. The controversial suggestion could upend strategies for preventing cases of rock snot.

When they’re not blooming, the soda bottle–shaped algae are the width of a human hair and lead solitary lives under river rocks. Researchers might have to scrub six to eight basketball-sized rocks to find one cell, says freshwater ecologist Brad Taylor of Dartmouth College. Didymo blooms, on the other hand, look dramatic. In a matter of days, the algae grow whitish stalks that can extend two to three inches. Cells clump at the top and form snotlike blobs. When the blobs merge, they form mats that look like shredded paper.

“It’s amazing that a little creature like that can produce three inches of stuff on the stream bottoms that can cover miles of a riverbed,” Taylor says.

AGGRESSIVE ALGA When Didymosphenia geminata blooms, the normally soda-bottle-shaped tiny alga transforms to a stalked, branching form (shown) that clings to rocks. B. Taylor

The mats look slimy, but they actually feel like wet cotton. Their forest of watery fibers creates a refuge for worms that carry a fish parasite. The mats are also a deadly snare for large insects that fish eat. The combination can devastate salmon and other fish populations.

With waterways and wildlife at stake, researchers and policymakers are anxious to stem what they assumed is a biological blitz. But Taylor and aquatic ecologist Max Bothwell of Environment Canada in Nanaimo aren’t convinced that didymo is invading. Together they examined fossil records, genetics and ecology of the alga and its bloom patterns. “None of it added up that [didymo] was spreading around,” Taylor says. Their analysis appears May 7 in BioScience.

The invasion explanation arose in the wake of blooms that appeared in the 1990s around Vancouver, where they had never been seen before. Starting in the mid-2000s, scientists had spotted didymo blooms in Europe, North America, South America, Asia and New Zealand. In many countries, policy makers hastily developed strategies to control the spread of the alga, assuming it was an invasive species.

But Taylor and Bothwell say that fossil records of didymo place the alga on multiple continents 10,000 years ago. In historical records, they found didymo had been in most of the bloom locations for decades, if not centuries, Taylor argues. And not all rivers with didymo today experience blooms.

If invasions didn’t trigger recent years’ deadly blooms, Taylor says, something else must have changed. Genetic analyses haven’t found a mutation linked to the snot formation, he adds. But many researchers, including Bothwell, have discovered that blooms occur only when a river’s phosphorus levels drop extremely low. Taylor and others think that when didymo is starved for phosphorus, it morphs into its long, stalked form to reach higher into the water column.

The surge in blooms does seem to be linked to river conditions, not introductions of foreign species, says algal ecologist R. Jan Stevenson of Michigan State University in East Lansing, Mich.

SLIME ON THE ROCKS Attached to rocks, blooming Didymosphenia geminate form stalked clumps that look mucus-like, giving the algal blooms the unofficial name “rock snot.” Sarah Spaulding/Fort Collins Science Center/USGS

Stevenson and Taylor believe that changes in pollution and climate are behind altered river conditions, which in turn caused the synchronized blooms across the globe during the 2000s. One way climate change lowers phosphorus in waterways is by interfering with how nitrogen moves through the environment. Emissions from power plants contain nitrogen, which falls to Earth in rain. When plants get more nitrogen, their metabolisms speed up, requiring them to take up more of other nutrients including phosphorus. With less phosphorus in the soil, less washes into streams where didymo can get it, the researchers hypothesize.

“It’s the one that makes the most sense,” Stevenson says of the hypothesis.

Taylor also speculates that increasing amounts of melting snow, due to warming temperatures, may have lowered rivers’ phosphorus levels. Like increased nitrogen, snowmelt speeds up plants’ growth, leading them to take up more phosphorus. Speedy snow melts are known to reduce nutrient input to rivers and lakes, says Taylor. “It’s just a question of whether this is occurring at all of these places.”

At least one place doesn’t fit the phosphorus-didymo pattern: New Zealand. Didymo has run rampant on the country’s South Island since 2004. However, says freshwater ecologist Cathy Kilroy of the National Institute of Water and Atmospheric Research in Christchurch, “good records of dissolved phosphorus in many rivers show fluctuations over time, but no sign of a general decline.”

Since the New Zealand rivers hit by didymo had low phosphorus long before the 2004 blooms, most researchers believe that didymo is an invader there. Officials have instituted firm restrictions to keep the algae from contaminating pristine streams and rivers.

However, Kilroy thinks changes in environmental conditions in places other than New Zealand probably caused the unpleasant blooms.

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