Application of environmental DNA analysis for detecting potential sources of invasive aquatic plant Egeria densa in a riverine network

Invasive aquatic plants can have various ecological impacts on river ecosystems. It is important to identify the spatial positions of the invasive aquatic plant sources in a mainstem-tributary network to understand the distributions and prioritize the removal areas of invasive aquatic plants. This study used environmental DNA (eDNA) analyses to identify the potential sources (invasive macrophyte patches that persist throughout seasons in a river segment and can provide the plant fragments to the downstream) of Egeria densa, an invasive aquatic macrophyte, in a riverine network. We monitored the seasonal changes (March, July, August, and October) of eDNA concentrations of E. densa in tributaries with their associated mainstem sites (total 16 × 2 = 32 sites) in the Gonokawa River, Japan in 2021 and identified the potential sources of the target species based on the spatial abundance patterns and persistence in the riverine network. Our results indicated that the eDNA concentrations of E. densa have decreased in the mainstem from March to October, whereas the eDNA concentrations in the tributary sites had opposite temporal patterns possibly because the artificial tributary sites could be less affected by the flood event. In addition, certain mainstem segments and tributaries had higher eDNA concentrations and persistence of E. densa than other locations and could be potential sources of the invasive macrophyte in the system. Our study helps to identify and remove the potential sources of invasive macrophytes and prevent the further spread in riverine systems.