Experimental assessment of downstream environmental DNA patterns under variable fish biomass and river discharge rates

The development of environmental DNA (eDNA) methods towards implementation as a cost-effective, nonlethal tool for fish biomonitoring in lotic environments requires insights on the temporal and spatial distribution of eDNA in river systems. Yet, little is known on how downstream eDNA dispersal is affected by the combination of river discharge and source biomass effects. In this study, we aimed at unravelling the effect of source- and system-specific processes on the stream reach of eDNA. We used a longitudinal cage study in two river sections characterized by a significantly different discharge rate, where two invasive and two native fish species were introduced under two contrasting biomass treatments. Using droplet digital PCR (ddPCR) analyses, we found that eDNA concentrations become strongly reduced 2 km downstream from the source, an effect that is strengthened with increasing river discharge and coinciding dilution effects. Higher discharge rates resulted in equal or even higher d...