Turning trash into treasure: Leveraging discarded filters for national-scale aquatic eDNA biomonitoring

Monitoring biodiversity changes over large spatiotemporal scales is critical for effective ecosystem conservation and management. This study investigates the potential of environmental DNA (eDNA) metabarcoding to enhance national-scale biomonitoring of freshwater diversity by leveraging discarded filters associated with routine water quality sampling from the U.S. Geological Survey (USGS) National Water Quality Network (NWQN). We tested 375 samples from 103 NWQN sites for eDNA of native and non-native fish and found that 52% of the filters yielded any fish eDNA for a total of 70 fish species detections. Of the filters that had fish eDNA present, filters had an average of 3.7 species detected per filter. Benchmarking these results to USGS Aquatic Gap Analysis (AGAP)- which includes both field-verified observations along with predictive models derived from fish capture and landscape predictor datasets-we found that eDNA from these filters detected only a fraction of the observed and expected fish diversity for these sites. Our results indicate that these discarded filters may not be sufficient for eDNA sampling of fish communities and posit that alternative filter types more appropriate for eDNA sampling would yield more valuable biomonitoring data. Nevertheless, we successfully demonstrated the potential of integrating eDNA sampling into existing monitoring frameworks, which could be a cost-effective strategy to enhance biodiversity monitoring at large scales.