Government of New Brunswick

Didymosphenia geminata
(didymo) is a member of the group of single-celled aquatic plants (freshwater algae) known as diatoms. Although it is microscopic, didymo can form dense colonies called algal blooms.

The following FAQs have been adapted from Government of New Zealand and Province of Québec sources.

Q. Why are we concerned?
A. A didymo bloom occurred in Québec in the summer of 2006 in the Matapedia River. This river is a tributary to the Restigouche, a boundary water between New Brunswick and Québec. Sampling results later confirmed the presence of didymo in several rivers in the Gaspé Peninsula and the Matane River in the Lower St. Lawrence.

The mere presence of didymo in a river does not necessarily mean that severe blooms, as observed in other parts of the world, will occur. However, there are concerns about potential impacts to plant, insect and fish communities and the aquatic habitats on which they depend.  

Q. Does didymo impact on human health?
A. Didymo is not considered a significant human health risk. According to available data, it does not render water unfit for consumption and is not toxic. However, people swimming in waters downstream from areas containing high concentrations of didymo have complained of eye irritations, which may be caused by the silica of the frustules. Didymo is not the blue-green alga that has been of concern in some lakes across the country.

Q. Has it been found in New Brunswick? 
A. By early fall 2007, didymo was discovered in the Restigouche drainage (Kedgwick, Patapedia, Upsalquitch, Matapedia, and the lower part of the Main Restigouche below Matapedia) and the Upper Saint John River drainage (Shikatehawk Stream).  

Q. What action is being taken?
A. The best course of action is education aimed at preventing the introduction and limiting the spread of didymo to other rivers or streams. Natural Resources and Energy Development is working with other provincial agencies, stakeholders, the federal government, and the Province of Québec. We are also making significant efforts to keep the public informed. This includes a notice in the angling summary, media interviews, the distribution of fact sheets to Crown Reserve anglers, posters in Crown Reserve camps, fact sheets in our district offices as well as web information.  

Q. How do I recognize didymo?

A. Young colonies look like raised pimples on the surfaces of river rocks. However, under optimum growing conditions, didymo cells ooze large amounts of a mucus-like substance (mucilage) which attaches didymo firmly to underwater surfaces. As the mucilage elongates to form stalks, the colonies form impenetrable mats which form thick strands and can cover all surfaces, including the stream bed, other plants, logs and debris. Only the cells of didymo are living; the stalks are formed of non-photosynthetic mucopolysaccharides and range in colour from brownish yellow to white. Diatoms are unique in that their external cell walls (frustule) contain silica (a major element in sand), which is why didymo feels gritty when touched. As opposed to slimy, this alga feels like wet cotton wool.

Didymo occurs in shallow waters. When the river level falls, mats of dried algae on rocks can be mistaken for strands of toilet paper or parchment paper, causing concerns about possible sewage discharges. In contrast to cyanobacteria (blue-green algae), didymo blooms are not caused by pollution or excessive phosphorus in streams. To the contrary, didymo thrives in oligotrophic or low-nutrient, waters.

Q. How did it get into the Restigouche system?

A. It is unlikely that we will ever know exactly. The most likely explanation is unintentional human transfer by recreational river users. In British Columbia, it has been shown that the appearance of didymo coincided with the introduction of felt-soled waders. Because didymo cells can survive for up to 30 days in wet felt soles, they can be transported from river to river. Migratory birds, pets and wildlife can also be vectors for the introduction and spread of the alga, as well as boats, diving equipment and any other equipment used in lakes and rivers.

Q. Where else is it found?
A. Didymo's native distribution is limited to cool temperate regions of the Northern Hemisphere. This includes the rivers of northern forests and alpine regions of Europe, Asia and parts of North America. It was first discovered in the Southern Hemisphere in 2004 on the south island of New Zealand. Over the past 20 years, its distribution appears to be gradually expanding outside its native range. Even within its native range, there have been reports of excessive growths in areas where it previously existed only in low concentrations.  
In Canada, didymo blooms have been observed in British Columbia, particularly in several central Vancouver Island rivers, and east of the Rockies in Alberta. A number of western U.S. states, including Montana, Dakota, Colorado and Utah, have also reported blooms. In Europe, didymo has been confirmed in Hungary, Ireland, Iceland, Finland, Poland and Romania. Our climate is very different from some of these regions, so there is no reason to believe that the alga will spread to the same extent in New Brunswick rivers.  

Q. What type of habitat does it prefer?
A. Didymo prefers the flowing waters of rivers. Environments suitable for its establishment include cool to warm water temperature range, a stable rocky substrate, high light exposure, moderate flow velocity, clear, low nutrient waters with a pH neutral or slightly alkaline. Rivers with easy access and significant recreational pressure are most at risk. There is a chance that any spread of this alga in New Brunswick may be tempered by our cold winters and ice regimes.

Q. Will didymo spread to rivers throughout N.B.?

A. Possibly. Research on the environmental variables which control its growth (water depth and flow rate, nutrients, light, invertebrate grazing, etc.) has helped identify which habitats and locations it is most likely to establish. Ongoing surveillance will help determine rate and range of spread. Human activities are considered the most likely source of spread of didymo between rivers and catchments.  

