Riverine distribution of mussel environmental DNA reflects a balance among density, transport, and removal processes

Sampling water for environmental DNA (eDNA) is an emerging tool for documenting species presence without direct observation, allowing for earlier detection and faster response than conventional sampling methods in aquatic ecosystems. However, current understanding of how eDNA is transported in streams and rivers remains imprecise, with uncertainty of how the unique transport properties of eDNA may influence the interpretation of a positive detection. To test the utility of eDNA sensing in flowing waters, we compared quantitative eDNA analyses to zebra mussel density surveys in a Danish river. Although flowing water complicates the relationships between eDNA production, transport, and removal, we found weak but positive relationships between eDNA concentration, zebra mussels, and biophysical parameters. For example, while zebra mussel densities were only moderately predicted by eDNA concentrations, eDNA was most strongly influenced by nutrient concentrations and water velocity. These results may be used to inform future sampling strategies, where hydrological variables could better constrain eDNA fate. We also modelled estimates for net eDNA transport, retention, and degradation to estimate the relative importance of these processes for removing eDNA from the water column. In our study system, physical retention accounted for c. 70% of removal when compared to degradation alone, making it an important process to consider when assessing downstream eDNA transport.