Effect of Distance From Source and Stream Flow On Edna Signal Strength in Two Headwater Streams

Recent years have seen an explosion of interest in the use of freely available DNA present in aquatic systems, otherwise known as environmental DNA (eDNA), as a tool for monitoring aquatic organisms.  However, much remains unknown about the behavior of eDNA over a range of environmental conditions.  This is particularly true in high gradient headwater streams, which have received less attention than other types of water bodies.  DNA concentrations in streams and rivers will depend not only on the equilibrium between DNA entering the water and DNA leaving the system through degradation, but also on downstream transport.  To better understand the dynamics of eDNA concentration in lotic systems, we introduced caged brook trout (Salvelinus fontinalis) into two otherwise fishless headwater streams, and took eDNA samples at evenly spaced intervals downstream of the cage.  This was repeated 19 times from mid-summer through autumn, over flows ranging from approximately 1 to 96 l/sec.  We used quantitative PCR to relate DNA copy number to distance from source for each of these 19 sampling events.  We found that within a sample, the DNA signal generally decreased exponentially with distance from the source, but trace amounts were frequently still detected at 240 m from the source.  We also found wide variation in signal strength, with high flows strongly reducing the eDNA signal.  This study demonstrates the localized nature of the eDNA signal in headwater streams and suggests that eDNA monitoring in headwater streams should be avoided during high water periods to minimize false negative results.