Oh No! Not Rock Snot! What Do You Know About Didymo?

Contact: Susan Rivers, Maryland DNR | srivers@dnr.state.md.us

Photo: Walt Butler, Maryland Department of Natural Resources (April 2008)

ANNAPOLIS, MD (May 4, 2008) – Didymosphenia geminata (known as Didymo or rock snot) has recently been found in a Maryland stream. This diatomaceous alga forms heavy mats during its growing period and can coat stream bottoms. It can impact fishing, boating, and water intake structures by fouling equipment with this tough mat material. It can disrupt the habitat of insect larvae and other organisms that live on stream or lake bottoms, but research has shown no major impacts on fish populations. The only threats to humans could be that it can mess up water supplies and is abrasive if rubbed against sensitive skin.

Didymo is a diatom that lives at the end of a robust stalk that it builds. Many of these stalks can weave together to form thick dense mats. The diatoms have a shell made of silica and the stalks of mucopolysacharide; both are very tough. The mats trail in the water and can be light brown to white in color. Some people say it looks like toilet paper flapping in the water. It looks slimy, thus spawning the nickname “rock snot.” In reality it is thick and rough to the touch – like wet cotton. If you grab a Didymo mass, it is very tough to pull apart and hard to detach from rocks. There are some other algae that can feel coarse and tough, but they have some green color to them. Tough, dense, abrasive material in the water in white or tan shades usually means you’ve got rock snot. The mats can persist for up to two months after the diatoms die.

Until recently, Didymo was a rare alga distributed across the United States. It had always been outcompeted by other algal species or excluded by environmental conditions. In the past, it flourished in waters that were cold, low nutrient waters. It originally was described in waters with these characteristics in the late 1800’s and early 1900’s in Scotland, Finland, Sweden and northern China. It formed heavy mats in these, but was never seen doing this elsewhere. In the early 1990’s, worldwide changes began to be seen in D. geminata populations. More and more areas began showing larger, more extensive blooms. Major blooms have been observed in New Zealand where the government has mounted serious gear restrictions and disinfection policies on infested rivers. The diatom seems to prefer regulated rivers, particularly below dams, but this may be changing as well. It now tolerates warmer temperatures and higher nutrient levels. Didymo has a low tolerance for silt or sediment and tends to prefer clearer water and rocks for substrate.

Didymo is not a true exotic in the United States because it can be found in historic records. In its history in this country, Didymo did not exhibit invasive tendencies. However, researchers are afraid that what is being seen in rivers now is a different substrain or altered version of the diatom. It is spread primarily by man on materials that can stay wet and are transported from one area to another. It is possible that animals can do this as well, but humans have the most impact. Porous materials, such as felt bottom waders and clothing have been implicated, since many of the waters that have developed Didymo problems in the last five years are major, premier trout fishing areas. The problem has existed on the West Coast of the US for a while, but has been discovered on the East Coast in the last five years. Research is ongoing and biologists are trying to determine why the diatom is becoming more of a problem. With these changes and the unknown factors with this and other invasive species, management agencies are making strong efforts to educate the public and encourage checking, cleaning and drying all items that come in contact with open water before moving to another area. The major impacts of the blooms other than being an eyesore and nuisance are that the growths alter macroinvertebrate and algal community structure and may impact water quality. To date, no studies have shown impacts on fish or other aquatic vertebrate species.

Prevent the spread of Didymo by being fastidious with your equipment. When you leave a stream, clean all dirt off fishing gear, canoes, inner tubes, etc. Clean the gear; many things kill Didymo including drying, scrubbing with hot soapy water, wiping down with hand disinfectant and freezing. The one item that is not as effective and is discouraged in Maryland is chlorine (bleach). Chlorine can be very toxic, but is also unstable and may not maintain disinfecting levels because of its volatility. It has also been shown that chlorine does not penetrate dense materials well unless it has soap added. Other prevention methods suggest moving away from using porous materials, including boot felts, around water, or having two or more sets of gear, so one set can be drying for at least five days at any time.

All state agencies experiencing Didymo outbreaks are using similar approaches – educate the public to prevent the introduction or spread of this and other agents from water to water, identify characteristics that may spur blooms and learn to manage around the problem. Once this has gotten into open waters it cannot be removed.

For more information about other Invasive Species of Concern, visit www.mdinvasives.org or call the Maryland Department of Agriculture at 410-841-5920.

For more information on the Internet:

Dealing with Didymo. Flyfisher. Leah C. Elwell (2006)


MPI Biosecurity New Zealand