Carbon export from land to oceans plays an important role in the global C cycle as well as coastal ecosystem sustainability. Despite making up only 0.3% of the global area, rivers tightly connect two of the largest carbon reservoirs on earth: terrestrial ecosystems and the ocean. Natural and anthropogenic factors are responsible for changes in riverine C fluxes, including atmospheric deposition, climate variability, hydrological characteristics, soil properties, and land use and land management practices. While considerable work has examined C export from one or some specific rivers, much less has been focused on continental-scale quantification of riverine C fluxes. A reliable estimate of the riverine C export from North America is lacking. This restricts our capability to quantify the full continental scale C budget (Butman et al. 2016), and to examine how climate change will impact greenhouse gas emissions from river system (Raymond & Cole 2003) and coastal ecosystem C dynamics (Gruber 2015). In this study, we used a process-based land biogeochemical model, the dynamic land ecosystem model (DLEM), in conjunction with several sources of data, to quantify the patterns of riverine C flux (DOC, TOC, DIC) from North America to the coast during 1980-2015. Specifically, we quantify 1) decadal patterns of river discharge and terrestrial C export from North America, 2) inter-annual variations in terrestrial C export in North America, and 3) seasonality of terrestrial C export. We also explore environmental controls on terrestrial C exports from North America.
Results/Conclusions
- The estimated mean total riverine annual C flux from North America during 1980-2015 was 94.3±7.4 Tg C yr-1, which is about 20% of land carbon sink of the continent. DIC represented 57% of total C flux, with the remaining flux as organic carbon.
- The mean annual total riverine C flux was larger during the most recent 6-year period (2010-2015) than any of the previous three decades.
- The magnitude and temporal patterns of terrestrial C export varied significantly among 10 sub-regions of North America. Total C export (~28%) and DIC export (~37%) was greatest from the Gulf of Mexico sub-basin and DOC export (~39%) was greatest from the Hudson Bay sub-basin, which contributed to 2nd largest total C export from the North America (~21%).
- The largest riverine C exports occurred in autumn, followed by summer, spring and winter from North America.
- Riverine C fluxes are strongly correlated with river discharge, which, in turn, is controlled by precipitation.