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 smother stream bottoms and 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.  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 concern that Didymo may reduce the quality of fish populations and therefore harm trout fisheries as well as non-sport and native fish species. 

Didymo was first identified east of the Mississippi River in Tennessee in 2004. To date, it has only been found in streams below dam tailwaters in Tennessee, but a single cell was found in a free flowing section of the North Fork French Broad River in North Carolina in 2018. To date, no mats have been observed in North Carolina, but more surveys are needed to better understand the current distribution of didymo in the region to better predict were mats may occur in the future.

Community Partnerships to advance didymo science

Map of Didymo Distribution in Tennessee and North Carolina

Markers denote locations of samples submitted by community volunteers and sampling by Tennessee Tech. 

Submitted Samples

Site                                                            Sample Result                               Collected By                          Collection Date

Watauga River, NC                                   Absent                                         Jacob Rash                                   8/8/2019

Watauga River, NC                                   Absent                                         Jacob Rash                                   8/8/2019

Mitchell River, NC                                     Absent                                        Kin Hodges                                   8/9/2019

Avery Creek, NC                                        Absent                                         Jeff Wright                                    10/2019

Davidson River, NC                                  Absent                                         Jeff Wright                                10/24/2019

North Mills River, NC  (1)                      Absent                                           Ben Hill                                       11/7/2019

North Mills River, NC  (2)                      Absent                                           Ben Hill                                       11/7/2019

Swannanoa River, NC (1)                      Absent                                           Ben Hill                                        12/7/2019

Swannanoa River, NC (2)                      Absent                                           Ben Hill                                        12/7/2019

Elk Creek, NC (riffle)                               Absent                                       Doug Wells                                    1/2/2020

Elk Creek, NC (pool)                                Absent                                       Doug Wells                                    1/2/2020

Big Creek, NC                                              Absent   Michael Bobay                             4/11/2020

Manistee River, MI       Absent                                   Brian Kozminski                           6/10/2025

What to look for when looking for Didymo