# A Multi‐Taxa Approach to Estuarine Biomonitoring: Assessing Vertebrate Biodiversity and Ecological Continuity Using Environmental DNA Metabarcoding in the Rance River (Brittany, France)

## TL;DR

The study used eDNA to monitor vertebrate biodiversity in the Rance Estuary, showing it can effectively complement traditional methods.

## Contribution

The study demonstrates eDNA metabarcoding as a non-invasive, multi-taxa tool for estuarine biodiversity assessment.

## Key findings

- eDNA detected 124 vertebrate MOTUs across marine to freshwater stations in the Rance Estuary.
- Community composition followed the estuarine gradient, with species turnover driving dissimilarity.
- eDNA metrics aligned with traditional surveys and revealed fine-scale structuring in actinopterygians.

## Abstract

Estuaries are ecologically vital yet highly impacted ecosystems that serve as transitional zones between land and sea. Monitoring their biodiversity is essential but challenging due to their dynamic nature and the transient presence of many species. Traditionally, actinopterygian monitoring in these systems still relies on conventional and intrusive methods such as gill nets and trawls. Environmental DNA (eDNA) metabarcoding offers a non‐invasive, multi‐taxa alternative that can complement these traditional approaches. Here, we applied an eDNA‐based metabarcoding approach to characterize vertebrate diversity in the Rance Estuary, located in the Brittany Region of France. Water samples were collected from five stations spanning marine to freshwater environments. Special attention was given to two stations located upstream and downstream of the tidal power plant (TPP) dam to assess its potential impact on ecological continuity. We detected a total of 124 distinct vertebrate MOTUs, comprising actinopterygians, birds, mammals, and amphibians. Taxonomic composition followed the estuarine gradient, with Jaccard dissimilarity increasing with distance from the sea and largely driven by species turnover. While taxonomic and phylogenetic diversity remained relatively stable across the vertebrate community, functional diversity revealed an increasing terrestrial influence. For actinopterygians, taxonomic diversity decreased upstream, whereas phylogenetic and functional diversity indicated fine‐scale structuring, even among nearby stations. This approach enabled the development of biodiversity metrics and facilitated comparisons with previous actinopterygian monitoring surveys in the same area based on conventional methods (scientific fishing using nets and dredges). Our results emphasize the potential of eDNA for holistic estuarine biomonitoring and establish a valuable baseline for future non‐invasive assessments.

We applied eDNA metabarcoding to assess vertebrate diversity in the Rance Estuary, France, across five stations spanning marine to freshwater environments, including areas upstream and downstream of a tidal power plant dam. A total of 124 vertebrate MOTUs were detected, with community composition reflecting the estuarine gradient and species turnover driving dissimilarity. Our results demonstrate that eDNA provides a non‐invasive, integrative approach for monitoring estuarine biodiversity and can complement traditional fish surveys.