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New Hampshire Department of Environmental Services

Didymo (A.k.A. Rock Snot)

Didymo (a.k.a. rock snot) found in Vermont and New Hampshire

Didymo is the common name for Didymosphenia geminata, an invasive freshwater diatom species (microscopic algae). Didymo can form extensive "blooms" on the bottoms of rocky river beds, essentially smothering aquatic life forms such as macroinvertebrates (aquatic insects), native algae, and other organisms. Additionally, the physical appearance of the bloom is aesthetically unpleasing, and can reduce the recreational values of a waterbody. Didymo uses stalks to attach to rocks and plants in a river system. The diatom creates these stalks, which can form masses 10-12 inches thick on the river bottom, and trail for lengths of 2-3 feet in the current. It is actually these stalks that are more problematic than the algae. The algae will eventually die off and decompose, while these stalks tend to persist for several months on the river bottom.


CHECK - Remove all visible clumps of algae and plant material from fishing gear, waders, clothing, water shoes and sandals, canoes and kayaks, and anything else that has been in the water.

CLEAN - Using HOT tap water and lots of soap: Scrub boats and other "hard" items thoroughly; Soak clothes, felt-sole waders and other "soft" items for 30 minutes!!!!

Sightings in New Hampshire
Amy P. Smagula Limnologist/Exotic Species Program Coordinator
NH Department of Environmental Services
29 Hazen Drive; PO Box 95
Concord, NH 03302-0095
(603) 271-2248
(603) 271-7894 (fax)

Sightings in Vermont
Didymo identification, Water Quality Division
103 S. Main St., Bldg 10N, First Floor
Waterbury VT 05671-0408
(802) 241–3770 or 241-3777

NH Department of Environmental Services | 29 Hazen Drive | PO Box 95 | Concord, NH 03302-0095
(603) 271-3503 | TDD Access: Relay NH 1-800-735-2964 | Hours: M-F, 8am-4pm

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