Spatial and temporal variability of pCO2 and CO2 efflux in seven Amazonian Rivers

Current estimates of CO2 outgassing from Amazonian rivers and streams have considerable uncertainty since they are based on limited-time surveys of pCO(2) measurements along the Amazon mainstem and mouths of major tributaries, using conservative estimates of gas exchange velocities. In order to refine basin-scale CO2 efflux estimates from Amazonian rivers, we present a long time (5-year) dataset of direct measurements of CO2 fluxes, gas transfer velocities and pCO(2) measurements in seven representative rivers of the lowland Amazon basin fluvial network, six non-tidal (Negro, Solimes, Teles Pires, Cristalino, Araguaia and Java,s) and one tidal river (Caxiuan), with sizes ranging from 4th to 9th order. Surveys were conducted from January 2006 to December 2010, in a total of 389 campaigns covering all stages of their hydrographs. CO2 fluxes and gas transfer velocities (k) were measured using floating chambers and pCO(2) was measured simultaneously by headspace extraction followed by gas chromatography analysis. Results show high CO2 flux rate variability among rivers and hydrograph stages, ranging from -0.8 to 15.3 mu mol CO2 m(-2) s(-1), with unexpected negative fluxes in clear-water rivers during low waters. Non-tidal rivers showed marked seasonal CO2 flux patterns, with significantly higher exchange during high waters. Seasonality was modulated by pCO(2), which was positive and strongly correlated with discharge. In these rivers k was well correlated with wind speed, which allowed the use of wind data to model k. We estimate a release of 360 +/- A 60 Tg C year(-1) from Amazonian rivers and streams within a 1.47 million km(2) quadrant in the central lowland Amazon. Extrapolating these values to the basin upstream of A"bidos, results in an outgassing of 0.8 Pg C to the atmosphere each year. Our results are a step forward in achieving more accurate gas emission values for Amazonian rivers and their role in the annual carbon budget of the Amazon basin.