Assessing the nonconservative fluvial fluxes of dissolved organic carbon in North America

Fluvial transport of dissolved organic carbon (DOC) is an important link in the global carbon cycle. Previous studies largely increased our knowledge of fluvial exports of carbon to the marine system, but considerable uncertainty remains about in‐stream/in‐river losses of organic carbon. This study presents an empirical method to assess the nonconservative behavior of fluvial DOC at continental scale. An empirical DOC flux model was trained on two different subsets of training catchments, one with catchments smaller than 2,000 km 2 (n = 246, avg. 494 km 2 ) and one with catchments larger than 2,000 km 2 (n = 207, avg. 26,525 km 2 ). A variety of potential predictors and controlling factors of fluvial DOC fluxes is discussed. The predictors retained for the final DOC flux models are runoff, slope gradient, land cover, and areal proportions of wetlands. According to the spatially explicit extrapolation of the models, in North America south of 60°N, the total fluvial DOC flux from small catchments (25.8 Mt C a −1 , std. err.: 12%) is higher than that from large catchments (19.9 Mt C a −1 , std. err.: 10%), giving a total DOC loss of 5.9 Mt C a −1 (std. err.: 78%). As DOC losses in headwaters are not represented in this budget, the estimated DOC loss is rather a minimum value for the total DOC loss within the fluvial network.