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

Abstract

The development of environmental DNA (eDNA) methods toward 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 unraveling 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 detection probabilities at increasing distance from the source. The introduction of high fish stock biomass resulted in an increase of eDNA concentrations, as well as detection probabilities and in parallel reduced the stochasticity of the measurements. A peak in eDNA concentrations at a downstream distance ranging between 300 m and 2 km confirms the complexity of plume-shaped downstream eDNA patterns. Our results showed interspecific variation in eDNA emission and suggest species-specific differences in eDNA persistence. This study underlines the impact of both river discharge rate and source biomass on downstream eDNA detection and dispersal patterns.