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How Didymo Became Rock Snot

Masses of didymo inspired its nickname: Rock Snot.

Masses of didymo inspired its nickname: Rock Snot.

Once upon a time a microscope-owning subset of the upper crust would travel hundreds of miles and pay a pretty penny in their esoteric pursuit of the glassy frustules of a Didymosphenia geminata. Although the coveted diatoms clung to rocks in cold, nutrient-poor waters throughout the northern hemisphere (including Lake Superior), they remained as rare as diamonds. Diatom collectors who owned a didymo slide owned a trophy.

Fast-forward about 100 years to now. The single-celled silica-shelled didymo is reviled as "rock snot", particularly in New Zealand, in certain rivers of North America, and in the media.

Didymo’s fall from grace started about 20 years ago in British Colombia, about the time the algae’s extracellular stalks began covering swaths of streambeds in slime that feels like wet wool and looks like fiberglass insulation or… nasal discharge. Since then excessive and persistent didymo blooms have been noticed throughout North America.

In 2004, didymo was reported for the first time in the Southern Hemisphere when it blanketed roughly 18 miles of river bottom in New Zealand’s Mararoa River. Experts speculate that it arrived there via felt-soled waders worn by North American fly fishermen.

Sarah Spaulding, an aquatic ecologist with the U.S. Geological Survey who studies didymo, speculates that a subtle change in the algae’s behavior has taken place and that a new variant could be allowing the species’ range expansion. Further research needs to be done to examine this hypothesis.

What to Know about Didymo

  • It is a diatom that is considered to be native to cold, oligotrophic waters in the northern hemisphere, including Lake Superior.
  • The morphology of Lake Superior populations is distinct from, at least, the western U.S. forms based on general observations.
  • The species appears to be expanding its ecological range into more mesotrophic waters throughout the U.S.
  • It has become an invasive species in the South Island of New Zealand.
  • It can produce thick mats on hard substrates and persist for several years in that form.
  • When kept damp, cells can survive for over 2 months.
  • Mean air temperature during warmest period and percent base flow are strongest predictors for the presence of didymo in the U.S.
  • The most likely risk for spreading didymo is on felt-bottomed waders.
  • The key to curbing the spread of didymo is educating fly fishers in particular about cleaning their equipment.
  • Nuisance blooms may taper off after 3-4 years.

"Until recently we thought of didymo as having narrow environmental tolerances," said Spaulding. "It appears that excessive growths are occurring in broader physical and chemical ranges. Additionally, in contrast to historical reports, current blooms generate masses of stalks that persist for months over huge areas."

Spaulding came to Duluth, Minnesota in April to discuss the status of didymo with area researchers and experts after didymo came back onto the Lake Superior radar screen in the autumn of 2008.

"We received reports of excessive algae from a few shoreline spots in Lake Superior," said Doug Jensen, Minnesota Sea Grant’s aquatic invasive species program coordinator. "Lake Superior is typically too cold and too nutrient-poor for algae to thrive en masse so the reports piqued our curiosity."

When Jo Thompson, a diatom expert from the Environmental Protection Agency, examined the algae samples she and Jensen collected from the shoreline of Lake Superior on a finger-freezing November day, she found didymo cells intermingled with other diatom species and other species of algae.

"We have historical records indicating that didymo is part of the upper Great Lakes diatom community, so finding it is not really news," said Thompson. "But given the current attention the species is getting, we want to keep track of it."

Notice how large a didymo cell is compared with the other diatoms that have attached to the didymo’s stalk.

Notice how large a didymo cell is compared with the other diatoms that have attached to the didymo’s stalk.

Thompson reports that didymo doesn’t appear to be colonizing Lake Superior’s tributaries. Also, the species’ tell-tale polysaccaride stalks, which can reach a frightening length of three feet in areas where the diatom has gone rogue, are not growing more than two inches along Lake Superior’s shores.

Didymo blooms are unlike other algal blooms, because they are associated with nutrient-poor waters and have occurred in stream habitats considered pristine or with limited ecological disturbance. To date, nuisance blooms are only known in streams and rivers. During a bloom, the diatoms excrete stunning amounts of extracellular stalk material.

Scientists theorize that certain diatom species create these extracellular stalks to glean nutrients from the current while remaining anchored. Since the stalks are apparently resistant to biodegradation by bacteria and fungi, they persist after the diatom is long gone. The dense matrix of stalks presumably changes the habitat for stream-dwellers, especially the species living on the bottom. The fibrous carpet catches fine sediments and may influence the algae and invertebrate species composition.

Didymo challenges aquatic ecologists' understanding of streams and rivers. It presents a paradox. The species can quickly produce excessive biomass in conditions where nutrients are scarce and temperatures are chilly.

Recently researchers discovered that the concentration of dissolved oxygen within the masses of extracellular stalks is exceptionally high. (The concentration of dissolved oxygen within algal mats formed by other species is typically low.) It could be that didymo mats house organisms that produce oxygen. Some suggest that a unique assemblage of organisms thrives on the high concentrations of dissolved nutrients produced at the bases of mats, and then transfers these nutrients to the didymo cells. Determining the source of nutrients and flux of oxygen within the algal mats is likely to reveal how didymo attains its impressive biomass.

Scientists, conservationists and natural resources managers are concerned about nuisance blooms of didymo and the change in behavior of this organism or appearance of a nuisance strain. Many are involved in working to curtail the spread of the diatom through educational campaigns. They recommend decontaminating equipment between uses in different freshwater systems to prevent the spread of non-native and nuisance species.

See the YouTube video produced by Southland Fish & Game Office of New Zealand, Introduction to Didymosphenia geminata (Didymo), at

By Sharon Moen
December 2009

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