1. Average of discharge from Ji-Paraná river (1978 to 2001) (Agência Nacional das Águas – ANA) Discharge (m 3.s -1 ) pCO 2 (µatm) JJ J F M M A AS O N D atm equilibriumtributariesJi-Paraná pCO 2 (µatm) Average COM-1 and PB-1 pH Low waterHigh water 2.7 38,0 24,3 4,2 pCO 2 (µatm) Average JIP-2,3 pH High waterLow water 177,5 166,8 24,2 240,5 pCO 2 (µatm) Average JIP-1, 4, 5 High waterLow water 156,9 84,1 24,4 185,9 CO 2 * (µM) HCO 3 - (µM) CO 2 * (µM) HCO 3 - (µM) pH Machadinho 105,4 29,4 87,1 84,3 pCO 2 (µatm) High waterLow water Pimenta Bueno -2 pCO 2 (µatm) pH High water 177,2 154,7 Low water 36,1 219,7 Base Saturation ( % ) Urupá pH 610,8 60,3 High waterLow water 215,6 264,3 pCO 2 (µatm) INTRODUCTION OBJECTIVE STUDY AREA Fev/00 Mar/01 Abr/02 May/99 Jun/00 Jul/01 Sep/00 Nov/99 Dec/01 The partial pressures of dissolved CO 2 (pCO 2 ) in the Ji-Paraná and its tributaries are a function of respiration and mineral weathering. Although rivers draining eutrofic soils (Rolim, Urupá, Jarú and Pimenta Bueno-2) show the highest DIC concentrations and outgassing (evasion) of CO 2, pH changes lead to a pronounced seasonality in evasion. The outgassing is higher during the rising water, because the dilution of ground water by rain water, and probably also changes in metabolism, result in lower pH values, thus increasing the partial pressures of dissolved CO 2 (pCO 2 ) and evasion process. In the falling water, groundwater stay longer in contact with the substrate and weathering, which consumes H + and converts carbonic acid into bicarbonate, increases pH and lowers pCO 2 and the evasion process. The rivers draining distrofic soils (Comemoração, Pimenta Bueno-1, Preto e Machadinho) show lower DIC concentrations and lower pH, and the potential evasion of CO 2 is limited by lower pCO 2. CONCLUSION The influences of total dissolved inorganic carbon (DIC) concentrations and pH on potential outgassing from rivers in Rondônia. Maria de Fátima F. L. Rasera 1 ; Alex Vladimir Krusche 1 ; Nei K. Leite 1 ; Jeffrey E. Richey 2 ; Anthony K. Aufdenkampe 3 1 Universidade de São Paulo –CENA. Lab. de Ecologia Isotópica. Piracicaba/SP – Brasil mrasera@cena.usp.brmrasera@cena.usp.br 2 School of Oceanography, University of Washington, USA 3 Stroud Water Research Center - USA Recent studies point to the importance of CO 2 outgassing from rivers of the Amazon, suggesting that a significant part of the carbon fixed by forest return to the atmosphere through this pathway. Gas exchange between the atmosphere and waters is a funcion of gaseous gradients across the air-water interface, and the water pCO 2 is strongly determined by the concentrations of dissolved inorganic carbon (DIC) and pH, which, in turn, are a funcion of physical, chemical and biological processes. This study focus on the influence of DIC concentrations and the pH on potencial CO 2 outgassing to atmosphere from rivers of the Ji-Paraná basin, Rondônia. METODOLOGY RESULTS River Water Field Laboratory Filtration Dissolved Inorganic Carbon - ( DIC ) Calculated Theoretical diffusion model : F = D.(C water - k.P air ) / Z (Broecker, 1974) Where: D = gas- and temperature-specific diffusion coefficient; C water = concentration of gas in the water; k.P air = concentration of gas in equilibrium with the atmosphere Z = thicness of the boundary layer Total Organic Carbon Analyser (Shimadzu, 5000A) Infrared gas analyser, non dispersive Thermodynamic equilibrium equations [CO 2 *] = DIC.(10 pH ) 2. (10 pH ) 2 + k 1.(10 pH ) + k 1.k 2 (Stumm & Morgan, 1996) DIC (µM) pH temperature pH pH meter condutivity Condutivity meter Add Thymol [pCO2] = [CO2*] k0 Acknowledgements CO 2 evasion (mol CO 2.m -2.d -1 ) AVERAGE HIGH WATER LOW WATER * D/Z = K (exchange coeficient) = 1,2 m.d -1 (Richey et al, 2002) SH 37.13 To expand the understanding of changes in pH and DIC concentrations in the Ji- Paraná River Basin and its consequences for the outgassing of CO 2.