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. Long term trout data in the region shows Didymo mats may be reducing the fitness of larger trout, but smaller cohorts show no, or a slight positive effect of Didymo. Our food web research has shown that trout in the South Holston and Watauga rivers Tennessee are not only consuming mat material (likely inadvertently), but also assimilating it.

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.


Project Objectives:

Determine habitat parameters that support the colonization, growth, and stalk formation in Southern Appalachian streams.

Assess the impact of Didymo on macroinvertebrate structure

Determine whether Didymo mats change which food resources macroinvertebrates are consuming, and if fish are effected by Didymo through changes in their food resources.

Develop an easy and effective Didymo disinfection protocol for recreation uses.