Invasive Algae Found In Maryland – Potentially Destructive Didymo found in Gunpowder Falls

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Annapolis, Md. — The Maryland Department of Natural Resources (DNR) today announced that a new invasive, non-native algae has been found in Maryland for the first time. The algae, commonly known as Didymo, was found by anglers in Gunpowder Falls in Baltimore County. Didymo mats, also called “rock snot,” look slimy, but feel like wet cotton or wool, and can be white, yellow or brown.

“This alga has the potential to disrupt ecosystems in waters it invades by choking out bottom-dwellers and removing food organisms for game fish and other aquatic species,” said Don Cosden, Assistant Director of DNR’s Fisheries Service. “When Didymo takes over, many species are impacted.”

Although there is no human health risk associated with Didymo, DNR is developing an aggressive plan of attack to deal with this invader, and asks anglers and outdoor enthusiasts who enjoy Maryland’s waters to use extra precautions when moving from one stream or lake to another. Once Didymo is established, it can cover and suffocate a stream bottom, and movement of a single cell can contaminate a new waterway.

Felt bottom boots and waders commonly used by anglers are the worst culprits in the spread of aquatic invaders. Anglers are strongly encouraged to replace these boots with non-porous materials. New boots made of a sticky rubber material are safer for the aquatic environment and are much easier to clean.

Anglers and other recreational users of Gunpowder Falls and surrounding waters are especially urged to make sure they don’t contribute to the spread of Didymo or any other aquatic invasive species. The public is asked to clean anything that comes into contact with stream water by scrubbing away all dirt and debris before leaving a stream. At home, disinfect equipment by soaking in a 5% salt solution (1 lb/ 5 gal) for several minutes, or scrub well with dish detergent and rinse well. If disinfection is not possible, let equipment dry completely for at least 48 hours. Anglers may want to consider having two sets of equipment in order to move safely from one spot to another.

Didymo is an algal diatom that forms long stalks which combine to form heavy, thick mats that can smother a stream bottom. The stalks can persist for two or more months after the diatoms die, causing habitat damage for an extended period of time. Originally found in Scotland and extreme northern Europe and Asia, Didymo has been transported worldwide. Recently, the species has been found in the northeast and mid-Atlantic regions of the United States. In many cases, anglers have unknowingly transported the diatom on their fishing gear.

DNR urges anyone who observes Didymo to contact Don Cosden at 410-260-8287 as soon as possible.

For more information on Didymo and other invasive species, visit