Antimony is a trace element ubiquitously present in the environment, but data are lacking on its spatio-temporal distribution in aquatic environments. Long-term records serve as essential tools to decipher temporal patterns, historical sources and sinks and background concentrations in an area. We characterise the temporal concentrations, transport and behaviour of antimony in the Garonne River watershed, the main tributary to the Gironde Estuary, the largest estuary in south-west Europe. AbstractKnowledge of the environmental chemistry of antimony (Sb) in aquatic systems is limited, and a better understanding of its geochemical behaviour is needed. Based on a fourteen-year survey (2003–2016) with monthly measurements of dissolved and particulate Sb at five sites in the Lot–Garonne river system, combined with daily measurements of water discharge and suspended particulate matter, this work characterises Sb behaviour in the upstream major river watershed of the Gironde Estuary. The survey provides a first regional geochemical Sb background in the Garonne River watershed for dissolved (~0.2 µg L−1) and Th-normalised particulate Sb (Sbp/Thp ~0.25) concentrations. Observed decreasing temporal trends (<1 ng L−1 in dissolved and <0.02 mg kg−1 in particulate concentrations per month) at sites representing natural concentrations probably reflect global atmospheric Sb dynamics at the watershed scale. Regular seasonal cycles of solid/liquid partitioning, with higher solubility in summer (matching high dissolved and low particulate concentrations), reflect water-discharge and suspended particulate matter transport dynamics and possibly seasonal (bio)geochemical processes. Furthermore, this coefficient decreases from the river to the estuarine reaches (from average log10Kd 4.3 to minimum 3.7 L kg−1), suggesting an increased solubility of Sb in estuarine systems. Flux estimates indicate the relevance of the dissolved fraction in Sb transport (with negligible influence of the colloidal fraction) and a total flux (dissolved + particulate) entering the Gironde Estuary of 5.66 ± 2.96 t year−1 (~50 % particulate). These results highlight the importance of timescales and environmental parameters for understanding and prediction of future Sb biogeochemistry.< Réduire