A history lesson in slime

Sediment records may give us a clearer picture of what is causing algal blooms in Canada's iconic salmon rivers.

The year was 2006. Anglers headed out for a regular day of recreational fishing on the Matapedia River in Quebec, not expecting to return home with a thick and slimy mat of algae covering their boots. It was a species of algae never before seen… or so they thought.

The substance, now observed more frequently in rivers in Canada and around the world, quickly became known as "rock snot" for its slimy appearance. It also became a special interest of limnologists (scientists who study inland waters) and watershed managers, who were eager to find out why these algal blooms were occurring and how they were affecting our freshwaters.

These are the questions being probed by Dr. Joshua Kurek, a professor at Mount Allison University in New Brunswick, who is studying algal blooms in Canadian rivers using lake sediment records. The research has implications for management of freshwater salmon populations of importance to the sport-fishing industry and Canada's Aboriginal population. The research is also highlighting the impacts of climate change on freshwater resources.


Growths of this bottom-dwelling alga (Didymosphenia geminata), often called didymo blooms, have been unprecedented in recent years in rivers previously considered pristine. The alga's presence is more than just an eyesore; it is cause for concern for vulnerable populations of juvenile salmon.

Dr. Kurek explains that rock snot cannot be considered the cause of declining salmon populations, but it may be an added stressor. "By coating the riverbeds with a thick algal mat, didymo blooms not only restrict access to the main prey item of the juvenile species but also introduce them to entirely different food sources that may differ in both quality and quantity."

Dr. Kurek's research also suggests the algal species is a marker of widespread environmental change. Earlier thinking maintained that algal species were spread by recreational anglers. However, in recently published research, Dr. Kurek discovered that this species, although only recently observed on such a large scale, has actually been a part of our ecosystem for hundreds of years.

Dr. Kurek and colleagues collected sediment cores from river systems where blooms are now commonly found, particularly in eastern Quebec. An examination of physical, chemical and biological markers in the dated sediment record allowed them to identify the algae and understand shifting environments over time.

"With the sediment cores we can reconstruct the past history of the environment in order to better understand and put in perspective recent environmental changes. Sediments are like history books of the environment and the surrounding area."

Dr. Kurek and colleagues found remains of the didymo blooms in their sediment records dating back decades. Paired with an extensive literature review, which revealed recorded evidence of the species from algal surveys of Atlantic Canada as far back as the early 1900s, there was enough evidence to prove that rock snot is not a new phenomenon but one that was brought on by recent environmental change favouring its growth.

According to Dr. Kurek, detailed studies looking at its growth over long periods of time are still needed to identify exactly what the main causes could be, but the usual suspects include regional climate change, shifts in nutrient levels and other indirect linkages with climate warming.

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Source: NSERC

Joshua Kurek