Average of discharge from Ji-Paraná river (1978 to 2001) (Agência Nacional das Águas – ANA)

1. 24,2 177,5 166,8 240,5 High water Low water 60,3 610,8 AVERAGE LOW WATER HIGH WATER Average COM-1 and PB-1 atm equilibrium tributaries Ji-Paraná CO2* (µM) CO2* (µM) pCO2 (µatm) pH HCO3-(µM) HCO3-(µM) pCO2 (µatm) DIC (µM) N J M A O M A J J S D F Average JIP-1, 4, 5 Machadinho pH pCO2 (µatm) pCO2 (µatm) 24,4 29,4 87,1 84,1 84,3 105,4 156,9 185,9 High water Low water High water Low water Base Saturation ( % ) Urupá Average JIP-2,3 Discharge (m3.s-1) pCO2 (µatm) pH pH Average of discharge from Ji-Paraná river (1978 to 2001) (Agência Nacional das Águas – ANA) 215,6 264,3 High water Low water CO2 evasion (mol CO2.m-2.d-1) Pimenta Bueno -2 Fev/00 May/99 Jun/00 Dec/01 Jul/01 Mar/01 Nov/99 Sep/00 pH pCO2 (µatm) pH pCO2 (µatm) Abr/02 * D/Z = K (exchange coeficient) = 1,2 m.d-1 (Richey et al, 2002) 2.7 pH meter Condutivity meter 4,2 36,1 177,2 38,0 154,7 24,3 219,7 pH condutivity High water Low water Low water High water Infrared gas analyser, non dispersive Total Organic Carbon Analyser (Shimadzu, 5000A) SH 37.13 The influences of total dissolved inorganic carbon (DIC) concentrations and pH on potential outgassing from rivers in Rondônia. Maria de Fátima F. L. Rasera1; Alex Vladimir Krusche1; Nei K. Leite1; Jeffrey E. Richey2; Anthony K. Aufdenkampe3 1 Universidade de São Paulo –CENA. Lab. de Ecologia Isotópica. Piracicaba/SP – Brasil mrasera@cena.usp.br 2School of Oceanography, University of Washington, USA 3Stroud Water Research Center - USA INTRODUCTION RESULTS Recent studies point to the importance of CO2 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 pCO2 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 CO2 outgassing to atmosphere from rivers of the Ji-Paraná basin, Rondônia. OBJECTIVE To expand the understanding of changes in pH and DIC concentrations in the Ji-Paraná River Basin and its consequences for the outgassing of CO2. STUDY AREA METODOLOGY River Water Field CONCLUSION temperature Add Thymol The partial pressures of dissolved CO2 (pCO2) 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 CO2, 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 CO2 (pCO2) 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 pCO2 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 CO2 is limited by lower pCO2. Dissolved Inorganic Carbon - ( DIC ) Laboratory Filtration Thermodynamic equilibrium equations (Stumm & Morgan, 1996) [CO2*]= DIC.(10pH)2 . (10pH)2+ k1.(10pH) + k1.k2 [pCO2]= [CO2*] k0 Calculated Theoretical diffusion model : F = D.(Cwater - k.Pair) / Z(Broecker, 1974) Where: D = gas- and temperature-specific diffusion coefficient; Cwater= concentration of gas in the water; k.Pair = concentration of gas in equilibrium with the atmosphere Z = thicness of the boundary layer Acknowledgements