Didymo Distribution in the Southeastern U.S.
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
Tennessee Tech University, North Carolina Wildlife Resources Commission, and Trout Unlimited have initiated a community science program to help determine the current distribution of didymo in the Southeast. This project allows anglers or other stream users to collect samples and send them to Tennessee Tech to analyze for didymo presence.
Collection kits and additional information can be obtained from Jeff Wright at Trout Unlimited (Jeff.Wright@tu.org) or Justin Murdock at Tennessee Tech (jnmurdock@tntech.edu, https://sites.tntech.edu/jnmurdock/).
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 Absent Michael Bobay 4/11/2020
What to look for when looking for Didymo
Didymo cells and stalks
Didymo is a diatom, which has a silica cell well, and excretes a carbohydrate rich stalk from one end. The stalks make up most of the mat biomass. Cells are too small to be seen with the naked eye.
Mats in the South Holston River, TN
The white areas are the non-living stalks and brown areas are the living didymo cells.
Mats
Mats have a cotton-like feel and are not slimy.
Mats in the Watauga River, TN
Note small tufts forming. Tufts are the early stages of mat development.
Blackfly larvae on mats in the Clinch River, TN
Mats are typically brown, orange, or white in color.
Sparse mats in the Clinch River, TN
