Didymo / Rocksnot


Didymo is a diatom, a type of single-celled algae with a stalk. The stalk may attach to rocks, plants, or any other submerged substrates. When the diatom cell divides, the stalk also divides, eventually forming a dense mass.

How it Spreads

Recreational anglers are suspected to be the primary vector for the spread of didymo. Cells can remain viable in cool, damp, dark conditions for at least 40 days. Fishing equipment, boot tops, neoprene waders, and felt-soled wading boots in particular, all provide a site where cells remain viable, at least during short-term studies.

Impacts to Rivers

This alga is capable of producing such stalk densities that the mats covering the streambed change the ecology of the stream, including invertebrate diversity. In addition,high growth rates and extensive mats of Didymo may impact nutrient cycling. Algal, invertebrate, and fish species diversity and population sizes may be altered as a result.

The introduction of didymo in Wisconsin may have a large economic impact. The 600-plus coldwater spring-fed creeks of the Dritftless Area (and the world-class trout fishery they support) are a large economic engine for southwestern Wisconsin and are vulnerable to Didymo infestation. Trout angling generates $1.1 billion each year in the Driftless Area, strongly benefiting the local economy.


River Alliance of Wisconsin Factsheet (Lower Wisconsin River Basin AIS Strategic Plan)

River Alliance of Wisconsin Factsheet