Aquatic carbon fluxes from the conterminous US and Alaska

Abstract:

In 2007, the First State of the Carbon Cycle Report estimated that rivers exported ~ 35 Tg-C yr^-1 to coastal systems and reservoirs in the US served as sink of ~ 25 Tg-C yr^-1 through sedimentation, each reported with 95% confidence that the estimate was within 100%. Significant progress has been made to constrain and improve these estimates by carefully considering how inland water ecosystems dynamically process, transport, and sequester carbon with attention given to the gaseous evasion of carbon across the air-water interface, a component that was not included in the 2007 estimates. As part of the U.S. Geological Survey’s LandCarbon program, we present the first integrated assessment of freshwater carbon cycling for the conterminous US and Alaska. We estimate that 147 (95% confidence interval of 101- 208) Tg-C yr^-1 is exported downstream or emitted to the atmosphere and sedimentation stores 22 (95% confidence interval of 10-68) Tg-C yr^-1 in lakes and reservoirs. We show that there is significant regional variation in aquatic carbon flux, but verify that emission across stream and river surfaces represents the dominant removal flux at 85 Tg-C yr^-1, or 58% of the total aquatic carbon flux. These new estimates for aquatic carbon fluxes indicate that inland waters must be considered in the context of national scale carbon accounting. For the conterminous US, we compare our results to the output of Terrestrial Biosphere Models. Analysis suggests that within the current modelling framework, calculations of Net Ecosystem Production may be overestimated by as much as 27%. Reconciliation of mass-flux interactions between terrestrial and aquatic carbon sources and sinks will require significant additional field data collection and modelling capacity.