The Pulse of the Amazon River System: How pCO2 Evolves, from Small Streams to the Atlantic Ocean

Abstract:

A long-standing paradigm of river networks is that they are minor components in the global carbon cycle, passively connecting the land and ocean reservoirs. But recent research in the Amazon and globally show that the opposite is true- that fluvial systems are highly active, dynamic processors of organic carbon of terrestrial and aquatic origin, releasing CO₂ back to the atmosphere. That said, knowledge of how the fluvial system processes carbon and nutrients requires being able to track carbon processes throughout a basin, not just at select points; i.e., how do the pathways and fluxes of organic matter, nutrients, and associated elements evolve through river corridors, from the land-water interface to the ocean? The problem in understanding fluvial carbon dynamics at scales larger than relatively easily measured discrete streams is determining the spatial and temporal distributions across such a range of environments. This is a mesoscale question, where it is necessary to understand the distribution of moisture regimes and biogeochemical processes at scales of ~10,000–100,000 km² and up. Here we examine the spatial-temporal distributions of pCO₂ across the Amazon River system. Because of the vast and remote nature of the Amazon basin, the logistics of establishing a comprehensive and representative sampling network are considerable. The Rede Beija Rio network was established to conduct such measurements, wherein each node is occupied by a team of researchers from that site. Results showed pronounced consistency. For example, pCO₂ tracked the hydrograph at all sites, with maximum concentrations at high water, and minima at low. pCO₂ at low water ranges from 500 μatm in the Rio Araguaia and Rio Ji-Paraná to 1000 μatm in the Rio Solimões, to 2000 μatm in the Rio Negro. High water concentrations exhibit a broader range and higher magnitude, from 3000 μatm (Rio Pimento Bueno) to 5000 μatm in the Rio Solimões and 7000 μatm in the lower Rio Negro. Interestingly, pCO₂ in the far upper Rio Negro, where pH is in the 3.5 range, had lower concentrations than at the mouth. The highest observed values were 20,000 μatm in Campinas, a blackwater tributary of the Rio Negro. These patterns are reflected in major and minor ions, nutrients, sediments and sediment composition.