No pump, no problem: evaluating passive eDNA sampling for marine biomonitoring of a nuisance macroalga

Efficient detection and management of non-indigenous species are critical for mitigating their ecological impacts. Environmental DNA (eDNA) techniques have transformed biodiversity monitoring by enabling sensitive and cost-effective surveys. This study compares the efficacy of passive eDNA samplers (PEDS) to conventional active filtration methods for detecting the cryptogenic macroalga Chondria tumulosa within the Papahānaumokuākea Marine National Monument, Hawai'i, USA. Three components of the species-specific eDNA assay were evaluated: clinical sensitivity, DNA yield, and overall performance. Site-level detection sensitivity of 15-minute PEDS deployments matched that of 2-L active filtration, with both methods detecting C. tumulosa in all cases where it was known to present. Site-occupancy models provided a robust framework for evaluating overall performance, offering critical insights into the tradeoffs of PEDS for detecting rare taxa. The success of PEDS is largely dependent on the increased number of qPCR replicates employed in this study compared to the previously developed eDNA assay for C. tumulosa. Passive method performance resulted in lower qPCR detection rates with higher probabilities of false-positives and false-negatives. Model estimates for C. tumulosa eDNA occupancy were similar between PEDS affixed to stationary buoys and PEDS attached to roving SCUBA divers. There was, however, a decrease in the eDNA capture rate among samples collected while on SCUBA. We also tested two passive membrane types-research-grade mixed cellulose ester filters and low-cost cotton rounds. The absorbent cotton rounds yielded greater target eDNA yields and were more reliable for inferring the presence of C. tumulosa. However, DNA yields from PEDS were consistently lower than actively filtered samples, indicating the importance of optimizing sampling and processing protocols to balance erroneous detections. Despite these limitations, passive sampling successfully detected C. tumulosa at low abundances (<1%), demonstrating its utility for uncovering cryptic taxa. PEDS are a cost-effective, versatile, and scalable alternative to active filtration, particularly in remote or resource-limited settings.