Broad-scale detection of environmental DNA for an invasive macrophyte and the relationship between DNA concentration and coverage in rivers

Environmental DNA (eDNA) techniques have the potential to become useful tools for investigating the distribution and biomass of aquatic organisms. Additionally, Unmanned Aerial Vehicle (UAV) photos have recently been applied to evaluate vegetation cover. We tested the ability of eDNA techniques to evaluate the distribution and biomass of an invasive macrophyte, Egeria densa (Brazilian waterweed). We conducted field surveys in two Japanese rivers to determine the abundance of eDNA from E. densa and compared it with macrophyte coverage by evaluating UAV photographs taken in the summer and winter. We found eDNA of E. densa in all sampled sites, including various sites with no macrophyte coverage, indicating that there was a broad-scale distribution of E. densa eDNA. In summer, we detected a slightly positive relationship between E. densa eDNA and macrophyte coverage. We did not find a clear relationship between E. densa eDNA and macrophyte coverage, probably because of their widespread and dense distribution and the flow of their materials in the rivers. The seasonal differences in the regression may have been caused by the seasonal growing period of E. densa in the summer, with a decline in their biomass in late autumn and winter. We confirmed the ability of eDNA techniques to survey this invasive macrophyte in rivers by comparing results with coverage measurements from UAV photographs.