Connecticut River Coordinator's Office
Northeast Region
Photo of didymo attached to a rock - Photo credit:  U.S. Environmental Protection Agency
Photo of didymo attached to a rock. Credit: U.S. Environmental Protection Agency

Didymo is a unique and very large freshwater benthic diatom that is native to Europe. The individual diatoms are microscopic in size. They are unique in having a stalk that separates into two branches when the cells divide such that the stock from one diatom becomes entangled in the next. This eventually leads to the development of mats of diatoms which spread across the stream bottom. The mats are strong and resistant to degradation. They are not slimy. Instead, they feel fibrous like wet cotton wool. And, they are pale yellow-brown to white in color.

The mats of Didymo are associated with increased invertebrate populations of chironomid larvae and oligochaetes. It is not harmful to humans. Except for the aesthetic appearance of the infested river, there does not seem to be a negative impact to the trout or salmon populations living in infested streams in Norway, Quebec, Scotland, Finland, Iceland or France.

This diatom prefers cold, oligotrophic streams with stable flows, hard/stable substrates, Ca >2 mg/l and a high nitrogen to phosphorous ratio. It can attach to plants. It is rarely found in lakes. It needs a lot of light and thrives under increased UV conditions. Floods seem to provide a natural control.

Didymo is found in Europe, Asia and North America. It is known in the UK, Ireland, Norway, Spain, Poland, Slovakia, Iceland, New Zealand, China, Canada, Pakistan, Turkey, California, South Dakota, Colorado, Montana, Utah, Vermont and New Hampshire. The diatom was first recognized in the Batten Kill and White River in Vermont and in the mainstem Connecticut River in New Hampshire in 2007.

The local pattern of distribution suggests that it is being spread by recreational anglers. The diatom is so small that it can be spread in a drop of water. The management control for human transfers is to (1) check all gear and remove all visible clumps; (2) clean all gear by soaking and scrubbing for at least one minute in either 140oF hot water or 5% solution dishwashing detergent (1 cup detergent per 1 gallon water) - Felt soled boots require a 30-40 minute soaking; and, (3) Dry all gear thoroughly - it must be completely dried and then dried again for another 48 hours before use. If neither recommended cleaning nor drying is possible, restrict use of gear to a single water body and use different gear for infested areas.

Links for more information:


Last updated: September 1, 2010
Connecticut River Coordinator's Office
Northeast Fisheries Resources Home
Northeast Region Home

U.S. Fish and Wildlife Service Home Page | Department of the Interior  |  | About the U.S. Fish and Wildlife Service  | Accessibility  | Privacy  | Notices  | Disclaimer  | FOIA