The Role of Rivers in the Regional Carbon Balance

Abstract

Through the evolution of the Large-Scale Biosphere-Atmosphere Experiment in Amazonia, fluvial systems evolved from being regarded as ecologically interesting, but not necessarily relevant to the carbon budget, to important systems outgassing a volume of CO2 roughly equal to the carbon sequestered by the forest. Resolving the role of fluvial systems in the carbon balance of the Amazon basin is a problem in scaling, from small seeps and springs to streams to larger rivers. Groundwater discharge of CO2 and its subsequent evasion is a significant conduit for terrestrially respired carbon in tropical headwater catchments. Hydrologic transport of dissolved CO2 was equivalent to nearly half the gaseous CO2 contributions from deep soil (>2 m) to respiration at the soil surface. At larger scales, the dominant feature was a clear relation between discharge and biogeochemical concentrations, with systematic variance among sites. Seasonal distributions of pCO2 rose and fell almost exactly with the discharge hydrograph, while pH decreased and dissolved organic carbon increased. This suggests a constancy of processes across systems. Gas exchange is greater than previously thought, primarily due to greater outgassing in smaller streams than expected. No single organic matter source consistently fuels respiration; instead, the δ13C of respiration-derived CO2 varies with time and space. Photochemical production of labile bioavailable compounds would appear to be limited to clear water conditions. Based on these results, the original Richey et al. (2002) estimate of outgassing of 1.2 ± 0.3 Mg C ha-1 a-1 is conservative; the true value is likely higher. © 2009 by the American Geophysical Union. All rights reserved.