Drivers and riverine fluxes of rare earth elements to coastal ecosystems across temperate, boreal, and subarctic ecoregions in Eastern Canada†

Although rivers are recognized as major transporters of REEs to coastal environments, estimates of REE fluxes are still scarce, and the underlying drivers remain poorly understood—particularly in temperate, boreal, and subarctic river systems. This limits our understanding of the global REE cycle and limits our ability to identify or predict increases in exports driven by climate change or anthropogenic activities, which could pose a threat to biodiversity. This study aims to calculate and compare lotic REE fluxes and yields in 40 rivers draining watersheds with different geologies and climates within the Province of Quebec (∼1.5 M km²), Canada. Furthermore, we observed their differences in REE composition as well as the main factors driving their concentrations. We estimated the annual export of total REEs flowing into the Hudson Bay (1078 t per year) and the Atlantic Ocean (2941 t per year), which could increase significantly with climate change effects on the northern ecosystems. From these watersheds, rivers draining the Canadian Shield geology, characterized by lower water temperature and pH, but higher concentrations of dissolved organic carbon (DOC) and iron (Fe), exhibited higher concentrations of REEs compared to the broader study area, especially filtered light REEs (LREEs). In southern Quebec, rivers with intensive agricultural erosion were identified as REE export hotspots relative to their watershed area, while rivers draining densely populated regions exported anthropogenic gadolinium (Gd) (0.2–134 kg per year), contributing significantly to their total filtered Gd flux (18–98%). These findings provide essential baseline data to predict regional impacts of climate changes and anthropogenic activities on REE mobilization across evolving landscapes and clarify the role of temperate, boreal and subarctic rivers in the global REE cycle.