Title: Spatio-temporal variations of strontium isotope ratios in the Mur River: a tool to support river management

Authors:  Ulrike Moser, Barbara Čeplak, Stefan Wagner, Shaun T. Lancaster, Martin Šala, Thomas Prohaska, Gorazd Žibret, Johanna Irrgeher

Abstract:
This study explores the application of strontium (Sr) isotope ratios (⁸⁷Sr/⁸⁶Sr) as geochemical tracers to understand hydrological connectivity and anthropogenic influences in a geologically diverse Alpine river catchment encompassing regions of Austria and Slovenia. Focusing on the Mur River and its tributaries, water samples were collected across 45 sites during three hydrological regimes to capture seasonal and spatial variability. In total, 28 tributaries were sampled to evaluate their influence on the isotope composition of the main river. Isotope analyses using multi-collector inductively coupled plasma mass spectrometry, supplemented by geospatial mapping and multivariate statistics, were combined with measurements of 26 elemental concentrations to investigate land–water interactions and human impacts. The results demonstrate substantial temporal and spatial variability in ⁸⁷Sr/⁸⁶Sr ratios (ranging from 0.70853 to 0.71322), reflecting both geological heterogeneity and varying tributary inputs. A preliminary aquatic isoscape was developed, enabling the monitoring of connectivity and the impacts of geological and human influences under varying flow regimes. Tributaries, such as the Mürz River, significantly modulate isotope signatures in the main channel, often overriding local geological signals. The application of isotope pattern deconvolution revealed a contribution of 17% from the Mürz River to the Mur River’s isotope signature downstream of the confluence. Correlations between ⁸⁷Sr/⁸⁶Sr and metal concentrations further suggest the utility of ⁸⁷Sr/⁸⁶Sr as a proxy for source identification and anthropogenic influence assessment. By constructing a preliminary strontium isoscape for the region, this research provides novel insights into riverine connectivity, catchment-scale processes, and ecosystem dynamics. The integration of isotope data contributes to interdisciplinary understanding across geological, chemical, and ecological boundaries, and supports the development of isotope-based tools for sustainable aquatic system monitoring and management, including potential applications in fish ecology and habitat tracking.