Didymosphenia Geminata Detection, Distribution, and Mat Development in the Upper Tennessee River Watershed

Bowman, Andrea

ABSTRACT: Didymosphenia geminata (Didymo) (Lyngbye) M. Schmidt is a benthic diatom native to high-latitude streams of the northern hemisphere that can produce thick mats on stream substrata. Mats negatively impact fisheries, recreation, and aquatic food web structure and function. There is a growing concern regarding factors that regulate D. geminata distribution and mat development due to increased incidences of mats outside its historical range. Didymosphenia geminata mats have been present in Tennessee since approximately 2004, but its regional distribution and local factors that control its growth are unknown. My project focused on D. geminata distribution and understanding parameters regulating its presence and abundance in Tennessee rivers; specifically, rivers with documented presence (i.e., Clinch, Holston, South Holston, and Watauga rivers) and streams associated with Great Smoky Mountains National Park. Stream water quality, habitat, and D. geminata presence and abundance were measured at 24 sites in the spring, summer, and fall of 2015. Environmental drivers of D. geminata presence were assessed using discriminant function analysis, and drivers of abundance were assessed using multiple regression with Akaike Information Criterion used to select the most parsimonious model. A larger regional assessment of D. geminata distribution was done in 31 sites across the region and included sites in Tennessee, Kentucky, Virginia, and North Carolina. This project also evaluated the efficacy of using environmental DNA (eDNA) as a detection method for this species relative to standard microscopy detection. Environmental parameters were also measured to relate stream conditions to cell presence. Microscopy confirmed D. geminata in seven streams, with this species occurring only in tailwaters below dams with hypolimnetic release. Overall, the strongest drivers of D. geminata presence were low temperature, neutral to basic pH, and a wide channel with little canopy cover. The strongest drivers of abundance and mat coverage were pH and canopy cover. Amplification of D. geminata DNA occurred in all samples, suggesting either contamination, cell presence at every site, or problems with the PCR amplification process. Microscopy was found to be more reliable for D. geminata detection. Currently, it is not clear if D. geminata is native or invasive in Tennessee. Therefore, it is important to identify impacted rivers and those susceptible to D. geminata colonization to prevent potential spread of this nuisance species and increase public awareness of its impacts.