title: Aquatic carbon cycling in the conterminous United States and implications for terrestrial carbon accounting.
abstract: Inland water ecosystems dynamically process, transport, and sequester carbon. However, the transport of carbon through aquatic environments has not been quantitatively integrated in the context of terrestrial ecosystems. Here, we present the first integrated assessment, to our knowledge, of freshwater carbon fluxes for the conterminous United States, where 106 (range: 71-149) teragrams of carbon per year (TgC⋅y(-1)) is exported downstream or emitted to the atmosphere and sedimentation stores 21 (range: 9-65) TgC⋅y(-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 flux at 69 (range: 36-110) TgC⋅y(-1) or 65% of the total aquatic carbon flux for the conterminous United States. Comparing our results with the output of a suite of terrestrial biosphere models (TBMs), we suggest that within the current modeling framework, calculations of net ecosystem production (NEP) defined as terrestrial only may be overestimated by as much as 27%. However, the internal production and mineralization of carbon in freshwaters remain to be quantified and would reduce the effect of including aquatic carbon fluxes within calculations of terrestrial NEP. Reconciliation of carbon mass-flux interactions between terrestrial and aquatic carbon sources and sinks will require significant additional research and modeling capacity.
authors.affiliation: US Geological Survey, Boulder, CO 80303; School of Environmental and Forest Sciences and the Department of Civil and Environmental Engineering, University of Washington, Seattle, WA 98195; dbutman@uw.edu.
authors.forename: David
authors.initials: D
authors.lastname: Butman
authors.affiliation: US Geological Survey, Denver, CO 80225;
authors.forename: Sarah
authors.initials: S
authors.lastname: Stackpoole
authors.affiliation: US Geological Survey, Boulder, CO 80303;
authors.forename: Edward
authors.initials: E
authors.lastname: Stets
authors.affiliation: Wisconsin Department of Natural Resources, Madison, WI 53707.
authors.forename: Cory P
authors.initials: CP
authors.lastname: McDonald
authors.affiliation: US Geological Survey, Denver, CO 80225;
authors.forename: David W
authors.initials: DW
authors.lastname: Clow
authors.affiliation: US Geological Survey, Boulder, CO 80303;
authors.forename: Robert G
authors.initials: RG
authors.lastname: Striegl
mesh_terms: D001272:Atmosphere; D002244:Carbon / Q000032:analysis*; D057486:Carbon Cycle; D060106:Lakes / Q000737:chemistry*; D045483:Rivers / Q000737:chemistry*; D014481:United States
chemical_list: D002244:Carbon
keywords: aquatic ecosystems; carbon; carbon flux; inland waters; terrestrial ecosystems