Why didymo? Contrary to other algae species, didymo represents an interesting paradox where didymo mats form under very low nutrient conditions. Also, managers continue to grapple with how best to address this species when faced with extraordinary mat formations. For the past decade, ISAN has had an interest in this unique species because of its management implications.

The alga, Didymosphenia geminata, commonly referred to as “didymo”, is a freshwater microscopic diatom. It is found in streams and rivers across much of North America. Didymo attaches to the streambed by a stalk and it forms thick mats that can alter aquatic ecosystems. These mats have a rough texture similar to wet wool and can look like strands of toilet paper.

In recent years, streams in New Zealand, North America, Europe, South America and Asia have noted unprecedented masses of “didymo”. This diatom is able to blanket up to 100% of stream surfaces by with thicknesses of greater than 8 inches, potentially altering physical and biological conditions within streams.


Cleaning equipment between uses on different freshwater systems is important to prevent the spread of didymo and other invasive species. If you can follow the simple Clean, Drain and Dry steps, then you will do a world of good.

For decontamination methods specific to didymo, you can learn more from the links below in the report “Decontaminating Equipment”. For more information on general cleaning practices, go to cleanangling.org.

Other resources of note include:





The Conference was held on March 12-13, 2013 in Providence, Rhode Island USA and proudly hosted by the Invasive Species Action Network and Northeast Aquatic Nuisance Species Panel.

The conference successfully brought together natural resource managers, researchers, conservationists, fishing clubs and others with an interest in learning more about didymo.


For management recommendations, read the Management Challenges of Didymosphenia geminata.

For a complete list of presenters and conference agenda, download the Conference Booklet.

Conference presentations:

  • Aunins – Genomic characterization of D. geminata: current progress and future directions.
  • Cary – A sensitive genetic-based detection and enumeration method for D. geminata.
  • Gills – Impact of D. geminata presence on juvenile Atlantic salmon: A project overview.
  • Kilroy – Plenary Speaker: Didymosphenia geminata: an extraordinary organism.
  • Klauda – Didymo infestation in Maryland, USA: A state agency’s reactions, responses and Felt/Results.
  • Kuhajek – Laboratory-based experiments to investigate didymo distribution patterns in New Zealand.
  • Montecino – On the biogeography of D. geminata in Chile: niche requirements and potential habitats.
  • Nelson – Identification, enumeration and measuring cell dimensions of D. geminata using an imaging particle analyzer (FlowCAM).
  • Pillsbury – Are the recent blooms of D. geminata in Lake Superior (USA) caused by an aggressive strain on environmental changes?
  • Richardson – Spatial distribution and ecosystem effects of a nuisance, bloom-forming diatom (D. geminata) in Catskill Mountain streams, New York.
  • Sanson and Gerbeaux – D. geminata in New Zealand:an update on current managment and research approaches.
  • Spaulding – Paleolimnological records of Didymosphenia geminata in North America.
  • Sundareshwar – Does sulfation of Didymo stalks facilitate iron adsorption and phosphorus concentration in mats?
  • VanPatten – Missouri’s proactive approach to didymo.
  • Zarnetske – Integrating invasion history with environment to model D. geminata hotspots across New Zealand.

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- Leah Elwell, Fmr. Executive Director