Aquatic biodiversity on Reunion Island: responses of biological communities to environmental and anthropogenic pressures using environmental DNA

Abstract

Island ecosystems, characterized by isolation and vulnerability, are subject to natural and human-induced pressures. Rapid and effective biodiversity monitoring is crucial for tracking these impacts and adapting conservation efforts. This study focuses on Reunion Island (South-West Indian Ocean), where aquatic biodiversity is threatened by habitat loss, invasive species, and climate change. Stressors, both environmental and human-caused, can affect aquatic community structures. To test this hypothesis, a comprehensive dataset was compiled from various aquatic habitats, including rivers, ponds, reefs, and coastal waters. Biodiversity data for bacteria, diatoms, invertebrates, and fish were collected using eDNA metabarcoding, while environmental and anthropogenic parameters were recorded through field measurements and local databases. Redundancy analysis was used to identify the spatial distribution patterns of aquatic communities and their variations in response to these parameters. Results showed a significant distinction between freshwater and marine communities, with rivers and ponds hosting fewer taxa than marine environments, reflecting unique ecological patterns. In freshwater systems, fish and invertebrate communities are significantly driven by conductivity, temperature, and metals such as arsenic and barium, while diatoms and bacteria are primarily influenced by oxygen levels, atrazine, and perfluorooctanesulfonate. In marine environments, community composition is primarily affected by turbidity and conductivity. This study demonstrated that eDNA methods are effective for routine monitoring of large taxonomic groups, enabling the detection of biodiversity changes related to water chemistry in watersheds. These approaches, commonly used on continents, are also effective in monitoring biodiversity on tropical islands threatened by human activities.