Didymo Invasive Species
Tuesday, February 9, 2016

Didymosphenia geminata

 Didymo Cell Didymosphenia geminata is a single-celled alga, within a group of algae called diatoms (Baccilariophyta). The individual cells grow a stalk and can form large colonies. The stalks, rather than individual cells, create negative impacts. Colonies are visible as white to brown tufts attached to rocks. As colonies develop and more stalk material is generated, growth will cover rock surfaces and trap fine sediment.


D. geminata tends to form excessive growths in:
  • Low, stable flow
  • Water 3 inches to 6 feet deep
  • Moderate to high current
  • Below outlets of lakes and reservoirs
  • Sunny open streams


In North America, D. geminata is expanding its geographical range and forming massive growths in streams. This nuisance organism grows attached in streambeds and may impact freshwater fish, aquatic plants, and aquatic insects.


Didymo impacts other stream organisms, and impacts are greater when large masses are present. 

  • Aquatic larval insects decline (caddisflies, stoneflies, and mayflies)
  • Other aquatic larval insects increase (midge larvae and worms)
  • Decline in large adult trout
  • Abundant young of the year trout
 Didymo in Rock Creek


Economic impacts include fouling of water intakes and a potential link to decline in fisheries. Impacts to aesthetics are common and recreationalists report unsightly masses that appear like strands of toilet paper or fiberglass. The growths become snagged in fishing gear and make swimming areas unappealing. Direct human health impacts are thought to be limited to eye irritation in swimmers.



Didymo feels like wet wool to touch, and is not slimy. Please report suspected masses of didymo. Help us confirm your report by collecting a small sample (put a pinch of the material in a business card and fold). Send samples with the date, latitude, and longitude (or detailed accurate site information).


Send reports and samples to:
Sarah Spaulding
USGS/ EPA Region 8
1595 Wynkoop St.
Denver, CO 80202

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