Whispers of Wildfire - Using eDNA Metabarcoding to Decipher Effects of Megafires on Watershed Biodiversity

Wildfire can dramatically influence terrestrial landscapes, but the effects on biodiversity beneath the water's surface are difficult to see. Following the 2020 Labor Day wildfires in Oregon's western Cascade Mountain Range, we sampled 25 Oregon streams and 6 Washington streams representing a gradient of fire severity and pre-fire forest stand age to determine the effects of megafires on watershed biodiversity. In each watershed we used environmental DNA (eDNA) metabarcoding and traditional field surveys (Surber or electrofishing) to assess biodiversity. Our results suggest aquatic taxonomic richness for macroinvertebrates and vertebrates in the first-year post-fire was unaffected by wildfire. Diversity responses to wildfire severity and pre-fire forest stand age were inconsistent across methods indicating decreasing diversity with increasing burn severity from electrofishing and decreasing diversity with increasing stand age from eDNA, but no reciprocal method changes were observed. eDNA aquatic insect detections revealed greater species richness in middle-aged to older forests. eDNA detected 158 organisms, 48 of which were found in all fire severity categories, but each category revealed unique species compositions. We demonstrate the utility of eDNA metabarcoding for a broader taxonomic assessment of post-wildfire disturbance facilitating an improved understanding of the effects of wildfire and forest management on watershed biodiversity.