Optimal Sampling Density for Catchment-Scale River Biomonitoring with environmental DNA

Abstract

River networks are vital ecosystems that face increasing threats from societal pressures and global environmental changes. Effective biomonitoring of these systems is crucial for maintaining water quality and biodiversity by guiding policy and management actions. Over the past two decades, environmental DNA (eDNA) has emerged as a promising tool, offering advantages over traditional biomonitoring methods. However, the downstream transport and decay of genetic material in river networks poses a challenge for the spatial interpretation of eDNA samples. This study follows from work of Carraro et al. (2021) and aimed to find the optimal sampling density to determine taxon distributions at the catchment scale based on eDNA sampling. This was done by means of a virtual experiment, where a large number of sampling campaigns were simulated over multiple synthetic taxon distributions and river networks. This enabled the development of generalized sampling principles irrespective of system-specific confounding factors. Results showed that taking at least one sample per 10 km2 of catchment area provides a reliable estimate for taxon abundance across the catchment. Taking more samples only marginally improved the accuracy of the predictions when it concerns taxa that are commonly found in different parts of the catchment. In case of finding rare taxa, a minimum of two samples per 10 km2 is necessary to ensure that the taxon is correctly located. This study also underscores the limited capability of eDNA in capturing fine- scale spatial patterns, suggesting its complementary role alongside traditional surveys. In summary, this study provides a generalized sampling guideline for eDNA biomonitoring as a tool for broad-scale environmental assessments and biodiversity conservation.