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Abstract Dissolved organic matter (DOM) is a heterogeneous mixture of organic compounds that is produced through both microbial degradation and abiotic leaching of solid phase organic matter, and by a wide range of metabolic processes in algae and higher plants. DOM is ubiquitous throughout the hydrologic cycle and plays an important role in watershed management for drinking water supply as well as many aspects of aquatic ecology and geochemistry. Due to its wide-ranging effects in natural waters and analytical challenges, the focal research questions regarding DOM have varied since the 1920s. A standard catchment-scale model has emerged to describe the environmental controls on DOM concentrations. Modest concentrations of DOM are found in atmospheric deposition, large increases occur in throughfall and shallow soil flow paths, and variable concentrations in surface waters occur largely as a result of the extent to which hydrologic flow paths encounter deeper mineral soils, wetlands or shallow organic-rich riparian soils. Both production and consumption of DOM occur in surface waters but appear to frequently balance, resulting in relatively constant concentrations with distance downstream in most streams and rivers. Across biomes the concentration and composition of DOM in flowing waters is driven largely by soil processes or directmore »inputs to channels, but high levels can be found in streams and rivers from the tropics to the poles. Seven central challenges and opportunities in the study of DOM should frame ongoing research. These include maintaining or establishing long-term records of changes in concentrations and fluxes over time, capitalizing on the use of sensors to describe short-term DOM dynamics in aquatic systems, integrating the full carbon cycle into understanding of watershed and aquatic DOM dynamics, understanding the role of DOM in evasion of greenhouse gases from inland waters, unraveling the enigma of dissolved organic nitrogen, documenting gross versus net DOM fluxes, and moving beyond an emphasis on functional ecological significance to understanding the evolutionary significance of DOM in a wide range of environments.« less

Mean annual temperature and mean annual precipitation drive much of the variation in productivity across Earth's terrestrial ecosystems but do not explain variation in gross primary productivity (GPP) or ecosystem respiration (ER) in flowing waters. We document substantial variation in the magnitude and seasonality of GPP and ER across 222 US rivers. In contrast to their terrestrial counterparts, most river ecosystems respire far more carbon than they fix and have less pronounced and consistent seasonality in their metabolic rates. We find that variation in annual solar energy inputs and stability of flows are the primary drivers of GPP and ER across rivers. A classification schema based on these drivers advances river science and informs management.