Rare Alga Morphs Into Widespread Nuisance

A once rare and innocuous freshwater alga seems to have recently mutated into an amazing menace. The previously sparse alga now produces copious quantities of brown slime on boulders and plants in rivers and streams. As water recedes, some slimy mats get left hanging to dry, looking like toilet paper draped on rocks and branches.

The alga's disgusting appearance has earned it the name rock snot. Still the species is more commonly referred to as didymo, an abbreviation of its scientific name Didymosphenia geminata.

For inquisitive scientists didymo is "an amazing biological phenomenon" about which little is yet known. However they, along with environmental managers, are also concerned that this alga has suddenly turned into a rapidly-spreading nuisance and an ecological and economic threat. Didymo mats detract from recreation, clog water intakes and may harm fish.

Didymo is native to North America, Europe and Asia. Until twenty years ago in Canada and United States, it was always scarce and seldom seen.

Massive blooms of didymo were noticed for the first time in 1985 within the Heber River on Vancouver Island, British Columbia. Since then it's quickly taken on this nasty habit in many parts of North America and Europe. Besides numerous BC rivers, blooms have appeared in Alberta and in US waterways from California and Montana to Virginia and Arkansas.

The alga also turned into an invasive exotic species when people inadvertently introduced it to New Zealand. Didymo first appeared there in 2004 and promptly migrated to a dozen South Island rivers. Notably, didymo has aggressively moved into warm and nutrient-rich waters, conditions it never grew in before.

Nearly everything about didymo is different. It proliferates in nutrient-poor rivers, where algae are normally not abundant. Despite looks, it doesn't actually feel slimy the way most algae do, but instead has the texture of wet wool. Its mats, sometimes growing 20 centimetres thick and extending for several kilometres, persist long after the organism dies.

Why didymo all of a sudden changed into such an irritating and invasive species, no-one has yet figured out. The most prevalent speculation is we're seeing the outcome of a biologically successful genetic mutation.

The mats themselves have huge ecological implications. They are mostly made up of small stalks extending outside of diatom cells. The stalks are composed of mucopolysaccaride, a complex chain of mainly sugar molecules. Bacteria and fungi don't readily degrade didymo mats.

The mats persist, smothering river bottoms, trapping fine particles and accumulating sediment. For some unknown reason, the mats also contain supersaturated concentrations of dissolved oxygen. The change in river habitat created by an influx of didymo has repercussions for other stream-dwelling organisms including fly larvae and probably fish.


Andrea E. Kirkwood, Troina Shea, Leland J. Jackson and Edward McCauley. 2007. Didymosphenia geminata in two Alberta headwater rivers: an emerging invasive species that challenges conventional views on algal bloom development. Canadian Journal of Fisheries and Aquatic Sciences. 64(12): 1703-1709.

Sarah Spaulding and Leah Elwell. 2007. Increase in nuisance blooms and geographic expansion of the freshwater diotom Didymosphenia geminate: Recommendations for response. United States Environmental Protection Agency, Region 8 and Federation of Fly Fishers.

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