Lakehead University Researcher Wants to Clarify Issues Associated with Lutsen Rock Snot
Posted on: November 14, 2018


This image of Didymosphenia geminate makes it clear where the nickname “rock snot” comes from. Credit: David Perez – Own work, CC BY 3.0,

“Rock Snot” a Misunderstood Diatom

Lakehead University PhD candidate Nathan Wilson recently brought to our attention a story we linked in the “Flotsam and Jetsam” section of Infosuperior’s newsletter about a type of alga—rock snot—appearing in Lutsen’s Poplar River. Nathan asserts that the article, which appeared in Minnesota’s West Central Tribune was missing a big piece of the puzzle because some important research is being left out.

Link to West Central Tribune article “‘Rock Snot’ algae confirmed North Shore stream in Minnesota” by John Myers

Didymosphenia geminate, known colloquially as didymo or “rock snot”, is a single cell alga (diatom) native to North America that can experience periods of rapid growth (blooms) forming large mats of brown-grey sludge on rocks and gravel in cold and shallow nutrient-poor waters. One such bloom was recently reported for the first time in the Poplar River, Lutsen.

News articles about the sudden appearance of didymo in Poplar River have suggested that blooms in new locations are due to introduction of the didymo cells to previously uninhabited areas and that the cells are brought in on waders and other fishing equipment. Nathan Wilson pointed out to us via e-mail that the idea of fishermen spreading didymo and causing blooms “was the traditional thought until research showed it occurs naturally and blooms when there are favourable nitrogen levels.”

The research that Nathan refers to is covered in “The Didymo story: the role of low dissolved phosphorus in the formation of Didymosphenia geminata blooms, Diatom Research” by Bothwell, Taylor and Kilroy, published in 2014 in the Diatom Research journal. It details the history of didymo research, through which we can learn how the misconceptions about fishermen causing didymo blooms began.


Unsightly and invasive didymo at Mararoa River in the Southland Region of New Zealand. Credit: Thorney¿? at the English language Wikipedia, CC BY-SA 3.0,


The Mystery of Didymo Blooms

It wasn’t until the 1980’s that dense mats of the diatom were reported in streams on Vancouver Island. Usually we associate alga blooms with the introduction of nutrients due to human activity; however, the didymo mats were appearing upstream of major human habitations. Early research could not determine why didymo, a native species on Vancouver Island, suddenly began blooming.

In 2004, didymo mats appeared in the Southern Hemisphere for the first time, specifically in New Zealand. The diatom was widely accepted as invasive to New Zealand and therefore must have been introduced by human activity. It was then suggested that the introduction of a mat-forming variety of didymo to Vancouver Island could explain the sudden appearance of blooms there decades earlier. The coincidental timing of increased fishing tourism, river use and felt-soled waders alongside the first appearance of blooms in Vancouver Island streams and rivers gave credibility to this idea.

The thing is, shortly after didymo began to bloom in New Zealand, it became apparent that simply introducing didymo cells to a new stream did not always result in blooms. From 2008 to 2010 Environment Canada and the New Zealand Institute for Water and Atmospheric Research collaborated on a research project to solve the Didymo problem for good and their results were published in three research papers referred to by Bothwell, Taylor and Kilroy as “the Didymo Trilogy.”

The Important Piece of the Puzzle

The results of the Didymo Trilogy showed that mats were formed when didymo cells colonized and grew taller than usual and that this type of growth could only occur when soluble inorganic phosphorous levels were less than two parts per billion. The authors suggest that this may be a strategy employed by the didymo cells to grow past the benthic boundary layer into the water column where phosphorous is more easily accessible.

Scanning electron micrograph image of a single didymo cell. The porefield is the area where the stalk is produced to grow taller cells. Scale bar is 50 μm. Credit: Sarah Spaulding, USGS. Public Domain,

Thus, it is true that didymo cells must be present to result in blooms, however the cause of blooms is a result of nutrient depletion in aquatic habitats. This is still something caused by human activity as our affects on climate, atmosphere and soil processes lead to acidification which causes nutrient depletion. Although didymo is not inherently bad for the water in which it is found and the transfer of alga to new streams will most likely not result in unsightly “blooms” when the soluble inorganic phosphorous levels are greater than 2ppb, it is still important to practice good cleaning habits with fishing equipment as many detrimental invasive species can be spread this way.

Nathan Wilson studies various aspects of lakes and lake management through the environmental biotechnology program at Lakehead University, working with Dr Carney Matheson, and Dr Rob Stewart. His focus is on examining lakes within Northwestern Ontario to better understand nutrients and cyanobacteria.



Max L. Bothwell, Brad W. Taylor & Cathy Kilroy (2014) The Didymo story: the role of low dissolved phosphorus in the formation of Didymosphenia geminata blooms, Diatom Research, 29:3, 229-236, DOI: 10.1080/0269249X.2014.889041


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