The role of gas exchange in the inorganic carbon, oxygen, and ²²²Rn budgets of the Amazon River1

Abstract

Dissolved oxygen, ***Rn, pC02, 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 222Rn into floating domes yielded an average boundary layer thickness of 78 pm, and oxygen mass balance calculations resulted in an average of 38 pm. Given a boundary layer thickness on the order of 50 pm, CO, loss to the atmosphere in the entire reach would have been 37.4 kmol s-l, which is about equal to the total tributary dissolved inorganic carbon (DIC) input (43 kmol s-l) and is about half of the total fluvial DIC input to the section (97.7 kmol s-l). Thus, CO, 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,. Dissolved ***Rn activities in the mainstem varied from 3.5 to 8.3 dpm liter-' and were always highly supersaturated with respect to the atmosphere. Dissolved radon was also not supported by decay ~f~*~Ra (0.59-0.70 dpm liter-') in the mainstem. A ***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.

Factors controlling nutrient concentrations in Amazon floodplain lakes1

“…In the Amazon River system, alkalinity is derived almost exclusively from the weathering of carbonates and evaporites in the Andes Mountains and Andean piedmont region (Gibbs 1967;Stallard and Edmond 1983;Devol et al 1987). Since the ,4mazon mainstem and most of the major tributaries we sampled (I& Jurua, Japura, Purfis, Madeira) have headwaters in these areas, their alkalinity levels were generally high ( Table 2).…”

Excess atmospheric carbon dioxide transported by rivers into the Scheldt estuary

“…Cependant, les fleuves sont aussi largement sursaturés en CO 2 vis-à-vis de l'atmosphère et transportent donc un excès de CO 2 atmosphé-rique, qui résulte de la respiration dans les sols et le milieu aquatique et peut représenter une fraction significative du carbone inorganique dissous (CID). Cet excès de CO 2 est en partie transporté longitudinalement par les eaux de surface et en partie transféré vers l'atmosphère, dans des proportions qui dépendent du débit, de la différence de pression partielle en CO 2 (pCO 2 ) et du coefficient d'échange gazeux eau-air K [1,11,22]. Dans les fleuves et les estuaires, les valeurs de K sont supérieures à celles de l'océan, pour une même vitesse de vent, car la turbulence créée par les courants favorise aussi la ventilation des gaz [1, 6, 10-13, 17, 21].…”

“…In pristine basins, soil respiration sometimes dominates [17] and sometimes not [11]. In polluted rivers, the contribution of aquatic respiration increases [16,21].…”

“…Moreover, gas exchange coefficients are higher in estuaries than in rivers [1], so that physical ventilation of the CO 2 input by rivers occurs in the estuaries [13]. Although it sometimes represents a significant fraction of the DIC, very few studies quantify the transport of excess CO 2 by rivers [11,22]. In this paper, we use a one-year data set of oxygen, pH, alkalinity and organic carbon in the five rivers entering a highly polluted European estuary, the Scheldt (Belgium, the Netherlands).…”

“…Consequently, a fraction of the dissolved inorganic carbon (DIC) in rivers is present as 'excess CO 2 ', and can escape to the atmosphere due to physical water-air equilibration. This excess CO 2 results from: (1) input from surrounding areas, including soils [5,17], wetlands [15], intertidal marshes [9], and urban areas [16]; (2) production by heterotrophic organisms and assimilation by phytoplankton and periphyton [5,11,13,21]; and (3) evasion to the atmosphere that is determined by both the pCO 2 difference between the water and the atmosphere and the gas exchange coefficient K. Many different empirical relationships are used, giving the value of K as a function of either the river discharge [17,21], the current speed and water column depth [6], the wind speed [10] or the wind and current speeds. However, these studies all show that because of the turbulence of the water, gas exchange coefficients are higher in rivers and estuaries than in the ocean for the same wind speed.…”