The role of gas exchange in the inorganic carbon, oxygen, and 222 Rn budgets of the Amazon River 1

Dissolved oxygen, 222 Rn, pCO 2 , alkalinity, respiration rate, and discharge have been measured at eight mainstem and seven tributary stations during February–March 1984 in a 1,700‐km stretch of the Amazon River between Vargem Grande and Obidos in Brazil. Air–water gas exchange rates were estimated two ways: measurements of the flux of 222 Rn into floating domes yielded an average boundary layer thickness of 78 µ m, and oxygen mass balance calculations resulted in an average of 38 µ m. Given a boundary layer thickness on the order of 50 µ m, CO 2 loss to the atmosphere in the entire reach would have been 37.4 kmol s −1 , which is about equal to the total tributary dissolved inorganic carbon (DIC) input (43 kmol s −1 ) and is about half of the total fluvial DIC input to the section (97.7 kmol s −1 ). Thus, CO 2 evasion is a major component of Amazon River DIC balance. Because gas exchange within the section was rapid relative to water travel time through the section, a quasi‐steady state was maintained between respiratory input and evasion of CO 2 . Dissolved 222 Rn activities in the mainstem varied from 3.5 to 8.3 dpm liter −1 and were always highly supersaturated with respect to the atmosphere. Dissolved radon was also not supported by decay of 226 Ra (0.59–0.70 dpm liter −1 ) in the mainstem. A 222 Rn mass balance indicated that direct groundwater input into this stretch of the Amazon mainstem probably accounted for no more than 1% of water discharge.