Didymosphenia geminata (Didymo) is a single-cell alga (a diatom) that can form thick blooms in streams and completely cover stream bottoms. The cells attach to rocks by long stalks, which can remain after algal cells have died, often littering stream banks and reducing recreation and aesthetic appeal. The magnitude of Didymo’s impact on stream health is not clear. Smothering the stream bottom and our research has shown that dense mats can alter the macroinvertebrate (bottom-dwelling insects) species composition by changing food availability. Since macroinvertebrates are a major food source for trout in Southern Appalachian Mountain streams, there is great concern that Didymo may reduce the quality of fish populations and therefore harm trout fisheries as well as non-sport and native fish species. In Tennessee, Didymo is only found below large bottom-release dams.  Macroinvertebrate appear to have adapted to mats, as mats have a small effect on macroinvertebrate community composition.

Didymo was first identified in Tennessee in 2004. To date, it has only been found in streams below dam tailwaters in the state in the Clinch, Holston, South Holston, and Watauga rivers. Recent surveys have also found Didymo in the Obey River below Dale Hollow Dam, TN, and in the Tuckasegee River, NC.

New research suggests that Didymo may be native in many areas, and changing environmental conditions may be prompting increased stalk production. At this point it is unclear whether Didymo is native or invasive in Tennessee.

Our research has shown that in Tennessee and North Carolina, the cause of Didymo colonization and mat formation can be controlled by several factors. Dissolved phosphorus which has been strongly linked to mat formation in other regions, is not a strong driver in the Upper Tennessee River basin. Instead, pH, water temperature, and several other ions have a stronger relationship than phosphorus, which is only correlated to mat coverage in the summer. Additionally the environmental factors most closely related to Didymo presence and mat coverage vary across seasons.