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Although the flux of dissolved organic carbon (DOC) through freshwaters is nearly equivalent to the net carbon uptake of all terrestrial ecosystems, uncertainty remains about how source processes (carbon production and location) and transport processes (hydrologic connectivity and routing) interact to determine DOC flux across flow conditions and ecoregions. This limits our ability to predict the fluvial carbon flux responses to changes in climate and land use. We used DOC concentration and discharge patterns with ensemble modeling techniques to quantify DOC flux behavior for 1,006 U.S. watersheds spanning diverse climate and land cover conditions. We found that DOCflux was transport-limited (concentration increased with discharge) in 80% of watersheds and that this flux behavior spanned ecoregions and watershed sizes. The generality of transport limitation demonstrates how coupling discharge models with widely available watershed properties could allow DOC flux to be efficiently integrated into landscape and Earth system models.

Plain Language Summary When water flows through ecosystems, it picks up dissolved organic carbon (DOC) from plants and soils, sometimes determining whether the ecosystem is a net carbon source or sink. DOC is also an important water quality parameter and understanding how it is produced and transported affects society's ability to provide water for industrial, agricultural, and domestic uses. Because DOC flux through rivers varies widely with flow and in different regions, DOC flux remains a major source of uncertainty in the global carbon cycle. Based on one of the largest and most geographically diverse analyses of river DOC dynamics to date, we found surprising similarities in DOC flux behavior. From southwestern deserts to northeastern forests, hydrologic flow, not DOC sources, determined DOC flux behavior in 80% of watersheds in the conterminous United States. In other words, DOC concentration systematically increased with river flow, even during large flow events, indicating that organic matter stocks provide ample DOC to maintain delivery to rivers. Additional analysis of this large data set identified several landscape and climate conditions that predict DOC flux behavior in watersheds. Together, these findings demonstrate that watershed DOC flux can be simulated across spatial scales using river flow and widely available watershed properties.