Unlocking the potential of hydrogen isotopes (δ2H) in tracing riverine particulate organic matter sources and dynamics

This study explores the efficacy of hydrogen isotopes in tracing the origins and dynamics of particulate organic matter (POM) in the Loire River, employing a novel dual vapor equilibration method to measure its non-exchangeable hydrogen (δ²H_n). By integrating δ²H_n with traditional carbon (δ¹³C) and nitrogen (δ¹⁵N) isotopes and C/N ratios of POM, we compared the ecological information using multivariate analyses. Fortnightly river sampling over 3 years showed seasonal patterns in the δ²H_n values of POM, highlighting two primary POM end-members: phytoplankton and terrestrial matter. Bayesian mixing models confirmed that using δ²H_n as a tracer effectively discriminated these riverine POM end-members, with phytoplankton predominating in spring–summer and terrestrial matter in winter. Redundancy analyses revealed the main environmental drivers of POM composition, identifying significant correlations between the POM sources, chlorophyll a, suspended particulate matter, and river discharge. Our findings demonstrate that δ²H_n, alone or combined with δ¹³C, δ¹⁵N, or N/C ratio, provided a new robust tracer for POM source dynamics, thereby offering valuable insights into riverine biogeochemical cycles and ecosystem functioning. Our study underscores the novel potential of δ²H_n as a tool in environmental and ecological research, advocating for its broader application across various aquatic ecosystems to enhance our understanding of organic matter dynamics.