Estimating the Surface Area of Small Rivers in the Southwestern Amazon and Their Role in CO2 Outgassing

Abstract

A recent estimate of CO2 outgassing from Amazonian wetlands suggests that an order of magnitude more CO2 leaves rivers through gas exchange with the atmosphere than is exported to the ocean as organic plus inorganic carbon. However, the contribution of smaller rivers is still poorly understood, mainly because of limitations in mapping their spatial extent. Considering that the largest extension of the Amazon River network is composed of small rivers, the authors? objective was to elucidate their role in air?water CO2 exchange by developing a geographic information system (GIS)-based model to calculate the surface area covered by rivers with channels less than 100 m wide, combined with estimated CO2 outgassing rates at the Ji-Paraná River basin, in the western Amazon. Estimated CO2 outgassing was the main carbon export pathway for this river basin, totaling 289 Gg C yr?1, about 2.4 times the amount of carbon exported as dissolved inorganic carbon (121 Gg C yr?1) and 1.6 times the dissolved organic carbon export (185 Gg C yr?1). The relationships established here between drainage area and channel width provide a new model for determining small river surface area, allowing regional extrapolations of air?water gas exchange. Applying this model to the entire Amazon River network of channels less than 100 m wide (third to fifth order), the authors calculate that the surface area of small rivers is 0.3 ± 0.05 million km2, and it is potentially evading to the atmosphere 170 ± 42 Tg C yr?1 as CO2. Therefore, these ecosystems play an important role in the regional carbon balance.