Didymosphenia geminata, commonly known as didymo or rock snot, is a species of diatom that produces nuisance growths in freshwater rivers and streams with consistently cold water temperatures and low nutrient levels.

[1] It is not considered a significant human health risk,[5] but it can affect stream habitats and sources of food for fish and make recreational activities unpleasant.

[1] Didymosphenia geminata is a diatom, which is a type of single-celled organism unique for their silica (SiO2) cell walls.

Even within its native range, there have been reports of excessive growths in areas where it previously existed only in low concentrations.

Occurrence, ecology, water quality, and scanning electron micrography of D. geminata from Irokinda Mine Site, Buryatia, John G. Aronson, North American Diatom Symposium, Sept, 2011.

[citation needed] British Columbia: D. geminata was first reported as a nuisance species in the late 1980s on Vancouver Island.

[13][14] Arkansas: In late spring / early summer of 2005, didymo was found directly below Bull Shoals Dam on the White River.

[24] In August 2007, didymo was found in New York State in a section of the Batten Kill, a Hudson River tributary, in Washington County.

The N.C. Wildlife Resources Commission is advising anglers to use extra caution when cleaning their equipment to prevent the spread of didymo, which was recently found in the Tuckasegee River in Jackson County.

[29] In 2013, PA Department of Environmental Protection put out an alert that it has now been discovered in Pine Creek, Lycoming County.

[31] Tennessee: Didymo was found in the tailwaters of the Norris, Cherokee, Wilbur and South Holston hydroelectric dams in 2005, the first U.S. finding east of the Mississippi River.

The sighting was reported by a fishing guide and confirmed by Dr. Sarah Spaulding, a didymo expert from Denver, Colorado.

If you find clumps later don't wash them down the drain, treat them with the approved methods below, dry them and soak them in bleach for at least 4 hours.

Clean: Soak and scrub all items for at least one minute in either hot (60 °C) water, a 2% solution of household bleach, antiseptic hand cleaner, or dishwashing detergent.

New Zealand and the U.S. states of Alaska, Maryland, South Dakota and Vermont have banned anglers from wearing felt-soled boots.

Given didymo’s habitat and environmental preferences, it was proposed that climate-related changes in regional rivers are likely an important factor that favors its proliferation.