Nuisance Algal Blooms in Montana

To a new eye, large blooms of bright green algae might raise an alarm. A nuisance algal bloom is a rapid growth of algae or cyanobacteria that affects the appearance and health of a waterbody. Most nuisance blooms do not produce toxins, but they can reduce water quality, create unpleasant odors, discolor the water, interfere with swimming and boating, and lower dissolved oxygen as the bloom breaks down. Nuisance algal blooms have been increasingly reported around Montana– in lakes, reservoirs, streams, and rivers.

Cladophora glomerata, a nuisance green macroalgae, in the West Gallatin River.

Didymosphenia geminata (didymo), commonly called rock snot, is a nuisance alga found in many Montana streams. Although it resembles a filamentous alga, didymo is actually a single-celled diatom. Individual cells produce long carbohydrate stalks that form the thick, fibrous mats covering stream rocks. The stalks—not the cells themselves—make up most of the material seen in a didymo bloom. Photo from the West Gallatin River.

Oedogonium is another type of filamentous, green algae that is found attached to rocks or free-floating and can look like a HAB. Remember, if in doubt, stay out!
Didymosphenia and Cladophora often co-exist and form thick, nuisance mats as seen here in the West Gallatin River.