Patterns and potential drivers of land-water productivity coupling across U.S. river systems

Watershed ecosystems rely on energy and nutrient exchanges between terrestrial and aquatic systems, which influence carbon and nutrient cycles, biodiversity, and overall ecosystem dynamics. The synchronization of terrestrial and riverine productivity, referred to as coupling strength (CS), provides a useful metric for assessing ecosystem integration and responses to environmental change. Despite its importance, the spatial variability of CS and its environmental drivers remain poorly understood, particularly across regions with diverse natural conditions and human impacts. This study quantifies CS across over one hundred river sites and their upstream watersheds in the continental United States. Using explainable machine learning, we identified key environmental factors, including water temperature, river width, leaf area index, and watershed area, that exhibit distinct nonlinear relationships with CS. Clustering analyses revealed spatially diverse coupling regimes, influenced by a combination of ecohydrological processes and anthropogenic activities. These findings advance the understanding of how environmental conditions mediate synchronization between terrestrial and aquatic productivity. The results provide a foundation for future research into the mechanisms of land-water interactions and their responses to environmental stressors. By integrating these insights into broader ecological and hydrological frameworks, this work can support the development of predictive tools for watershed management under changing environmental conditions.