Q. Could it spread to lakes throughout N.B.?
A. Yes. Although didymo prefers a river environment, blooms have occasionally been reported along the wave-swept margins of lakes. Lakes which catch affected rivers will be at highest risk.  

Q. What effect can it have on stream habitat & aquatic organisms?
A. There have been limited scientific studies on the effects of didymo in areas where algal blooms have been reported. In extreme cases, didymo blooms are believed to impact stream food webs by reducing algal diversity and altering the species composition of invertebrate communities. There are reportedly more snails, midges and tubifex worms, but fewer caddisflies and mayflies. In addition, a higher density of organisms is typically observed, but their average size is smaller. Large mats of algae covering riverbeds could also cause changes in stream flow.  

Q. Will our Atlantic salmon stock be affected?
A. Despite the fact that the presence and spatial coverage of this alga have increased in recent years in some European countries (France, Iceland, Ireland, Scotland, Finland and Norway), the presence of didymo does not appear to have had an impact on the fishery or the maintenance of Atlantic salmon populations. Local fisheries managers indicate that no impacts have been observed or even suspected to date on Atlantic salmon, either adults or juveniles.
In early September 2006, the ministère des Ressources naturelles et de la Faune in Québec carried out an electrofishing survey at sites in the Matapedia River where a significant quantity of didymo had been observed, and at other sites where a small quantity of the algae had been reported. The results of the study did not show that the algae had a measurable impact on the abundance of juvenile salmons (parr).

Q. Are fish and other food taken from affected waters safe to eat?
A. Yes.

Q. What is the potential impact on infrastructure and recreation?
A. Growing mats of didymo or algal material drifting in the current can sometimes clog water intakes. The equipment of anglers, kayakers, canoeists or other recreational users of rivers can also be made foul with didymo mats. The mats can affect the aesthetic quality of rivers, making them look as though they were polluted.

Q. Can it be eradicated?
A. Past experience with invasive species indicates that eradication of didymo is unlikely. This may be in part due to its widespread distribution and because it is considered a native species in some countries. However, eradicating any microscopic organism from a natural environment is difficult, especially in water. The environmental impacts of any large scale river treatment could be severe, perhaps even greater than the impact of the organism. In certain rivers on Vancouver Island, the didymo situation has improved considerably. This decline in population could be tied to the rivers' natural control mechanisms, which have not yet been identified. A decline was also noted in some infected Québec rivers in 2007.  

Q. What can be done to prevent its spread?
A. To prevent the introduction or spread of didymo, it is generally recommended to restrict the use of equipment, boats, clothing and other items to a single waterway wherever possible. If you have to move from one waterway to another, the following methods, developed and tested by New Zealand authorities, have proven effective:
1. Check: Before leaving the river, check your boat and equipment carefully and remove all obvious clumps of algae, taking care to look for hidden clumps. Leave clumps at the affected site. After leaving the area, if you find any clumps, do not wash them down the drain, but rather throw them in a garbage can. Treat your equipment as follows.
2. Clean: Any item that was in contact with water.

Non-absorbent items:   
Soak and scrub all items for at least one minute in one of the following:

  • very hot water kept above 60 °C (hotter than most tap water), or for at least 20 minutes in hot water kept
  • above 45 °C (uncomfortable to touch).
  • a two per cent solution of bleach (200 ml and water added to make 10 litres);
  • a five per cent solution of salt (500 ml or two cups and water added to make 10 litres);
  • a five per cent solution of antiseptic hand cleaner (500 ml or two cups and water added to make 10 litres);
  • a five per cent solution of dishwashing liquid (500 ml or two cups and water added to make 10 litres).

Absorbent items:  
Items like felt-soled waders require longer soaking times to allow thorough saturation. Leave absorbent material to soak:

  • at least 40 minutes in hot water kept above 45 °C; 
  • at least 30 minutes in hot water kept above 45 °C containing a five per cent dishwashing detergent solution.

3. Dry: If equipment cannot be cleaned adequately, dry it completely and then allow it to dry for an additional 48 hours before using it in another lake or river.
When applying these decontamination methods, it is recommended that you:

  • Use biodegradable products.
  • Use phosphate-free detergents.
  • Do not dispose of cleaning wastes in lakes or rivers.
  • Use a cleaning solution recommended above that will not alter your equipment. 
  • Follow the product manufacturer’s safety recommendations.

Freezing: Freezing any item until solid will also eliminate didymo cells.

