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Abstract Inland waters emit significant amounts of carbon dioxide (CO₂) to the atmosphere; however, the global magnitude and source distribution of inland water CO₂emissions remain uncertain. These fluxes have previously been “statistically upscaled” by independently estimating dissolved CO₂concentrations and gas exchange velocities to calculate fluxes. This scaling, while robust and defensible, has known limitations in representing carbon source limitations and spatial variability. Here, we develop and calibrate a CO₂transport model for the continental United States, simulating carbon transport and transformation in >22 million hydraulically connected rivers, lakes, and reservoirs. We estimate 25% lower CO₂fluxes compared to upscaling estimates forced by the same observational calibration data. While precise CO₂source distribution estimates are limited by the resolution of model parameterizations, our model suggests that stream corridor CO₂production dominates over groundwater inputs at the continental scale. Our results further suggest that the lack of observational networks for groundwater CO₂and scalable metabolic models of aquatic CO₂production remain the most salient barriers to further coupling of our model with other Earth system components. 

Abstract Rivers and streams play an important role within the global carbon cycle, in part through emissions of carbon dioxide (CO₂) to the atmosphere. However, the sources of this CO₂and their spatiotemporal variability are difficult to constrain. Recent work has highlighted the role of carbonate buffering reactions that may serve as a source of CO₂in high alkalinity systems. In this study, we seek to develop a quantitative framework for the role of carbonate buffering in the fluxes and spatiotemporal patterns of CO₂and the stable and radio‐ isotope composition of dissolved inorganic carbon (DIC). We incorporate DIC speciation calculations of carbon isotopologues into a stream network CO₂model and perform a series of simulations, ranging from the degassing of a groundwater seep to a hydrologically‐coupled 5th‐order stream network. We find that carbonate buffering reactions contribute >60% of emissions in high‐alkalinity, moderate groundwater‐CO₂environments. However, atmosphere equilibration timescales of CO₂are minimally affected, which contradicts hypotheses that carbonate buffering maintains high CO₂across Strahler orders in high alkalinity systems. In contrast, alkalinity dramatically increases isotope equilibration timescales, which acts to decouple CO₂and DIC variations from the isotopic composition even under low alkalinity. This significantly complicates a common method for carbon source identification. Based on similar impacts on atmospheric equilibration for stable and radio‐ carbon isotopologues, we develop a quantitative method for partitioning groundwater and stream corridor carbon sources in carbonate‐dominated watersheds. Together, these results provide a framework to guide fieldwork and interpretations of stream network CO₂patterns across variable alkalinities.