Additional precautions must be taken for the following items:

Motor boats: Clean boats, both inside and outside, as well as mechanical parts (including propellers), various compartments and trailers with one of the above cleaning solutions. Clean boats and trailers before moving from one watercourse to another.
Kayaks, canoes and dinghies: Scrub the exterior of the craft vigorously with one of the recommended cleaning solutions for at least one minute, then fill the interior of the craft with the same solution and place all equipment, gear and clothing used in the boating activity into the boat. Immerse the equipment completely and stir the solution for at least one minute, then rinse with tap water.
ATVs/vehicles: Carefully examine vehicles to remove all clumps of algae that may be attached. Thoroughly clean the underside of vehicles, tires and parts that come into contact with the water with the cleaning solution. Leave the solution on for at least one minute. Commercial car washes with an underside spray are suitable for this purpose.
Fishing equipment: Clean all rods, reels, fishing lines, flies, tackle boxes and waders and rinse with tap water. If it is not feasible to clean fishing equipment using the suggested methods, anglers can choose to restrict the use of their equipment to a single lake or river. It is important to choose one of the cleaning methods that is effective but also the least harmful to delicate gear such as salmon flies, neoprene or other synthetic materials. The simplest solution would be to allow the equipment to dry out completely.  
Waders: Anglers must pay particular attention to their waders, especially neoprene waders and waders with felt soles. Felt-soled waders are felt to be an important vector in the transmission of didymo. It is very difficult to effectively clean or dry the felt soles. Studies in New Zealand have demonstrated that didymo cells can live for up to 30 days in damp felt soles.  
Pets: Wash pets for at least one minute with an animal shampoo or disinfectant. If they cannot be washed, they should be held in an area away from waterways for at least 48 hours.
Swimmers & divers: Wash yourself with soap or shampoo and dry yourself completely before entering another lake or river. Potentially contaminated clothing such as bathing suits, wet suits and gloves, must be washed with laundry detergent and dried thoroughly before being reused. Shoes, masks, fins, etc. should be scrubbed or sprayed with one of the suggested cleaning solutions for at least one minute.

Q. How long will cleaning solutions remain effective?

A. The effective concentration of cleaning solutions can change when exposed to the elements (sunlight, rain, etc.). Additionally, solutions may become diluted or contaminated after frequent usage. It would be prudent to change solutions frequently to maintain their effectiveness as a disinfectant.  

Q. What do I do with the waste water after cleaning my equipment?
A. Do not dispose of cleaning solutions or wash water in any natural watercourse. Ideally, they should be diluted with clean water and disposed of in an area far from water and where they can be filtered through vegetation and/or soil. If large quantities of chemical solutions are used, they should be collected and disposed of at an appropriate wastewater treatment facility.

Q. Why do I have to clean between EVERY river?
A. Didymo is hard to detect in its microscopic form and therefore it could be in rivers that have yet to be confirmed. Because of this, we need to treat all waterways as if they might have didymo. People need to check, clean, and dry every time they use a waterway.

Q. If other creatures can spread it, will it really make a difference if I clean?
A. Scientists believe that humans are the biggest risk of spreading didymo. Everyone needs to do as much as they can to minimize the spread.

Q. If areas are low risk – or if it's already there – why should I clean?
A. In its microscopic form, didymo is hard to detect so we need everyone to treat all waterways as if they are affected This is not just about didymo, it is about protecting all of our waterways. Didymo has been the wakeup call that we must do more to protect our waterways from invasive pests.

Q. If I think I see didymo, what should I do?
A. All river users are asked to keep a lookout for signs of didymo. If you see something you suspect is didymo, you can report the location at a Natural Resources office.

Q. How do I know if it is didymo or not?
A. Definitive identification requires microscopic analysis. However, didymo can be distinguished from other species of algae on the basis of:

  • Colour – didymo is beige/brown/white but not green. 
  • Touch – although it looks slimy, it doesn't feel slimy, but rather spongy and scratchy like cotton wool. 
  • Odour – live didymo has no distinctive odour. 
  • Strength – didymo is securely attached to river stones and does not fall apart when rubbed between your fingers.    

SOURCE:  BIOSECURITY NEW ZEALAND, 2006. “Didymosphenia geminata”, on the Biosecurity New Zealand web site, [Online]. (accessed Dec. 4, 2006).

MDDEP-MRNF Scientific Advisory Committee on Didymosphenia geminata, 2007. What Is Didymo and How Can We Prevent It From Spreading In Our Rivers?, Québec, ministère du Développement durable, de l’Environnement et des Parcs et ministère des Ressources naturelles et de la Faune, ISBN: 978-2-550-49391-4 (PDF), 10 p

BOTHWELL, M., 2006. “Blooms of Didymosphenia geminata in Rivers of Vancouver Island 1990 to Present: A Sign of Environmental Change or a New Invasive Species?”. Paper presented in Québec City, November 17, 2006, at INRS-ETE.

KILROY, C, 2004. A New Alien Diatom, Didymosphenia geminata (Lyngbye) Schmidt: its Biology, Distribution, Effects and Potential Risks for New Zealand Fresh Waters, National Institute of Water and Atmospheric Research, New Zealand. Client Report: CHC2004-128, 34 p.

KILROY, C, 2005. Tests to Determine the Effectiveness of Methods for Decontaminating Materials that have been in Contact with Didymosphenia geminata, National Institute of Water and Atmospheric Research, New Zealand. Client Report: CHC2005-005, NIWA Project: MAF05501, 30 p.

BRITISH COLUMBIA MINISTRY OF ENVIRONMENT, 2006. “Water Quality, Didymosphenia geminata in British Columbia Streams”, B.C. Ministry of Environment, website [Online]. (accessed Dec. 4, 2006).